Thought this was interesting.
Excerpt:
New Code Would Allow Unvented Attics
Asphalt roofing manufacturers oppose the change, fear trapped moisture and rot
Builders, designers, and manufacturers have long debated whether and when to vent attics, and a recent change to the International Residential Code aims to put that debate to rest. The 2004 code supplement allows unvented roof assemblies if two conditions are met: The interior surface of the ceiling has no vapor retarder and the underside of the roof deck is insulated with impermeable insulation with enough R-value to keep its monthly average temperature at or above 45¼F. The new provision will tentatively be included in the 2006 version of the IRC.
Proponents of the change say it will save energy by bringing ductwork inside the home’s conditioned space, and reduce mold and mildew problems in humid climates where attic vents can bring humid air into the building envelope.
But the new code language is not without its opponents. Earlier this year, the Asphalt Roofing Manufacturers Association petitioned the International Code Council to repeal the change. Dave Roodvoets of DLR Consultants, which represented ARMA in the effort, says the issue isn’t roofing, but moisture problems. He worries that requiring “air-impermeable” insulation could actually make a home more susceptible to rot by trapping moisture inside, even though it is keeping exterior moisture out. “Ventilation compensates for a lot of sins,” he says. And he claims that proponents don’t have enough research to justify the change. “Even the best researchers have only a few years of data on unvented attics in humid climates.” ……..
Edited 5/7/2005 10:52 am ET by csnow
Replies
ya, it is.
be vented
sobriety is the root cause of dementia.
So Imerc's attic, insulated per building design scientist, would not have passed this code.
I dunno about that particular case. Many ways to skin a cat.
>> two conditions are met: The interior surface of the ceiling has no vapor retarder and the underside of the roof deck is insulated with impermeable insulation with enough R-value to keep its monthly average temperature at or above 45¼F.
Looks like something designed by a committee with a computer modeling program: a great big hum and how the heck do you actually use it?
>>Monthly average temp at or above 45¼F.
Now there is an easy requirement to meet!
Are they going to speciy we only allow mold which won't grow when the condensation occurs only because of (i) a swing below average and (ii) an impermeable insulation installation with no mistakes made?
>> Impermeable? So we line the roof sheathing with a pool liner? Or the hull of a ship?
Are there any building products which are impermeable?
And perfectly installed?
I'm guessing the draftsman wasn't around when we started opening up all of those "superinsulated walls" we first put in after the oil embargo.
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Sojourners: Christians for Justice and Peace
The interior surface of the ceiling has no vapor retarder and the underside of the roof deck is insulated with impermeable insulation
If I'm reading that right, that seems to say only if the ceiling is unpainted (and/or there's no latex admixture in the popcorn); and a sprayed-in-place closed cell foam is the only permissible insualtion.
Except, some BI might (just might) let a person use rigid foam, foamed "closed" to the rafters. The rafters being the "trick" of the definition--some AHJ are going to include rafters in the "deck."
I know I'd like a simple answer; and there aren't many. Every place is different, and for different reasons. When it's 98ºF at 67% RH, there's not a realistic air volume, at near-full saturation, that has any cooling capacity "left." Particularly when it's versus a sun-heated asphalt shingled roof deck.Occupational hazard of my occupation not being around (sorry Bubba)
Latex paint is considered "permeable" and could be used on the ceiling no problem.Also remember that most rooms have at least one hole in the ceiling for a light fixture or an exhaust fan or a recessed light. These all "leak" to and from the attic space. Case studies have shown the attic measures within 10F of the house air with this system. Stu
Latex paint is considered "permeable"
Except the "VB" paints that are now out. Some AHJ employing people with only moderate managerial skills (or out-and-out PHB); said folk might not tollerate the 'confusion' of vb versus non-vb paint. (Not that I've sufferred bitterly at the hands of bureaucrats, or anything . . . <g>).
I've also seen some textured ceilings that were pretty much vapor proof . . . (even if not meant to be).Occupational hazard of my occupation not being around (sorry Bubba)
Yes there is an air impeneterable (spell?) building material. That is spray foam insulation. Either 1/2 or 2 lb.No air movement in the insulation and a very effective air infiltration blocker.The most effective way to use it in this manner is without the vent channel under it. Buildings have been made this way for years. BTW, how is every insulated flat roof in the world vented? They are not! The air right under it is 'conditioned' just like this change will make effective in residential construction.Stu
"spray foam insulation....The most effective way to use it in this manner is without the vent channel under it."
Why?
This keeps being repeated in this forum. Why?
Putting impermeable insulation under the roof deck directly contravenes hot roof designs by building scientists, and contradicts current thinking at buildingscience.com. Yet it keeps being repeated here as the only way to do a hot roof. Why?
http://www.buildingscience.com/housesthatwork/default.htmThey have a number of designs, in both hot and cold climates, that eiterh unvented cathederal roofs or conditioned attics.I don't have time to read through, but I believe that the only difference between the two is whether the bottom of the roof is finished or not.http://www.buildingscience.com/housesthatwork/default.htm
I looked at the building science site.
It calls for permeable insulation in the rafters in every case of a conditioned attic that I looked at.
Cold climate: Conditioned attic: low density spray foam insulation, 10 perms. Another system (vented cathedral) has permeable insulation over baffles over vents.
Mixed-humid: all vented.
Hot-dry/mixed-dry: all vented.
Hot-humid: permeable insulation such as cells, to allow drying to inside.
Perhaps the "Mooney ceiling" is not so off the mark.
Someone explain to me again why the only way to insulate an attic is impermeable foam insulation over the sheathing.
Taylor, allow me disagree a bit... and maybe have another look at BuildingScience.com's roofing insulation documentation. Here are some selective quotes from the article:
Where rigid insulation is installed above the roof deck, or an impermeable spray foam is applied under the roof deck, condensing surface temperatures are said to be controlled
So, even if Icynene and the like are great for exterior walls, Corbond and it's closed-cell cousins are a better choice for roof decks. Having a built-in vapor barrier works both ways, and there is no reason to require a permeable insulation layer.
Asphalt roofing shingles require special attention when installed on unvented roof assemblies in hot-humid, mixedhumid and marine climates due to inward vapor drive from incident solar radiation. A 1 perm or lower vapor retarder (Class II) as tested by the wet-cup procedure should be installed under the asphalt roofing shingles to control this inward drive.
Wood shingles or shakes, require a minimum 1/4-inch vented airspace that separates the shingles/shakes and the roofing felt placed over the roof sheathing for similar reasons.
So, yes, some roofing materials need a small air layer under them to control the deck temperature/humidity, but that has to do with the materials and the local climates, not the insulation. An example given were wooden shingles and asphalt tiles, where moisture might be driven towards the roof deck as a part of the normal operation of these materials.
Whenever there is the possibility of a lot of humid, warm air to interact with the roof deck, insulating it from the outside makes a lot of sense. The diagrams in the article detail many such possibilities, including hybrids of interior and exterior insulation to keep the whole system happy. I maintain that if your AC system is installed in a sealed attic, it is trivial to have that system move warm moist air back down to the rest of the house by having an inlet up there and just cycling the fan from time to time in concert with the HRV.
Naturally, closed-cell foam by itself may not be the only answer. If the climate is extreme enough, you may need additional exterior insulation to prevent ice damming and the like.
Lastly, applying closed cell foam should not occur in shots thicker than 4" to prevent the stuff from frying itself. Thus, it may be cost effective to have a mixture of board and in-situ applied foam to optimize the cost of insulating your home.
Edited 5/15/2005 1:49 pm ET by Constantin
At the Oak Ridge National Labs (ornl) site they have a program to tell you whether you will have a successful unvented low slope roof or not.http://www.ornl.gov(the calculator is in their somewhere)The idea goes something like this (for cold climates). Stack effect will drive vapour from inside to outside during the heating season and the other way during the summer (although not as much).The calculator looks at the permeability of the materials at various levels of your roof and based on your climate (and normal RH during the winter) gives you an idea of how much water is going to go into your roof over the winter.Then, the calculation turns around and calculates how much drying you can expect during the summer.The various assemblies they suggest work with no vapour barrier because the roofs are designed to dry inside and outside when possible.Anyway, the site gives a much more detailed treatment of this and gives the formulae to calculate whether a particular design is going to work. You could use it online or just program it into a calculator.
If my house wasn't a 100 years old... there would go a really fine hobby.
Taylor & All:
I can't get the open cell foam people to address my concern about giving interior moisture, which is going to migrate into their "conditioned attic", a way to get out. It seems they want to spray the underside of the roof sheathing just to save themselves a second trip after the ceilings are installed.
When you have large volume attics under steep pitched roofs, you are now needing to insulate a lot more sq.ft. than the ceiling area, not to mention the additional gable end walls. Since we pay by the sq.ft., its obvious why the Icynene folks recommend this. But by sealing over the soffit & ridge vents with foam, don't we create a "slightly permeable" plastic bag over a house?
It seems this highly touted, state-of-the-art insulation method violates the cardinal rule of giving moisture a path to the outside. Keeping an attic above 45 degrees doesn't matter if if moisture can't get out.
Actually, it does matter what the temperature of the attic is WRT condensation soaking the woodwork. If the attic surface temperatures are similar to those found inside the home, condensation will not occur on any surface inside the attic. However, if there are permeable surfaces and moisture entrained in warm air penetrates them, then the moisture will condense out as the air that carries it cools down.This is why having an impermeable moisture barrier is so important and why buildingscience calls for a vapor barrier to be painted on some low-density foam installations (icynene, et. al.) - to prevent moisture permeation. Now, the Icynene folks retort that if the interior Icynene temperature remains high enough and/or the Icynene is thick enough, that it's impermeable enough to prevent water damage.As long as the attic is as warm as the rest of the house, the air in it will be just as resistant to condensing as it will be in the rest of the house. If you make the attic part of your AC/HRV circuit, you can recirculate the air into the rest of the house. I could be wrong, but I presume the reason that roof venting is so important with fiberglass insulation is because most fiberglass installs do little to nothing to prevent air movement within them. Plus, fiberglass loses its insulative value rather dramatically as it loads up with water molecules, further accelerating the formation of ice dams and other symptoms of warm roofs in the wintertime.
So far nobody has answered the question of what happens to the moisture in an unvented attic. Unless you provide vents (i.e. provide it with recirculation down into the conditioned living areas) that moisture will just build and build and build and grow mold & rot.
The stipulation in giving this method new code-approval is that the attic always stays above 45 degrees. And it's clear that requirement is to keep moisture in the attic air from condensing. But dew will form at 60 degrees as well if the RH is high enough. How will this moisture ever get out in an unvented attic once all vents to the exterior are sealed? (Hello Icynene engineers?)
Part of the issue is controlling moisture within the envelope. That needs to be addressed as part of the whole house issue. We can't just put a bandaid on a nicked artery. T'aint not going to do the job.
I installed AC in my house a few years after I built the house. As part of the installation, the second floor air handler and ducting was going to be in the attic. Prior to the install I wanted to bring the attic inside the thermal envelope of the house.
I used 2" foil-faced polyiso on the faces of the rafters, from ridge to sole plate. Put 1" ff polyiso on the gable end walls. A few other details along the way, but the net result was that an attic that had previously been I don't know how cold in the winter, but as hot as 127 degrees in the summer, remained about 64-66 degrees in the winter and 76-78 degrees in the summer. That's with no heat or AC in the attic.
The addition of the polyiso gave me a rock-solid VB, prevented any air infiltration/exfiltration whatsoever, and also gave me a barrier preventing radiant energy from entering the attic in the summertime.
Relative humidity was in correlation to that in the rest of the house. No excess build-up.
No condensation on the gable end windows in the attic.
The attic was not isolated from the remainder of the house. There is a full set of walk-up stairs with an open doorway from the second floor up into the attic.
I forget the exact number, I have it on file somewhere, but I used less oil to heat the house with in subsequent winters as well.
Thanks for your explanation of including the attic area into the overall conditioned living space. Evidently the "bleed off" from having the HVAC equipment in your attic was a key element in your moderate temps & conditions after the foam board on the underside of the rafters.
But what if your HVAC equipment is in the basement, you have zero ductwork in the attic, AND you will double the volume to be heated/cooled/dehumidified by adding all your attics to the living space? Why can't the 1/2 lb. foam be applied directly to the attic side of the drywall ceilings?
Edited 5/24/2005 4:45 pm ET by Latvian Pete
Bleed off is not an issue.
The house is heated with RFH, so the attic gets no heat in the winter as there is nothing blowing through the ducts.
In the summer the attic got nothing as well out of the AC system The ducts are sealed with mastic and Al foil taped, plus wrapped with foil-faced insulation. Tight as you can get.
Regarding the foam board, when the sheets of polyiso were installed, I gapped them by about 3/8ths of an inch. Came back later ang gunned the gaps with canned foam, then Al foil taped over those seams. Tight as you can get.
Building code often lags behind building science. Code often tries to protect against the lowest common denominator in construction.
Builders screw up today's allowed methods of construction. I'd hate to see what some yahoo could do with foam wihout knowing what he was actually doing.
As I see it, it's up to you to chose where you want the thermal boundary of your home to stop and start. If you can seal all the floor penetrations, etc. going into the conditioned space, I imagine that you can save some material by blowing into the attic floor joist bays rather than the rafter bays.However, considering how difficult it can be to properly seal the conditioned space from the attic (i.e. via doors, attic stairs, etc.) I'm not sure that the minimal difference in Corbond/Icynene material quantity is worth it. Plus, if your house is already standing, it's usually much easier applying foam to the (typically open) rafter bays than the (typically closed) floor. In such a case, the additional labor of coring openings into the floor and the very real risk of an uneven fill make rafter-bay applications a safer and perhaps more economical choice.Besides, a usable attic will add to the resale value of your home while also giving you an opportunity to store items and other things up there that would otherwise have to stay inside the lower floors. One aspect of Corbond that I like a lot is its compressive strength, which also reinforces the walls it is blown into by making them stiffer.
Why can't the 1/2 lb. foam be applied directly to the attic side of the drywall ceilings?
Well, it can--you just have to make provisions around everything that's through the ceiling. Can lights, bathroom vent fans, etc. It's doable, it just wants attention to detail.Occupational hazard of my occupation not being around (sorry Bubba)
Constantin, Mongo, and CapnMac-
Bravo to you all!
I went back and reviewed some of the early threads on this topic and I am truly impressed by the level of knowledge and civility that this group has risen to since then. I have enjoyed the discourse so far. Should we take it to exterior air barriers next?
Thanks for the kind words! I also appreciate the fantastic insights you have brought to this discussion...I always love to learn more about this topic, even though the insulation in our home is pretty much set in stone at this point (or behind plaster, to be precise). Foam sure is amazing stuff... I talked to my energystar.gov consultant yesterday and discovered that our 0.40ACH rating was at 50pascals (?). Not bad for a 133-year old girl!
Edited 5/24/2005 8:48 pm ET by Constantin
That can't be. I just can't believe that an old house could test like that. How was the test performed? That number, if true, would make your house one of the tightest houses in the world. If that number was at ACH Natural, it would not be shabby for an old house.
Well, I have very little expereince in this field, but that claim made me scratch my head also. I'll go back and ask for confirmation. After all, I have a 4" hole to the outside in the basement where the combustion air comes from. Given that the room where it goes into isn't sealed from the house yet... Nevermind all the historic glass all over the house that is simply supplemented by Harvey Tru-Channel storms...Unfortunately, I missed the test when it happened. Perhaps I'll be able to convince them to come back once the basement utility room is done and the windows are all installed...
I went from 8.5ACH (50Pa) to 3.0 ACH (50Pa) in my century home. Estimated leakage before; 356 sq. in., after 96 sq. in.If you did better than that you must have had terrible humidity problems last winter...This is the whole thing that got me seriously into building science.I got my air sealing by replacing windows (small effect), spraying the rim joist with 3" polyurethane and sealing to the foundation wall, weatherstripping all doors, weather stripping and insulating the attic door, sealing up all gaps/holes in the masonry from the outside. Not too many and mostly in the foundation wall section (which is limestone rubble). There were a few holes that my cat could have gotten through. Amazing.In the course of replacing the furnace, of course we lost a chimney. Everything is sealed combustion now. That is how I got my air sealing done. And, I consider that pretty good for a 100 year old house. Now, after I do the attic properly and finish up the basement we will see about R2000...Exterior Air Barriers
Isn't the whole point of the exterior air barrier to just be part of the whole ABS and make conditions easier on the components farther into the envelope (say house wrap). Or, are you talking about completely air sealing the outside of the envelope? I guess it all depends on your construction method.No Vapor Barrier Approach
The idea is to air seal perfectly (or close to it) and then allow the small amounts of moisture that want to diffuse through the material (and condense under some conditions) and eventually dry out to one side or the other. This depends on controlling interior humidity fairly well and making sure that your building materials are capable of handling some moisture storage. Then the lack of vapour barrier encourages a good amount of drying in the warm months (I am in a cold climate). Moisture storage ability is why I am seriously looking at cellulose for my attic. There is a neat set of calculations over at the ORNL site for nonvented low slope roofs... Given climate conditions, building material properties, interior RH, it is possible to gauge whether or not you can use an unvented low slope roof for your project.My roof is a cathedral ceiling. Definately not low slope. My assumption so far has been that this likely puts me ahead of the game when I use similar calculations as what was laid out very clearly at ORNL.http://www.ornl.gov/sci/roofs+walls/calculators/wetroof/index.htmlIs this assumption correct? Am I missing something fundamental about the properties of a low vs. highly sloped roof. I had assumed that a highly sloped roof would be less vulnerable to leakage from outside and that the snow load would be less. Both good things as far as water storage go. I was going to use similar calculations as above (with different materials) and make sure that I design everything so that I am very far into the safe zone (or maybe I will decide it isn't possible).If my house wasn't a 100 years old... there would go a really fine hobby.
DgH-
Way to go. You did very well with your old house. I love meeting people that take such an interest in these topics and in their home as a system. There is so much more to learn. Learning about the characteristics of the water molecule and moisture transport is key to starting to learn about building science. After that, there are a host of building failures to study in order to understand what doesn't work. From there, we can start to develop methods that may work better(scary part) and test them to see if they work.
I got involved in this end of the business when I realized that we were driving cars and using appliances that were inspired by genius but were living in homes that were dumb by comparison. It has been a great ride and has changed the way we do things.
Exterior Air Barrier Approach-
I'd like to take this to a new thread. Should it be here or in the energy section?
ORNL calculators-
I would not use these calculators as the sole means for determining the safety of a roof or wall design. The most powerful of these to my knowledge is WUFI(developed by Hartwig Kunzel). I believe that you can still download a free evaluation copy of WUFI from the ORNL website. Even though these are powerful tools in helping us to understand the movement of heat and moisture within a system, in reality they are simply a set of algorithms that are fed data based on assumptions that may or may not be accurate. My own experience seems to indicate that they tend toward greatly minimizing spikes in the results, thereby failing to accurately predict problems that can occur during anomolies. I must apologize to all the dedicated building scientists that work on these programs to constantly improve them(HK, AK, et al). They have my utmost respect.
With respect to your question about the effect of different roof slopes; the slope of the roof and it's orientation has a great deal to do with the amount of solar heating that will occur on it's surface. A north facing, steeply pitched roof, will get almost no solar gain in winter. This will affect it's performance in ways that the writers of the program could not could include in their formula.
I would definitely not use cellulose insulation in the unvented rafter bays in your climate. We often talk of stepping away from the edge when discussing systems that are on the threshold of being problematic. When I think of cellulose in the unvented rafter bays in your climate, I get a picture of the coyote who has just run off the edge of the cliff and is now spinning his legs madly in space in a vain attempt to get back on solid ground.
You've done a great job so far. Congratulations.
hi ..ray....
we've been developing our insualtion techniques since the mid'70's..
here's a job we took over after the 2d floor was framed and furred
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this is coastal RI.. 6000 DD heating...
extremely humid summers... but maybe only a week of over 90 deg.
Mike Smith Rhode Island : Design / Build / Repair / Restore
Edited 5/25/2005 9:16 am ET by Mike Smith
Ray,
A new thread would be appropriate, and the Energy folder would be just the right place for it. I'll look for it.
Cellulose can have limitations, but dense packing during the installation makes it more attractive.
And, welcome to the fray!
Mongo
"I would definitely not use cellulose insulation in the unvented rafter bays in your climate. We often talk of stepping away from the edge when discussing systems that are on the threshold of being problematic. When I think of cellulose in the unvented rafter bays in your climate, I get a picture of the coyote who has just run off the edge of the cliff and is now spinning his legs madly in space in a vain attempt to get back on solid ground."Your opinion is that cellulose in an unvented rafter space in a cold climate is a bad idea...If it makes a difference I would be blowing 3.5 lbs per cu. ft. and going to R60.Now, I hear from time to time that this is bad because of no venting. However, in a cold climate when the stack effect is working "up" the air is too cold to actually provide any kind of reasonable drying. The only time the roof gets to dry out is in the summer when the air can absorb moisture. And, in the summer, the stack effect is "down". Good thing I'm not planning a vapour barrier. Could you please explain to me how the venting is going to help?If my house wasn't a 100 years old... there would go a really fine hobby.
Actually, The more insulation you put in, the worse the condensation will be from the standpoint of the temperature of the first condensing surface. That roof deck is going to be really cold. Cellulose will allow water vapor travel right through to condense on the cold decking.
No matter how tightly you pack the cellulose, it will still settle. I had to have that proven to me. I thought that as the water collected on the bottom of the deck, that the cellulose would wick it back out. I was wrong. It will however, catch the drips. In your climate, the moisture collection will most likely be in the form of frost. It will melt the next day and reform at night. Also, consider that the moisture storage capacity of the cellulose is a two edged sword. The moisture that is stored in the cellulose is available to the cold surface.
The cold outdoor air in winter has a very low dewpoint. Venting above the cellulose will allow the moisture that is created and stored within the house to escape to the dry exterior air. Moisture moves in the direction of warm to cold and wet to dry. Foam is a much safer alternative and foam above the roof deck is the safest. We in the south can get away with a little more than you can up north due to our milder winters.
If you must use cellulose, you should vent above it and vapor retard below it. The argument has been going on for some time now. Here is a link to a brief history on the subject compiled by Bill Rose.
http://brc.arch.uiuc.edu/billrose/history_attic_vent.pdf
Here are two more links of interest.
http://alcor.concordia.ca/~raojw/crd/reference/reference000083.html
http://www.buildingscience.com/resources/presentations/Roof_and_Attic_Ventilation_Issues_in_Hot-Humid_Climates.pdf
There is one other way to make the cellulose work without venting. Move south. Way south.
Constantin,Do you have Corbond behind original plaster walls? If so, did you remove siding and sheathing?Any tricks you want to recommend for tightening up an old house? When I gutted my bathroom, I could see daylight in several places in the stud bays....
Unfortunately, we had to gut the whole house. Not my first choice, that's for sure. However, with all the utilities, etc. to deal with (i.e. replace)... Anyway, so we got access to the stud bays via the inside. They all got filled.On the outside, we had to take the clapboards off because most of them had not been properly maintained about 30 years ago - present paint was OK, but the underlying wood was not. Tyveked the place in the process. We also built a new foundation (i.e. jack the place up, demo, build) that is unlikely to be leaky since it consists of cast concrete, new windows, etc. The only hole down there (besides the stuff for utilities, HRV intakes, exhausts, etc.) is the combustion air supply.The third floor windows had to be replaced because our dear friends the demo dumbos decided to rip out the frames (against our explicit instructions). Lack of supervision in this area and elsewhere cost the demo company its entire profit margin...As far as tricks go, I would pay very close attention to thinking of all the places where you application of foam, cellulose, etc. may not reach. For example, our stairway had a diagonal stuctural brace in the wall behind it that prevented foam from filling the wall behind the baseboard running up with the stair. Once we took the baseboard off, we could fill those stud bays as well.Another area where I found significant "holes" is where fireblocking fiberglass had been applied w/o thought and hence harbored large areas of free space behind it. This is particularly true for new foundations that tend to be a lot thicker than the brick walls that used to support older homes. Lastly, I highly recommend renting a thermal camera on a cold day to see where there are open joists, cold spots, etc. to investigate later. I haven't done that yet, look forward to it next winter.
Should we take it to exterior air barriers next?
Yeah--but it will want its own thread i nthe Energy Conservations, I'm thinking.
I'm remembering a line similar ot "Knowledge is experiential; one increases the other" or words to that effect. Wish I could cite that properly.
This specific topic may lend its self to good banter, too. Some of the climate extremes in, say, Ontario are going to be similar to those in, say, Hutto or Round rock. The difference will be in the AHJs.
Also, as you pointed out, far too much "common knowledge" on insulation is from a centralized government source. Common and locally correct can be mutually exclusive.
There was a house built on an in-fill lot near my folks up in Dallas. It was built, to the letter, following the 1980 guidelines for building a sensible house. It was just built 20º of latitude too far south. It was interesting, though--a stuccoed salt box meets mock spanish mission. I always wanted to find out what all of the glass on the two story due-south elevation ran the a/c bills up to. Last time I was up to Dallas, the big windows were being ripped out for smaller and two roof overhangs added.
Such is life.Occupational hazard of my occupation not being around (sorry Bubba)
It was built, to the letter, following the 1980 guidelines for building a sensible house. It was just built 20º of latitude too far south.
If you take global warming seriously, then potentially a catastrophe will be unfolding in the housing stock in coming decades.
The thread on exterior air barrier systems has been started in the energy section.
If the attic is a part of the conditioned space in the house, how will the temperatures and RH rise to the levels you are describing, unless the whole house experiences them?Have a look at the Las Vegas research over at BuildingScience.com. They show that the leakage from AC systems in unvented attics is sufficient to ensure that the attic space stays at a constant temperature and RH, just like the rest of the house. In our home, we have one AH in the attic and no insulation between the attic and the spaces below for this very reason.I don't think you need vents into the attic or anything near that, as the natural forces of vapor pressure, etc. will ensure that the moisture will migrate until it equalizes around the house. Once the whole house is at one temperature, then it also ought to have the same RH, unless you have extenuating circumstances like indoor jacuzzies, humidifiers, etc.
Hmmmm, how about the Northern Cimes?
Enough sub-roofdeck insulation to keep the 'average' attic temp at 45? This sounds like crossing a stream whose average depth is 2 feet.
Ventillation is what stops ice dams, Snow on the roof deck will not melt and form ice dams if the roof deck is kept the same temp as the ambient air. So unless the homeowner wishes to install an elaborate system of cooling coils, attic (or roof deck) ventillation is the only means of preventing ice daming. Does the new IRC address this?
BruceM
The insulation under the roof deck also keeps the roof surface at ambient. Therefore no uneven melting and refreezing and damming.Can't you guys see that the proposal gives you an attic that is just like conditioned space? There's nothing wrong with a properly done attic conversion.
I'd love to insulate my attic, now I've made the mistake of putting an air handler in there. I'm just not convinced that we know how to do it right yet, let alone how to do it for the long haul (as climates change with global warming). Seems to me the sensible compromise is vents, baffles, insulation over baffles. Yet the debate here is between venting and complete impermeable insulation. Looks like we'll end up with two codes for attics, both of which are broken.
Well said Kevin. This is nothing revolutionary here.It's like having 100% cathedral ceiling. What's the problem?
"Hmmmm, how about the Northern Cimes?
Enough sub-roofdeck insulation to keep the 'average' attic temp at 45? This sounds like crossing a stream whose average depth is 2 feet.
Ventillation is what stops ice dams, Snow on the roof deck will not melt and form ice dams if the roof deck is kept the same temp as the ambient air. So unless the homeowner wishes to install an elaborate system of cooling coils, attic (or roof deck) ventillation is the only means of preventing ice daming. Does the new IRC address this?"
In this design, the attic plane is not sealed and insulated (it's conditioned space), so 45 degrees is no great trick. Many attic spaces WITH an insulated attic plane would meet this temperature without venting. It is not so different from an attic conversion with a cathedral ceiling.
Also, you will not be likely to get ice dams because this design creates a continuous 'cold roof'.
Sorry...hate to go on-'n-on about this, but its an important topic.
I think I was unclear. For cold climates, the back of the roofdeck must be kept as close to ambient as possible. Practically, this can't be done with standard insulation technique, unless infiltration from the heated space is sealed and air-tight. The reason is snow is a great insulator...so if its zero outside with 24" of snow, the temp where the snow meets the roof may be only 28..and not much heat from the hard-working heating system below will need to infiltrate existing insulation to cause melting/ice daming. And is highly unlikely batts or cellulose will be able to prevent this kind of infitration.
The earlier post of creating a venting space behind the roofdeck (soffit to ridge) and then blowing foam into the remaining rafter space seems like the best approach.
And as mentioned, this would also allow the radiant heat thrown off by the underside of the roofdeck a means of escape during those hot summer days.
BruceM
I've done a LOT of reasearch into his and concluded that there is no better alternative than a spray-in-place urethan foam insulation tight to the inside od exterior wall sheathing and held 1 " below the underside of roof sheathing with ventilation baffles - not to disspiate vapor but to maintain the roofing warranty by disspating heat.
Andrew
Note though, that the roofing manufacturers opposed are not saying that heat buildup is a problem. A change from their old argument which seemed a bit less than genuine from the start.
So you are running the ventilation baffles all the way up the sheathing to the ridge and installing a ridge vent also to allow a flow of air from the soffets to the ridge under the foam insulation right? This sounds like a solid idea that addresses many peoples idea of a potential problem with the spray foam application directly to the roof sheathing.
I have not seen this construction method anywhere to my knowledge. Where did you come up with it and has it been successful? Also, what venting product are you using since the run is not typically that long for vent baffles as I have seen them used.
thanks
Andrew,
Welcome to Breaktime.Please fill in your information so we know where your located.BTW, I agree with everthing you say except the vent in the roof part. The best method is coat the entire inside envelope of the house with SPF insulation.Stu
Buildingscience.com recommends min 2" of venting space."Impermeable" foam is not impermeable. It just permeates very slowly. It seems to me that that means that over the long haul it may accumulate a lot of moisture and give it up very slowly. What is that a recipe for?
I can only share what I heard explained to me by the engineers from Icynene: They explained that their product is all but impermeable to air molecules and so vapor (air-born moisture) can not penetrate. There is no dew point in the material and there is no possibility of condensation within the insulation. I assume that this has to do with the random and relatively rapid movement of air molecules being unable to make their way through the material, because solid-state water can pass through it - if there is a roof leak, they said that water will work its way through in (order-of-magnitude) 2-3 weeks time. Water molecules pulled by gravity obviously move very differently than air molecules.
Since water vapor can not pass through the material, they recommend application directly to the underside of the roof sheathing, foregoing the traditional ventilation above the insulation. But because of the potential damage to the roofing from excessive heat build-up, I prefer both - placement of ventilation baffles (either the molded foam plastic baffles or a home-made variety) on the underside of the roof sheathing to help keep the roofing cool, and then application of the spray foam urethane directly on the underside of that. There is SO much heat loss directly through the interior framing from stack effect pressures (as in heat loss through all the necessary holes drilled through the top plates by electricians and plumbers) , that including the attic space within the insulation envelope is less costly as less wasteful even if it is increasing the conditioned volume of space.
Andrew Gil
Architect/ Builder
Edited 5/13/2005 9:06 am ET by Andrew
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I like the sound of your vent baffles under the foam but this does not seem to be the recommended method for Hot and Humid environments specifically per Building Science. The reason for this is the entire reason for the air seal is to prevent Hot and Humid air from entering the home and mechanically controlling the humidity from within. The Foam sprayed directly to the deck prevents hot humid air from having the opportunity to condense on the sheathing. It seems that if you allow the moist humid air to enter between the sheathing and the foam insulation you are providing it with an opportunity to condense and eventually affect your sheathing. The spray foam directly on the sheathing would lead to a hotter roof but it should keep condensation from humid air from occuring and potentially rotting the deck.
In my opinion, the baffles would be perfect in a Hot /Dry climate where moist air from outside is not a primary concern. But hey, I am no Building Scientist.
TXKEVIN,
You raise some really good points. I doubt that there is any solution out there that answers ALL possible short-comings (at least not without an awful lot of energy-consuming mechancial air conditioning). While your main concern of hot humid air entering between roof sheathing and vent baffle can't be argued with, what would make it "condense" there ? For the sake of arguement, say that it does - even then, the way this vent space works is that any sun hitting the roofing turns this space into an oven and natural convection draws outside air (cooler by definition) in through soffit vents and out through a ridge vent. Because it is cooler, it should absorb moisture as it heats up and carry it away (by way of this scenario, I wouldn't expect to ever find any "condensation" in the first place). But, like you, I'm not a building scientist. Arguing ventialtion/ condensation/etc is like politics or religion ........lots of different points of view, ya?
Andrew
natural convection draws outside air (cooler by definition) in through soffit vents and out through a ridge vent
Yeah, but empricially, you run into tow factors; real air flow, and how much heat can ambient air "absorb."
I've been up on these roofs here in riverine Central Texas, the heart of Hot/Humid summertime climate. I've put blowers in the attic to demonstrate that no realistic air volume will cool either the attic or the roof deck--not at 98º DB/ 75º WB conditions. The roof mass (shingles, felt, deck) is just greater than the 0.001± cfm/lf ventilation will cool.
With attic temps around 135-140º, you'd expect a hot plume from a squirrel-cage or rdige vent--but that's not generally the case. Even with thermal imaging, there's some heat "bloom" from the vents, on the lee side. But it's often hard to pick out from the radiation off the roof itself.
I'm not being a naysayer for argument's sake here; I want a solution that actually works. It's just tough on a person when a "built right" attic is just as hot as the "not right" one across the street.Occupational hazard of my occupation not being around (sorry Bubba)
CapnMac,
I agree, and can't imagine how ANY space can be cooler or dryer than ambient atmospheric conditions without conditioning the air. That said, the traditional details should still evaporate any unwanted condensation ; even in the desert there's a time when a hot sun rises but the air is still relatively cool, no?
Andrew
Edited 5/17/2005 9:27 am ET by Andrew
there's a time when a hot sun rises but the air is still relatively cool, no?
Ah, that's just it, perhaps in a dessert, hot/dry environment. As a for instance, the 0700 report this morring here was 62º, the dew point was 59º giving an RH of 90+% (it's come up to 74º, & RH of 73% as of the 1000 report). The daily average will stay around 65-67%.
To use the hvac term-of-art, it's the pounds of water vapor per air volume--and that stays about the same the whole day long. That water vapor becomes a significant heat sink. There's even a joke about how to know it's how here--it's "eighty at eight" (80º at 0800).
Back in grad school, there was some talk about researching an alternate answer, the "refrigerator under an umbrella," but that needed funding it wasn't going to get in the early 80's. That was going to be a heavily insulated "box" with a very lightweight roof "floating" over that.Occupational hazard of my occupation not being around (sorry Bubba)
We've been debating vented/unvented for years now.
Same arguments, but only the names have changed.
"I've put blowers in the attic to demonstrate that no realistic air volume will cool either the attic or the roof deck--"
Once people understand the thermodynaics of what yo just wrote, they'll have a grasp of the essence of what this argument is all about.
When the sun heats a roof, the roof warms and the underside of the roof sheathing becomes one monster radiant panel. The radiant energy wamrs the gable end walls and the attic floor via RADIANT gain. The warmed surfaces in turn warm the air via conduction/convection. Thus the hot attic.
Blow the air out via a vent, then turn the vent off. Within seconds the attic is again hot. Ventilation cannot kill radiant energy.
Same idea in my house, heated with radiant floor heat. Open the windows/door in the middle of winter, and yes, the room cools. But as soon as you close the windows/doors, the body is warm due to the mass of the structure and the radiant energy that the warmed mass provides.
In a forced air house, the house will be cold/cool for several minutes while the cold air is replaced with warm air via the forced air system.
Control RADIANT energy in the attic and you've won the war. Fiberglass insulation will not stop radiant energy, it passes right through. Cellulose will stop radiant energy. So will rigid foam board insulation, as well as a properly detailed radiant barrier (although thses do have some limitations in the practical world).
How do you spell relief?
R-A-D-I-A-N-T
Good post, CapnMac.
Radiantly Yours (in cyber-terms only<g>),
Captiain Mongo
Where is FRED when you need him?
Such praise, oh my, I fear my hat may not fit properly <g> (work will cure that, soon enough).
Within seconds the attic is again hot. Ventilation cannot kill radiant energy
Now, if only the AHJ who require venting could understand that . . .
Oh, and I actually did use blowers & thermographs to get a City engineer to grant a variance on a too-thin for mandated R30 and roof venting cathedral roof the one time. And that was west of here, where it's drier on average (just not that much).Occupational hazard of my occupation not being around (sorry Bubba)
Problem venting cross-gabled roof. My house (Sacramento; hot/dry, so I have GOT to keep out the summer heat) has two intersecting gables. I have practically nowhere to put in soffit vents because I have almost zero horizontal eave sections. I wouldn't be able to get continuous airflow through baffled gap between roof sheathing and insulation in rafter bays. For the last two days I've been reading all threads related to radiant barriers, and concluded that I would like to do Paul Hayden's foil-draped-over-the-rafters method. But when I realized my lack of ventability, I thought, well, looks like I'm gonna have to spend the big bucks and either polyiso foam or urethane foam the whole enchilada. 37 squares x 3" thickness x $1.13 per board foot (using Fomofoam's do it yourself pricing for their largest quantity available) totals $12,500. The shingles will be fibercement (Re-con's Firefree Plus Quarry Slate in dark gray. Hope fibercement can take the extra heat. And with 3" of foam on the underside of the roof deck, I shouldn't expect any radiant heat buildup in the attic, should I? I would love to post photos of my house (drawings, since it’s not built yet), and have tried for the last 2 hours to get the photos attached here. I’ve read the threads on how to post a photo, but no luck for me.
Anyway, hope someone has some advice. Thanks.
Barbara,
Perhaps the foam pricing has changed a lot since I got quotes last year as oil prices have spiked a great deal since then. However, I was getting quotes for less $/boardfoot of Corbond installed by a pro than what you are quoting for the DIY kits. So, perhaps going the DIY route is not that cost competitive...
What climate are you located in?
You really don't reject much heat in an attic through convection. The radiant heat gain will quickly overwhelm the ability of ventilation to flush attic heat. The reason for attic ventilation is to remove moisture from below a cold roof deck surface in the winter months. Some heat is removed of course, but attic ventilation requirements came out of studies in the thirties in Minnesota that were conducted to find ways of preventing roof deterioration caused by frost forming below the cold roof deck. A national building code requirement was enacted in the late thirties as a result of these ASHRAE sponsored studies. Ever since then, we have been required to ventilate attics in Texas in order to avoid a roof in Minnesota from rotting.
Attic moisture is a problem if not considered in the construction of a building but there are other ways of dealing with it besides using attic ventilation. Climate must be considered in any questions involving wall and roof design, unless a form of PERSIST is used. PERSIST can be used in any climate without fear of condensation. Exceptions to this would be extreme cases such as steam showers perhaps. I'm not sure I can even come up with an example where it would not work.
If you deciide that attic ventilation is the way to go on this project, you might consider some form of low roof vents or low vents on the gable ends near the plate line. I have not done this but it will work if you can use a vent that will keep rain out of the attic. As with any attic ventilation, you will be introducing moisture into the attic in certain weather conditions, such as high dewpoint or fog or drifting mist.
Let us know more about your situation.
This is a great thread but obviously a controversial topic. I'm very interested because I have a couple projects coming up (in Eastern MA). I don't want to interject an opinion, but tell me if I'm on the right track. In using the icynene sprayed in insulation in rafters, creating a conditioned space in the attic, there seem to be two basic concerns:
1. Moisture - to the first order, the sprayed in foam creates a vapor barrier so that there is no warm moist air from inside the house hitting the cold ambient conditions, thus no condensation. From what I read in this thread, there is 'some' permeability of the foam, but not enough to worry about, so in net, in my climate, the foam does NOT create a moisture problem.
2. Heat - A thin (1") air gap between the foam and the roof sheathing, with eave vents connected to ridge vents through this space, is recommended by the roofing industry (without which you void the warranty on your shingles?), but is considered by some here to be ineffective and unnecessary (I must say, CapnMacs argument is pretty compelling). Nevertheless, it wouldn't be that hard or expensive to add the air gap.
I guess there is a third, and maybe dominating issue, and that is the what do local codes require.
Does that sum it up?
Hi Bill, I'm just a homeowner, but the local authorities (Cambridge, MA) had no issues whatsoever with a sealed attic. We used Corbond, a better material for roof decks, IMO, because it has a high R-value/inch and is quite impermeable compared to Icynene. In our climate, the slightly-higher temperatures that a non-vented sealed roof entail are probably of no consequence to roofs built with quality roofing materials. Even in Las Vegas, the total delta-T between a non-vented vs. a vented roofdeck was a mere 17 degrees F in building science corp. testing, and well below the temperature limits imposed by the roofing companies. Besides, if you've ever read a roofing material warranty, it becomes pretty clear that they don't cover much of anything. At the end of the day, most of them require only the replacement of material, which, usually, is the smallest component of the total installed cost. There are some notable exceptions, but I wouldn't hold my breath. Researching the materials to ensure that they'll stand up to the local climate is a much better insurance policy.FWIW, I like DuraSlate recycled plastic roof slates. If this coming summer shows me the error of my ways, I'll be happy to report that also.
Bill W,
I admit that CapnMac's situation in Texas is beyond my experience (even if my mother did come from 200 years of Texans). As I grew up in Mass., and currently work in Upstate New York, I am familiar with your eastern Mass climate. The architectural company that I work for now has looked into Icynene for 3 recent projects. The first two times, the Icynene engineers came down to our office to present their product and discuss. Yes, their product is an effective vapor barrier, so you needn't add anything in addition: They recommend that there be NO attic ventilation (as mentioned in an earlier post, you can get warm, moist outside air bringing moisture in, and even though the attic will be within the building's 'conditioned envelope' you won't actually want to fully heat it(to dry out outside moisture coming in)): Icynene still recommends using a air barrier on the exterior (e.g. "Tyvek") - they considered it a good drainage plane: Many building codes won't accept Icynene's (or other open cell foams) installation not because of the product but because so many codes have mandated ventilation on the cold side of insulation.
My personal preference is to put in ventilation baffles between the rafters and then spray directly to that (creating an inch of ventilation between the insultation and the underside the sheathing), using typical soffit and ridge vents. It is virtually the same thing as a vaulted ceiling with batt insulation between rafters with a vent space above the insulation. This detail (a) helps keep a roofing warranty intact (if the manufacturer requires ventilated cooling); (b) helps prolong the life of the roofing regardless of whether the warranty is being compromised or not by dissipating the heat absorbed from the sun; AND (c) keeps the code officials happy, by not deviating from traditional details. If you're wanting to follow the Icynene standard, the International Building Code (IBC) approved unvented attics a year or 2 back. Icynene can get you the documentation, an maybe you can persuade the local building officials to accept it.
'luck,
Andrew
Architect/ Builder
CapnMacs argument is pretty compelling
Hey, careful <g>, I have not nearly enough experience to opine on cold climates (it was shirtsleeve weather around thanksgiving).
The thermal mass of a roof ought (that's just ought) to work in cold climates, too. It should become a heat sink. A coat of standing snow would be a decent insulator, too. So, unless there's a simple way to keep the snow off, it would seem like separating the roof deck mass from heat sources would be the answer.
But that's pure speculation on my part--I've not had to wear a coat and a tool belt in an attic at the same time. Camelback of ice water, yes, just not a coat.Occupational hazard of my occupation not being around (sorry Bubba)
Man, I have to say, you folks who live in climates with these temperature extremes sure have issues to deal with that we don't here in the Pacific Nothwet. It's all interesting reading, but just about enough to make this layman's head spin.
What is at the center of the debate? Preventing condensation as warm air moves through the wall or roof, thereby protecting the building from molds, or water damage?
Hi Jim,The issue we're debating about has to do with building physics. Specifically, we're concerning ourselves with water transport that may or may not result in condensation, wetting, and hence rot in the roof deck. For a exhaustive discussion look into the buildingscience.com web site, which has a lot of research, results, and recommendations for different climates, building materials, etc.At the heart of the condensation issue is whether warm moisture can make it to the part of the roof that is cold enough to make it condense. Ridge and soffit vents are one attempt to encourage circulation via convection to help carry water away. However, with an impermeable sealant/insulation like Corbond being applied in-situ, there is no way for the moist air to make it to a cold surface. However, depending on your locale you stil have to overcome the concern of local code officials, roof material suppliers, and sceptical builders who are unwilling to take a risk.
Hey, Bill, I think Constantine did me one better.Occupational hazard of my occupation not being around (sorry Bubba)
jb, this is "vented vs unvented".
Please, please, don't try to put a centered focus on the discussion. That'd take all the fun out of it.<g>
This comes down to the most basic debate in insulation: stop the air leaks vs allow the house to breathe. Current thinking seems to be in the direction of air-tight sealing with permeable insulation that allows drying, at least for walls. For some reason attic venting at least as championed on this board wants impermeable insulation....Disclaimer: Haven't had time to grok all of the material posted here recently....
It is a difficult and often times conflicted agrument.
Personally, I'm a proponent of air sealing, first to prevent moisture-laden air from being drivendrawn into and through the house. Starting with the basement.
Secondly, it is nice to have the wind whistling outside and to not feel it, or hear it, inside.
Air movement through insulation absolutely kills any perceived R-value.
Which is why FG, in my very small world, has limited to no value.
Well it's looking like there are 3 camps:1. The "let it breathe" camp2. The "seal it up" camp which splits into:
a. the "make it impermeable" camp (big VB fans)
b. the "leave it permeable" camp I think the 3rd camp is arguing that, as long as you don't have air leaks, VBs are not so important. And with almost every climate having a mix of heating and cooling, ability to dry is important. Buildingscience is in 2(b) on walls, and depending on what you read is either in 1, 2(a) or 2(b) on attics. You pays your money, you takes your choice.Now this current discussion is 2(a) vs 2(b) on attics....
Ah yes 2B or not 2B, that is the question.
I think the 3rd camp is arguing that, as long as you don't have air leaks, VBs are not so important. And with almost every climate having a mix of heating and cooling, ability to dry is important.
And lest we forget: another argument for permeable in attics is that (in some climates) heat from the sun will drive moisture (from rain) into the roof deck, so shingles will need to dry to the interior. The solution that was developed for one roof was open cell foam insulation. But now we hear that an open cell foam company claims they are impermeable given sufficient thickness. But we also hear that buildingscience recommends a coat of imperm paint to make a VB over open cell. But if we look through the archives we hear that someone claims imperm paints are not imperm. But......
I'm starting to appreciate why people stay with FG insulation and vented attics.....
"Air movement through insulation absolutely kills any perceived R-value."
Isn't that why we use a vapor barrier on the warm side of walls and ceilings? To prevent that movement?
This discussion has taken on pretty much every angle where the roof system is working as advertised. But what happens to an unvented roof when the roof develops the inevitable leak?
Part of me thinks if water can get in water can get out. This may be more true in a heating climate where the water has a chance of being vapor and moving back out to the cool, low humidity outside, but a cold roof may trap the water . In an AC environment either water or vapor could get in and could find a cool spot to condense on and then never leave.
Essentially this is a question of how robust is the system when the vapor barrier fails in a big way.
My understanding is that this is an advantage of cellulose over closed cell foams and some open cell foams. The cellulose has a higher permeability to allow the vapor to find a way to get out yet is an effective air flow barrier.
Block the air, allow vapor a way out, get your R value, don't see any reason to vent.
eric
http://www.buildingscience.com/housesthatwork/buildingmaterials.htm
Material Perm-inch
gypsum 40 (dry cup)
XPS 1 (wet or dry cup)
EPS 3.5 (wet or dry cup)
Fiberglass 120 (dry cup), 168(wet cup)
cellulose 75 (wet cup)
icynene(open) 16 (dry cup)
what happens to an unvented roof when the roof develops the inevitable leak?
That's an interesting question. Hmm, if it's a baffled and foamed roof, and it leaks through the deck, it drops into the vent channel and out to the soffit. So the first clue to the HO is when the soffit vent rusts through or the soffit rots. Less than good.
If it's a fully foamed roof, the leak has to get through the deck and then the foam, coming out who knows where. So, the first clue is either a rotten rafter caved in from above, or a mysterious spot o nthe ceiling. Nope, that sounds under-good as well.
[rude brit term beginning with "b"!] No good answer, there. It would be harder to assess if any reroofers in the future do a good job, too (unless the nails blow out the foam).
I dug out some of my old research & notes--some of that data wants to suggest that the ends of the rafters ought to be covered, at least in hot climates, so as to not be radiators of conductive heat from the roof deck.
And, as was pointed out in a previious post the "Witchita to Witchita Falls" problem remains--how to best address 120/120 hdd/cdd climates? They almost "want" two VB.Occupational hazard of my occupation not being around (sorry Bubba)
I believe that DJMerc said that in his house he went with open cell foam for the roof because he though that a leak would show up inside on trhe ceiling.However, if it is like my experience with open cell styrofoam, in a much different applciation, the water will be build up for years and you wan't know it until you have a 500 lb mass of socked foam that lets loose and falls through the ceiling.
foam that lets loose and falls through the ceiling
Wonder if installing masonry-style weepage wicks would almost make sense . . .Occupational hazard of my occupation not being around (sorry Bubba)
I believe that DJMerc said that in his house he went with open cell foam for the roof because he though that a leak would show up inside on the ceiling.
One of the points that Engineers from Icynene brought up every time I heard them present is that water from a roof leak will eventually pass through open cell foam (as in 'after a few week's time') allowing you to locate a roof leak. Sure, it might also run down a rafter, or the underside of the foam, giving additional stains, but enough should pass straight through the foam to leave a sufficient trace.
I'd hate to leave out the vent baffles because I feared the certainty of a roof leak and worried about the soffit vents rusting. To me, that feels like staying home because I might have a car accident.
Andrew
Edited 5/20/2005 12:42 pm ET by Andrew
icynene(open) 16 (dry cup)
Doesn't that mean 4" of icynene is more permeable than 1" of EPS?
What are the icynene engineers talking about, impermeable?
Hello All-
This is a subject that is near and dear to my heart. This is my first post here and my first visit to the site. I have read this thread with interest and would like to jump in here but where to start?
There is some confusion about the term impermeable and some manufacturers are partly to blame. Vapor permeability refers to the movement of molecules through a material. Water molecules move through materials through diffusion. Diffusion is a very slow process even in highly permeable materials because it happens at the molecular level. It is insidious and given enough time it can move a troublesome amounts of water vapor. Air movement can carry orders of magnitude greater amounts of water vapor. Molecules carried in this way can quickly deposit or adsorb (different from absorb) onto cold surfaces. Spray foam insulations greatly retard air movement but are highly permeable in half pound per square foot formulations. Closed cell spray foams are much less permeable.
I will generally speak towards hot humid climates but the same principles hold true for all climates; although they are often applied differently depending on the system being anylized. I will jump around some; because, I type much slower than I compose. I have delivered this material many times but usually orally and with slides to keep me focused.
Buildings should be built tight, very tight. They should also be mechanically ventilated in a controlled manner. Depending on poor air sealing practices for the occulpants' fresh air supply is a poor and outdated approach. You can't control the air in a building until you control the envelope. My home tested at .76 ACH50 or .038 ACHNat. Further air tightening would be too costly to pursue and would have negligible benefit. More on my methods to obtain these values will follow.
Unvented attics are highly desireable in hot humid climates where the HVAC equipment and distribution are located within the attic. The approaches used must be understood in order to avoid problems. Air movement must be eliminated at the bottom surface of the roof deck to avoid moisture buildup on this condensing surface in the winter. The slow vapor diffusion through 1/2 lb foam has not been found to be a problem due to the rate of movement and the ability of the wood components to store these tiny amounts of moisture for the duration of the winter. As the roof warms up this small amount of water vapor dries back out through the spray foam and is removed to the interior.
There are several methods of insulating at the roof line. Fiberglass batts should be avoided, period.
Spray foam insulation has three drawbacks. Cost is one, the need for fireproofing is the second, and the R-value is the third. Fireproofing is required by code regardless of the manufacturers claims. Thankfully, many Building Inspectors are misinterpretting the exceptions in the code and are allowing it to be sprayed without the extra expense of a combustion barrier. The safety of this is another long thread but it is much safer than a vented attic with the insulation on the ceiling. Currently the manufacturers only allow 5-6 inches to be sprayed because that is the thickness that it was fire tested at. This is not enough insulation. I have used this method on several homes.
Another method is to cavity fill the rafters with cellulose insulation contained by a sheetgood that is vapor permeable such as Commercial Tyvek. This is cheaper but has long term problems and is not my current choice. I used this method in my own home and have closely monitored it with sensors and visual inspections for two years. I have experienced outstanding energy performance from this method but would not use it in an attic without the ability to inspect for tears in the Tyvek. I reinforced the tyvek with nylon duct strapping tape and stapled it every one inch. I installed the cellulose to a level of 3 pounds per cubic foot. There are other issues with this method but it is very inexpensive compared to foam insulations.
The third method and my favorite to date is to install a 4" polyisocyanurate sheet with a layer of OSB applied to it on top of my plywood roof deck. The drawbacks are cost, difficulty in applying it to some complex roof designs and the thickness of the finished roof profile. This is a very safe and durable apllication that minimizes thermal bridging and keeps all of the roof framing within the hygro thermal envelope of the building. The finished R-value is superior and there is no fire issue in the attic.
This is a complex issue and I have not touched on many of the implications but I am tired and have typed enough for now. I will continue later if ya'll are interested.
I will close by saying that I have been involved in these issues for several years now and am continuing to learn every day. Every building scientist I know is still learning. The field is young and exciting. Discussion is the most important part of learning. The tests we run to prove our theory come in a close second.
Thanks,
Ray
"Another method is to cavity fill the rafters with cellulose insulation contained by a sheetgood that is vapor permeable such as Commercial Tyvek."Have you looked into the netting made just for this purpose?And what about high density PU foam?
The netting is designed for 16" centers, which limits it's application in truss roof systems which are typically installed at 24". It is not designed for the depth being discussed, and will sag even more than commercial tyvek.
The Panels above the roofline are polyiso which is very dense. Closed cell applied underneath is the extremely costly, especially at the R-values proposed here.
Ray, many thanks for your insights into the water/vapor transport mechanisms within insulation. Your suggestion of a tyvek barrier shielding a thick layer of cellulose is a really good one, as long as the attic is accessible and the barrier inspectable. However, your statement that foams don't have the R-value we need has me a bit puzzled. If I were to apply 6-8" of Corbond (two layers), I get an R40+ attic ceiling, a built-in vapor barrier, etc. all in a package that then disappears behind sheetrock or a fire-resistant paint. It's a more expensive approach, but it seems like the kind of product that you could install and forget. Plus, what is the marginal cost of better insulation within the overall budget of a project? Homeowners don't seem to have any issues dropping $50k+ into their kitchens, why not extra $ for the parts of the house where it really makes a difference?I like your idea of externally-applied insulation and presume that SIPS could be used as a roof deck in a similar manner. Getting the dimensions/angles on SIPS just right can be a challenge but if the house was built plumb and square, it should be possible.
The R-value issue is related to spray applied 1/2 lb. foam. The manufacturer of Sealection 500 states that it may not be applied thicker than six inches. This is equal to an R-21.6. I prefer at least R-30 in my climate, especially when it is thermally bridged by the rafters.
We can't decide what the priorities of the client are so we must always attempt to moderate the additional cost of new insulation systems. We are trying to sell a premium insulation system that is being cost compared to fiberglass. Cost offsets like eliminating venting costs and IC rated recessed cans and using smaller HVAC systems can offset the upfront cost altogether if we keep our insulation upgrade cost in line. The cost of foam makes this difficult to do. Selling a usable attic is also a good approach if the architect will give a real access to the space and make it of usable dimensions. Pull-down stairs are too much trouble to be of much use. A real stair and a large door can make the unvented attic a much easier sell. Architects seem to have difficulty embracing the idea, not unlike their reluctance to give up enough space for closets.
Sips are more difficult for sure. The one sided SIPS panel screwed to a primary roof deck is easier to accomlish in the field and gives an easier means of supplying a shear diaphram to the roof structure.
This is equal to an R-21.6. I prefer at least R-30 in my climate, especially when it is thermally bridged by the rafters.
I appreciate your thinking, but disagree. The codes that specify R-30 or R-38, or whatever R, are mainly designed for FG. IMO, FG rarely, if ever, performs up to its nominal R-value. Get it to a Delta-T of 70F. or so, and FG offers very little R-value. Foam and cellulose both improve in R-value at a similar Delta-T. The point is that in real performance, R-21.6 of foam will probably outperform R-30 of FG, hands down.
AndyAndy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Hi Andy,
Did I meet you last August?
If I might try to explain my reasoning a little, I am not trying to meet code. I am trying to meet a consumption performance level that is not obtainable by meeting code or matching it's performance criteria.
Possibly - Where would that meeting have been?
AndyAndy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Joe's living room, listening to a great musician. I might have the name wrong. Might be another editor.
No, it wasn't me. Probably Dan Morrison.
AndyAndy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Ding Ding Ding
That rings a bell. Thanks.
"However, your statement that foams don't have the R-value we need has me a bit puzzled.".....
I feel like an apprentice on this subject compared to Ray, but on this I have to put in my 2 cents. Having run through several multifamily projects recently involving (1) spray foam insulation and (2) major sources of grant funding dependant on energy modeling using an approved modeling program, I can testify that the spray-in open cell foams, which are only marginally better in R-value per inch than FB, are HUGELY more energy efficient because of the impermeability to air. Setting the energy model "air infiltration" factor to zero shows an enormous savings in energy. This has been demonstrated by both consulting engineers to us on one project and by State-hired consulting Engineers on another.
Andrew
Architect/Builder
Edited 5/20/2005 12:45 pm ET by Andrew
Edited 5/20/2005 12:45 pm ET by Andrew
This is true Andrew, but I feel that the air tightening and an R-30 or greater will achieve superior results. Using the benefits of air tightening to overcome a lower R-value is an argument I've had with the foam people right along. I love my foam guy though.
With my level of air tightening which is at least three times tighter than what is achieved by spray foam alone, my home achieves a consumption rate of less than 1/4 KWh per square foot per month. I feel this is minimal standards for modern homes but few can meet it.
I aim to build every one of my projects to be able to run on solar panels, once they become economically viable. Unvented attics and airtight construction is part of the equation that allows that to happen.
Ray-
Thank you very much for your post. The very slow rate of air movement and subsequent moisture buildup within a 1/2 lb foam is an excellent description.I would very much like to hear more. I enjoy learning every day too, especially about this topic. Thanks,
Stu
In this field as in life, more will be revealed.
Another method is to cavity fill the rafters with cellulose insulation contained by a sheetgood that is vapor permeable such as Commercial Tyvek. This is cheaper but has long term problems and is not my current choice.
What are the long term problems?
An issue I have with foam is the weight of DW on the rafters. If I read it right, you regard it as a good thing if AHJ does not require thermal barrier over foam in the attic.
I agree that thick layers of foam can be heavy and a significant contributpr to dead weight. A structural engineer or Boss Hog might be able to tell you just how much you can load up the roof (and the supporting walls) with foam. Our house had a slate roof when it was built, the combination of Duraslate on the outside and foam on the inside is likely to weight less than the original roof.Oh, and the slate roof had been ripped off by the time we got the house... just some traces left in the attic. When we started our project, we had three layers of asphalt roofing up there... I wonder what the insulative value of that stuff is considering that we had over an inch of material up there.
Long term problems include tearing and stretching of the Tyvek, settling of the cellulose and possible condinsation on any section of roof deck where the Tyvek is compromised, allowing air to freely contact the underside of the roof deck from the living space in winter. The level of concern is dependant on the roof temperatures in the winter and the moisture levels in the interior space.
I do not feel that it is necessary to fire protect certain types of foam in attics. Yes the foam will combust if it has a continuous heat source but in a sealed attic it will quickly be limited by the supply of oxygen. The sheetrock ceiling in an unvented attic will survive substantially longer than a sheetrock ceiling that has a layer of insulation laying on the top of it. The insulation on top resists the radiation of heat to the attic and caused the heat to build up in the rock more quickly, leading to faster failure of the rock. Once the rock fails, the vented attic will supply the oxygen needed to sustain the fire. I have not burned down any houses to test this theory but I believe it to be sound based on testing of the component parts of the theory.(studies on fires with and without insulation on the ceiling sheetrock)
Certain foams have acquired exceptions to this requirement as long as four criteria are met. Building officials that interpret this to need only one criteria to be met allow foam to be used without a fire barrier. Unfortunately, according to the man responsible for the language in this section of the UBC, all four requirement must be met. One of those requirements is that the attic be vented. This makes no sense but it is the reality. His response is that the building codes are designed to address fire and human safety. Energy concerns are a very distant second. Since no real world testing has been done to prove a foamed attic doedn't present a safety hazard when unvented, the venting requirement stands as is. Our best bet for today is that the building official misinterprets the exceptions to allow for a foamed unvented attic without the prohibitive cost of a combustion barrier. For me the problem is unneeded cost that serves to block the use of a superior system. The best way around this to use the foam panels on top of the roof.
Yes the foam will combust if it has a continuous heat source but in a sealed attic it will quickly be limited by the supply of oxygen.
Hmm, if we are insulating because of air handler in the attic....what about all those ducts bringing air from the envelope up into the attic?
Not trying to be difficult, I am finding your posts absolutely fascinating.
The attic is within the envelope. Ductwork can provide a path to the attic but the attic will contain the fire for a time. These ducts will always provide this path but in a vented attic they will provide a powerful stack effect to draw in fresh oxygen from below. The time to flashover in an sealed attic will be extended by lack of oxygen replacement and the roof deck will be insulated from the heat for a time.
Another concern that is often raised is that of the gasses produced when combusting the foam. The fire marshal in my town has stated to me that in any fire, the CO will kill you before that becomes an issue. CO often kills or debilitates even after you survive the fire.
I appreciate the discussion.
Ray, many thanks for your insights. I too worried about fires, considering that the walls are filled with petrified Dino juice. However, the fire first has to spread into these spaces... and how many fires are started inside the conditioned space vs. inside walls or other inaccessible spaces?For the conditioned spaces, I have a sprinkler system. In new construction their cost is reasonable...if you can find a professional that likes to do residential work... many don't. Tack that cost onto the mortage, and you get an instant 30% discount, while the annual savings of 10% on the insurance premiums do not get taxed and add up. Plus, you sleep better at night (our third floor only has one stairway).Even though the fire engines are but a 2 minute drive away, I doubt that they'd get to the site before the sprinkler system had finished the job. One further tidbit to consider in any residential install is tying a fail-safe shutoff into any HRV or AH that is coupled to the whole-house alarm system. That way, once a smoke goes off, your IAQ system won't transport soot and smoke throughout the house.
From Post #1 in this thread:
Dave Roodvoets of DLR Consultants, which represented ARMA in the effort, says the issue isn't roofing, but moisture problems. He worries that requiring "air-impermeable" insulation could actually make a home more susceptible to rot by trapping moisture inside, even though it is keeping exterior moisture out. "Ventilation compensates for a lot of sins," he says. And he claims that proponents don't have enough research to justify the change. "Even the best researchers have only a few years of data on unvented attics in humid climates."
Some of us have been wondering about this, and permeable vs impermeable foam under the roof deck. I'm guessing your answer is that the attic should be mechanically ventilated like the rest of the house. Low-cfm fan outside for those retrofitting an existing house?
An issue that no-one has addressed is whether permeable foam may be preferable to allow the roof deck to dry to the interior. This could also be accomplished with baffles.
Mike Smith has a wall assembly he calls the Mooney wall. 2x2 furring strips, DW, dense-pack cells. Reduces thermal bridging, I think a reaction to 2x6 stud walls. What would you think of this for roof deck insulation? Sturdier than your Tyvek scheme, still cheaper than foam if AHJ requires thermal barrier, might be more likely to show up roof leaks than foam. No vapor retarder AFAIK.
Edit: Oh just saw this, I think that answers the last question.
Edited 5/21/2005 5:22 pm ET by Taylor
The terminology of air impermeable is easy to confuse with vapor impermeable and they are not mutually inclusive. I might infer from the statement, "ventilation compensates for alot of sins" that the author (paid by the roofing association) thinks that little roof leaks are not a problem as long as they can dry out. He never mentions condensation of interior created moisture. I easily made that leap though, the first time I read it. If we jump to the conclusion that mr.Roodvoets is talking about any moisture other than a roof leak, then we are indeed jumping to a conclusion. I tend to agree that little roof leaks that can dry out are not a huge problem. By insulating in the rafters we are reducing the drying potential of the roof deck. We are also increasing the drying potential of the ceiling surface. I would of course prefer a perfect roof and usually get them. This is a genuine concern and one more reason to use the rigid foam panels above the roof deck.(rafters can dry easily in case of a roof leak and damage is confined to non structural members.)
As for interior produced moisture condensing in winter, it all relates to the climate both indoors and out. In my climate(hot and humid, with an average Jan. temp. of 45 F), I used cellulose in my attic. I maintain low RH inside(<50%RH) and the high maisture storage capacity of the OSB and celulose gets me over the winter moisture hump. I would not use this method in a colder climate. Interior foam is safer as there is no condensing surface or air movement. I use a vapor barrier above the roof deck to stop sun driven moisture from the exterior. Drying is to the inside. Safest is rigid foam panels above the roof deck, where the first condensing surface is kept at room temperature.
The attic is part of the house. Ventilation is not required separately as the attic environment is almost identical (+3 degrees) to the living space. A small amount of return air is taken from near the ridge.
In my climate, when a vapor barrier is used above the roof deck, 1/2 lb. foam allows easy drying to the interior. Cellulose and Tyvek accomplish the same thing, but the air gap caused by settling is a step towards the cliff. If there is a path for moisture to cycle into this gap on an air current, moisture collection will occur with cold roof temperatures. This is not too troublesome in my climate but is possible.
I have wandered and need to go spend time with my dear wife anyway. Sorry for my disjointed post.
Thanks for the questions.
Can you cover the entire roof with water & ice shield to prevent water from leaking thru the sheathing and soaking the foam??. I have been following this thread because I am building a 47' x 84' ranch in SE Connecticut using 6/12 trusses. I plan on using 1/2 lb foam sprayed directly on the Advantech roof deck. I am not concerned about fire in the attic, I plan on installing smoke detectors in the attic and tieing them into the HVAC/ERV system to shut them down in case someting happens. Any constructive input would be appreciated. Thanks
The short answer is yes.
The long answer is that most roof leaks have to do with flashings and transitions. Complicated roof designs require careful detailing by intelligent professionals. Membrane systems leak when poorly done. There is no panacea.
What is your roofing system? It matters. I would not bother with that under composition shingles. I would however use an underlayment with a <1 perm rating under comp shingles.
I have a concern about thermal bridging through the rafters in cold climates. Are you worried about icecycles and ice dams caused by the warmer rafters? I do not have personal experience with this nor have I discussed sprayfoams with my northern BS buddies. Maybe I'll ask Joe and John about this.
It comes down to the ability of the rafter to insulate well enough on it's own to prevent snow melt during below freezing conditions. I would have reservations myself. I regularly see the pattern of heat loss in frost and heavy dew conditions showing through my insulated roof deck covered with comp shingles.
Will you be spraying over truss chords as well? What R-value will you be applying?
Edited 5/22/2005 9:30 am ET by RayMoore
Ok, if I'm reading right, your system is using a near-empty framing, and moving the insulation to the exterior of the framing. I have to admit to being very intrigued by that idea.
It suggests some desing considerations to me that are very interesting. It segues nicely into a detail I've been toying with to create a mechanical (emphasis on wiring) chase as thick wainscot detail, too.
With your approach, we could reduce the framing to its post and beam minimums, particularly if we use SIPS as the spanning panels (I'm using SIPs as an example--any sort of panelized substrate to carry the isnualtion ought to work as well.) You will have met clients in the Austin area that are fond of the "bones" of houses--but not often of the work it takes to get them in the house.Occupational hazard of my occupation not being around (sorry Bubba)
Don't forget that with a PERSIST system, the sheathing goes on, then the membrane and then the foam. It is not really a SIPS.
It is not really a SIPS
Well, yes, by definition (it's really better). But, one sometimes has to "sell" the contractor a system he "knows" (Kevin H could probably tell a tale or two, out of school, as it were <g> . . . )
More, I was waxing poetic a bit on the though of divorcing the structural system from the insulation & weather barrier systems.
One thought I had last night was that it really would be a way to execute Usonian design. Although, it would be hard (somewhat) to get the razor-edge-thin profile lines FLW always wanted and would never quite compromise about. Most of those lines always looked too skinny to me--but I like properly insualted structures.Occupational hazard of my occupation not being around (sorry Bubba)
On my current project, I managed to achieve a 5" roof edge profile by replacing the 4" polyiso with two layers of 1 1/8" sturdy floor run in alternate directions to achieve a 16" cantilever outboard of the insulated structure. I'm sorry I don't have time to post a picture of this. We used angle iron on the outer edge to help maintain a straight roof edge.
to achieve a 5" roof edge profile
That's not bad at all. Especially considering that the roof system kind of precludes stopping at the wall line. The long overhangs our lattitude "wants" can make for complicated details which want more time detailing & coordinating with builders.
That's life though.Occupational hazard of my occupation not being around (sorry Bubba)
I don't want to hijack this thread and I have enjoyed the your posts but would you mind discussing your approach to insulating the house other than the roof?
Thanks
FF
I don't want to hijack this thread and I have enjoyed the your posts but would you mind discussing your approach to insulating the house other than the roof?
Thanks
Perhaps we should take that to a new thread instead. Would that be better for all involved?
Hey there BigMan,
Concerning the "moisture drive" from the external of the roof... isn't regular tar paper directly on the roof deck sufficient? I believe this is a moisture barrier itself. Don't really see how rubber would be much better considering it would have just as many nail holes in it anyways. I can only hope that your roof is so well insulated with the foam that the 10" high x 1.5" wide joist transfers heat through in such a focused manner as to cause a 1" wide ice dam. Really I think this is over-thinking the issue a little bit. Consider this: would you get ice dams if you had a 10" thick solid wood roof deck? Probably not. Let me see if I understand correctly... anyone please correct me if I misunderstand. I always want to learn more. If a cathedral ceiling is build with traditional insulation like cellulose and has a vapor barrier correctly installed on the "warm in winter" side (the inside) and a roof leak happens, then what? I figure if the vapor barrier if such a barrier then the water would run all the way down the roof line to the wall and then comepletely down the inside of the wall since this has a vapor barrier also. In the mean time the insulation is soaked and deamed essentially useless and needs to be replaced. Is this leak possible to pinpoint? Now the same ceiling and wall are insulated with foam. A leak happens and eventually the water works it's way through the open cell (half lb density) foam. I would figure this would show up slightly down hill from the leak, on the ceiling. Now simply repair the roof. The foam dries out by the moist vapor being driven out through the roof and the ceiling sheetrock. How is a foam insulated cathedral ceiling or roofline so much worst when a roof leak happens? This I need help understanding. Any help? thank you,
Stu
Man this is confusing, so many posts and opinions, Okay im using 5/8 Advantech roof sheathing, I have no idea what the perm rate is.
I am using Certainteed 40 year light colored architectural shingles.
I am still debating thickness of 1/2 lb foam somewhere between 6" (R20) and 10" (R30) either thickness should completely cover the top chord of the truss so I shouldnt have a major problem with thermal bridging.This being a cold climate, and as I have read on the building sciences website this should be no problem.
In researching SIPS used in roofing I found quite a bit of info about the OSB rotting into mush because the water had no place to go, so the leak went undetected!!
I cant see adding a 2nd layer of sheating on firring strips over the Advatech to provide an "airspace" to protect the shingles, never mind the cost, what about the weight??
I wish someone could give a straight scientific answer without their opinions of a product clouding their judgment
Is Advantech simply a type of OSB? I am not familiar with it and a visit to their website didn't tell me much.
Of course, if you are burying the top chord completely with spray foam, the problem of thermal bridging is prevented. The more foam that is applied below the top chord, the closer it will stay to the exterior temperature.
On the subject of concealed leaks, a bad leak will run through 1/2 lb foam, but a small leak will simply be held in the foam and the rafter will stay wet. The foam can hold alot of water before it will run through to the ceiling. It will run down the surface of the foam and drip off at a bulge in the foam. On a steep roof it may make it to the wall before it drips off. There are alot of variables. Being able to inspect the structural members is a plus for the rigid panels above the roof deck. Having said that, I have more buildings with spray foam than I do with rigid panels. I now prefer the panels, as long as the roof is not too complicated.
The heat in the shingles issue is ridiculous when you consider the black roofs used in Houston. If the roofing manufacturers were really concerned about heat in the shingles, they would not make a black shingle available to that market. The thermal degradation of the roof decking under a black shingle in that climate is more of an issue.
Most of the heat loss from a roof is not to the attic anyway. Most of the heat that is built up in a roof is radiated back into space. On a clear day, the temperature that the roof sees overhead is quite cold. The hot shingle radiates its heat back to space. On a hazy day, I often see roof temperatures in the afternoon that are higher than on a clear day, all else equal. If I use my IR temp guage at the sky in july on a clear day it will read around 15 degrees F. If I point it at the sky on a hazy day, it will read around 70 degrees F. In this same circumstance, if I point my sensor at the top of the insulation in a vented attic, it will read around 140. The assertion that unvented attics lead to premature shingle failure was initially overblown and has since been relaxed due to extensive testing and careful and thoughtful lobbying by the building science community.
I have only been using these methods for four years. Blame my cross over to the dark side on Straube and Lstiburek and Makepeace. Since incorporating their suggestions and developing a few modifications of our own, our results have been phenomenal. We do ongoing monitoring of our homes to collect data on their performance and durability.
The short answer is, it sounds like you are going to be fine with your plans and I recommend further research on your part to cover as many bases as you can.
Hey Ray, Advantech is a composite sheathing like OSB go to huberwoods.com to check it out, it has a 50 year warranty I have seen this scraps of this stuff used on a jobsite as a walkway to get thru the mud, water and ice, in the water for weeks and it does not absorb any moisture, absolutely no change in dimension!! we are wiring an 11,000 sq ft house by the ocean that they used OSB for the decking and the crap swelled so much they had to bring in a floor sander to level it off!!
Maybe I should staple the styrofoam vent panels to the underside and foam over that? at least when I saw a rusty soffit vent I would know I had a leak ;-)
Soffit vent?
If you look back in the thread, there was some discussion of putting baffles over vents and then foaming. Some worry that you wouldn't know you had a roof leak until you saw your soffit vents rusting. But that scenario might be less troublesome than having your roof deck rotting behind foam insulation.Do I have this right? Would the situation with closed cell foam be much worse? A leak would not be able to permeate through the foam fast enough, whereas there is at least some hope with open cell? Closed cell is attractive for the higher R-values, and I get quotes for Corbond here that are lower than for Icynene...
I don't recommend the baffle and spray approach. The finished product is more coastly for very little gain and ends up in a job that is totally reliant on the foam for airtightness. The decking is not as troubloesome to me as the structural members are.
It could be worse with closed cell foam but the variable are many in any discussion of roof leaks and spray foams.
I'm surprised that you can get closed cell foam cheaper than 1/2 lb. Icynene is an expensive brand name compared to other 1/2 lb. products. What are you paying for closed cell?
I got quoted $2/bf for Corbond, $4/bf for Icynene. Latter was in stud bays (and not open cavity spraying), but I don't think that's $4/sf.
If that was not a sq ft price, then it is a total ripoff. Even at a sq ft price it is way too high for 1/2 lb. I pay 41 cents per bd ft for 1/2 lb. Demilec and $1.25 per bd ft for closed cell.
I want to spray at those prices!! Corbond should be ~$1 bf and Icynene cheaper by quite a bit.I'd check those numbers again. Stu
Concerning the "moisture drive" from the external of the roof... isn't regular tar paper directly on the roof deck sufficient? I believe this is a moisture barrier itself. Don't really see how rubber would be much better considering it would have just as many nail holes in it anyways.
Tar paper does little to stop vapor drive and when it gets damp, it's perm rating goes up substantially. It is marginal as a drainage plane. A rubberized membrane is a vapor barrier and is much better at self sealing around penetrations.
I can only hope that your roof is so well insulated with the foam that the 10" high x 1.5" wide joist transfers heat through in such a focused manner as to cause a 1" wide ice dam. Really I think this is over-thinking the issue a little bit. Consider this: would you get ice dams if you had a 10" thick solid wood roof deck? Probably not.
Why do you think the snow melt water above the rafter would stay in a nice stripe on the roof? Would it not start to form small ice dams which would spread outwards until they formed a continuous dam? As I said, I don't know and have not spent time discussing this with the northern experts except to listen to them talk about it in conferences. One on one, I focus my studies on my climate. I have a good understanding of the issues, but lack experience. Have you built this type of roof in this climate where the bottom of the rafters were exposed? Over thinking rarely gets me in trouble, but I am often guilty of it. As for the solid 10" thick roof deck, in Bigman's climate, you would get ice dams. The r-value of wood range from .7 to 1.4 Rs per inch. That is not enough insulation I don't believe and since he is using trusses, I would assume the top chord to be 3.5 or 5.5 inches so an R-value of between 3 and 7.5.
Let me see if I understand correctly... anyone please correct me if I misunderstand. I always want to learn more. If a cathedral ceiling is build with traditional insulation like cellulose and has a vapor barrier correctly installed on the "warm in winter" side (the inside) and a roof leak happens, then what? I figure if the vapor barrier if such a barrier then the water would run all the way down the roof line to the wall and then comepletely down the inside of the wall since this has a vapor barrier also. In the mean time the insulation is soaked and deamed essentially useless and needs to be replaced. Is this leak possible to pinpoint?
Lot's of variables here but I would not use cellulose and a vapor barrier in a cathedralized attic in this climate. There are better, safer systems.
Now the same ceiling and wall are insulated with foam. A leak happens and eventually the water works it's way through the open cell (half lb density) foam. I would figure this would show up slightly down hill from the leak, on the ceiling. Now simply repair the roof. The foam dries out by the moist vapor being driven out through the roof and the ceiling sheetrock. How is a foam insulated cathedral ceiling or roofline so much worst when a roof leak happens? This I need help understanding. Any help?
It depends on the rate of the leak.
One of the better properties of Icynene (and, I imagine, other spray foam insulations) is that it sticks to everything; Insulating that ductwork was never so easy, and air--tight too.
Andrew
See posting #35, May 15th
Andrew
Yes, vapor barrier is used to prevent air movement. Yet, what do you do in a climate like the middle of the US where the Vapor barrier is needed on alternating sides of the exterior wall as the season shifts from cold/dry to hot/humid?Furthermore, to the best of my klimited knowledge, FG does not stop the movement of air in itself, just slow it, unlike Icynene or other foams where practically no in-wall air movement occurs. Convective currents within a cavity will cut down on the net effectiveness of fiberglass in ways that the current testing standards do not cover. Using just R-values to calculate the net temperature flux is an approximation at best. It's a bit like electrical circuits where Resistance, Induction, and Capacitance can all have an effect on net impedence. Similarly, the resisitve, convection, mass, etc. properties of an insulation will work together to create a net temp-flux resistance that may change as a function of the temperature/time/etc. Plus, even when the insulation contractors are very conscientious, I have yet to see a wall cavity where the whole cavity is truely 100% filled the way in-situ applied foams and dense-pack cellulose treatments allow. Hence, I am biased towards such 100% cavity-filling insulation systems since they seem to guarantee no thermal short-circuiting, infiltration resistance, etc. Best of all, cellulose systems are a great way to recycle newsprint and have a low barrier to entry compared to FG.
Yes.
But it doesn't address air washing over/through FG batts laid in an open attic.
Nor does it address improperly installed batts (friction-fit batts with the sides pushed/tucked back allowing you to see the sides of the studs, etc) where mini-convective loops can set up inside of a stud bay, or air movement allowed due to poorly detailed FG around junction boxes, or all the holes in an improperly detailed VB due to partition walls, poorly detailed seams in the VB, poorly detailed VB at junction boxes or other perforations.
Most of the problems are operator-induced. A caring hand can cure a lot of ills in construction, but more often than not the lowest common denominator sets the performance bar in much of today's construction.
Gotta drag my soapbox upstairs, time to start another load of laundry...
Yeah, well, but as you pointed out before, not even the worlds best installer can make FG opaque to radiant...Andy Engel
Senior editor, Fine Woodworking magazine
An updated profile is a happy profile.
Other people can talk about how to expand the destiny of mankind. I just want to talk about how to fix a motorcycle. I think that what I have to say has more lasting value. --Robert M. Pirsig
Isn't that why we use a vapor barrier on the warm side of walls and ceilings? To prevent that movement?
As if this discussion hasn't shown how complicated and difficult VB's and insulation already are, many building science experts are warning that mold may become our generation's 'asbestos': Everyone is still building today with the same principles as 25 years ago, and yet air conditioning has become MUCH more prevalent. The result is that, in all of the "mild climate" areas where air conditioning in used in the summer, the vapor drive created by the cool, dry inside air sucks the moisture from the outside air through the walls until it hits the first cooled surface, where it condenses. This would presumably be the outside surface of the VB, except that the installation of VBs AND insulation is usually badly compromised around electrical outlets, where the insulation rests loosely against the sides of studs, and so on, and so the condensation often happens when the moist hot air hits the backside of the air conditioned gyp bd, which happens to be an excellent material for growing molds on. The result is wall cavities full of mold of a toxic (sometimes fatal) nature. The same building scientists that I have heard speak on this recommend that air conditioned buildings be built with a 1" rigid insulation on the exterior of studs (especially metal studs) affording a vapor retarder on the summertime- hot side of the insulation.
Andrew
Architect// Builder
Edited 5/20/2005 1:18 pm ET by Andrew
Instead of trying to develop a prescriptive answer to vapor barriers, it is much more helpful to think in terms of controlling the temperature of the first condensing surface. This could mean that a vapor barrier is appropriate in the middle of the insulation. An example would be an R-11 in the cavity and an R-7 of foam on the outside of the vapor barrier.
The best method I have found is actually safe to use anywhere and is the best affordable method. It is more expensive than batt insulation but I can't waste time comparing to that.
My wall would start at the interior with 5/8" drywall. I would then use a 2x4 stud with no insulation in the wall cavity. Next comes a layer of 7/16" OSB. I then cover the entire house with a 40 mil peel and stick rubberized asphalt membrane which is also used to provide pan and head flashings. This is followed by a 2" layer of polyisocyanurate with a foil outer skin. An airspace is left adjacent to the radiant barrier and then masonry veneer or siding on furring strips. This is the perfect wall. It is affordable and bulletproof. Air sealing is from the exterior. All lumber is inside the hygro-thermal envelope. Remodelling and adding wires in exterior walls is a snap. Plumbing in exterior walls is a non issue and no codensation can occur within the walls because they remain at room temperature year round.
I just highjacked the thread. Sorry.
Think of the roof as a sloped wall. The drainage plain and veneer may vary but they are quite similar in their design needs. Does this mean I only sort of highjacked the thread?
Ray-
...An airspace is left adjacent to the radiant barrier...
What is the depth of the airspace?
"I can't say I was ever lost, but I was bewildered once for three days."
The airspace is 3/4" typically. I spoke with the owner of the company that makes solar shield and he said that it requires a minimum of around 3/8" to be effective and that the airspace does not need to be vented to be effective. There is always great debate on that one.
My roof is built with two layers of decking. The top layer is an RBS (radiant barrier sheathing. I included this layer because I used a dark green composition roof on my house to be kind to the few rural neighbors where I live. My house blends into the hillside very nicely. They rewarded me by using a bright shiny metal roof. BTW The airspace between the two layers is created with 1x2 furring strips. The space is not vented or sealed. The first layer is sealed and functions as my air barrier at the roof plane. I have routinely found shingle temperatures to be in the 175 degree F range. My datalogger which is located just below the first layer of decking has never recorded above 132 degrees F. This is a temperature that could be expected on a clean white roof but I don't have to look at a white roof.
A local architect of some standing in the green building community insists on venting this space but I disagree. The cost of doing this is unrecoverable and is not as clean looking as a vent free eve and ridge. I hold this man in high regard but we disagree on many issues such as this one. I will always try to minimize the cost of my building envelope system. I spend money only where it makes sense and try to offset costs where possible to eliminate higher upfront costs. That way the energy cost savings go in the owners pocket from day one.
My august and september electric consumption in my 4200 square foot home were 1200 and 1100 KWhs per month. This includes pumping and purifying the rainwater. I also have a 150 KWh per month penalty for my 8 year old refrigerator. I average right at 1000 KWh per month. I use a 14 SEER heat pump and dehumidify to 48% RH year round. I ventilate at 50 cfm continuous through a 4" 95% pleated filter. My IAQ is better inside than outside all but 20 days a year perhaps. I build all my houses to be able to accomplish this. After that it is up to the homeowner to turn off the lights when they leave the room, etc.
OK a little long winded answer for the question you asked but there is a lot of material to cover and this seems to be as close a thread as any to attracting those interested in these topics.
Is that to say that you have a datalogger located between the top RBS and a subdecking such as plywood, that datalogger therefore being also found somewhere alongside the 1x2 furring strips?
What product is used in the top layer radiant barrier sheathing?
I guess I'm am trying to see what the roof is actually composed of? Going out: rafters to bare plywood to furring strips to RBS to shingles?
"I can't say I was ever lost, but I was bewildered once for three days."
Yes that is the construction and the datalogger is between the bare plywood and the insulation.
I'm curious as to what the name of the product used as the top layer radiant barrier sheathing?
"I can't say I was ever lost, but I was bewildered once for three days."
Techshield(formerly coolply)
Isn't that why we use a vapor barrier on the warm side of walls and ceilings? To prevent that movement?
....in all of the "mild climate" areas where air conditioning in used in the summer, the vapor drive created by the cool, dry inside air sucks the moisture from the outside air through the walls until it hits the first cooled surface, where it condenses.
I worry about bathroom showers with tiled walls. First thing you do is put a VB up to protect the studs from the water. Course you have to give a drying route for any air vapor that gets behind the VB. Even if you use the Kerdi system (4 perms), porcelain tiles are going to be pretty impermeable, and if you use epoxy grout.... I put XPS in my stud bays, now I plan to rip it all out and put in cotton insulation with Tyvek on the outside of the bays.... Cut down the air flow and rely on drying of any vapor that comes in via diffusion....
I worry about these old houses with plaster walls, surely the plaster and layers of paint are a pretty good internal VB.
Ray's solutions only require completely replacing all of the siding, roof and external trim.... Sigh.....
One solution to the shower on the exterior wall problem is to sheetrock the wall, furrout the wall with vent paths to the attic or ceiling cavity and then install your shower surround. This method will provide a path for the small amounts of moisture that get in the wall to dissapate to the attic. There will be a possible concern from the building inspector about fireblocking but hopefully reason will win that argument.
Plaster is vapor permeable as well as being hygric buffer. If permeable paints are used, there is not a vapor barrier concern.
My methods are best suited to new construction, where costs can be saved in other areas to help pay for the more expensive methods. In a retrofit, cost analysis may prevent their use in certain cases. This issue must be addressed as a system approach with costs and cost offsets all being considered together. There are ways to incorporate these methods into a remodel but it is more of a challenge.
Ah.....sorry Jim, but I gotta disagree with you. I moved to upstate New York from living in the Willamette Valley ( 'bout 90 miles south of Portland, Oregon) for 10 years. Nice 'mild' climate, but you want to talk about condensation problems! WOW!- if you turn your head and blink, anything and everything starts rotting (your house, your clothes, your furniture, your car....)
Andrew
From condensation?
It seems that if you allow the moist humid air to enter between the sheathing and the foam insulation you are providing it with an opportunity to condense and eventually affect your sheathing.
I'm sorry but this just sounds like total nonsense. The whole idea of venting is to draw away the condensation. Why is condensation more of a problem in the baffles than it is on the roof deck? In fact matters would be worse on the deck with heat from the sun driving the moisture into the sheathing. And finally, the whole idea of permeable insulation is to allow drying to the interior.
I can only share what I heard explained to me by the engineers from Icynene: They explained that their product is all but impermeable to air molecules and so vapor (air-born moisture) can not penetrate. There is no dew point in the material and there is no possibility of condensation within the insulation.
That sounds like poppycock to me. My understanding is icynene is open cell, it is not impermeable. If anything, some argue that it requires a VB.
OTOH being permeable, icynene would be suitable insulation for a roof in a cold climate according to building science, but for different reasons from what you quoted from the icynene engineers. But that would probably also depend on the depth of the foam, permeability is halved for every doubling in its depth. But that's my WAG, BS don't have details yet on materials....
"I can only share what I heard explained to me by the engineers from Icynene: They explained that their product is all but impermeable to air molecules and so vapor (air-born moisture) can not penetrate. There is no dew point in the material and there is no possibility of condensation within the insulation.That sounds like poppycock to me. My understanding is icynene is open cell, it is not impermeable. If anything, some argue that it requires a VB. "Miss stated, but not complete poppycock.Yes, icynene is permeable and will allow some movement of gasses water molecules. However it does not allow for much movement of air.And it has been found that in building construction that not much moisture moves through building products. But rather it is carried by the move of air. Inhibit the movement of the air with a continous, unbroken mass of insulations, such as icynene is, and you stop the flow of moisture.
Dear Taylor,
Like I said in an earlier email, taking about insulation, vapor barriers, and building science has become harder than talking politics or religion. Most everyone has strong opinions, but too often they argue on the strength of their conviction, and little more.
From one of my most favorite sources, BuildingGreen.com: "Third, Icynene is very effective at sealing air leaks in buildings. In fact, the company has promoted the product as much for its air-sealing properties as for its insulating properties."
From http://www.icynene.com/assets/documents/PDFs/Spray_and_Pour_Formulas.PDF:
"[Icynene]...[creates] assemblies with very low air permeance. No additional interior or exterior air infiltration protection is necessary. AIr permeability of core foam, ASTM E283 data = 0.0049 L/S-m2 @ 75 Pa for 5.25" and 0.0080 L/S-m2 @ 75 Pa for 3.25"......Because of its low air permeance, Icynene is not infiltrated by moisture-laden air....Traditional building envelope moisture concerns are addressed with Icynene because it reduces air transfer, which accounts for 99% of moisture migration and subsequent problems like mold growth. Icynene’s hydrophobic properties also means that the material is unaffected by wetting and drying. Its open-celled structure allows water to drain right through the material. " (emphasis added)
Andrew
Edited 5/16/2005 9:13 am ET by Andrew
Andrew, yeah, I personnally do not have a dog in this race, I just want to understand what is right and get around the ill-informed opinion. There is great information here, just have to check the sources....And of course the building science also changes...
Taylor,
I couldn't agree with you more, about everything you said.
Andrew
How is a one inch air barrier going to be a better dissipator of heat? The reasone insulation is used is it slows down/ prevents rapid tempature swings from one area to another. An air space is supposed to allow the free movement of air across the underside of the roof deck to cool it down and remove latent moisture.
But has anybody done scientific studies to see how much air actually is exchanged? I doubt there is little if any. I'm sure you have all been in an un insulated attic in the summer that has gable vents and or ridge vents. Yet its still hot as hell. What good are those vents? Some could argue that they allow cool moisture laden evening air into the attic space to cause potential damage. Granted different climates are going to have problems.
If an air space is the better way to go then why are beer coolers made this way? Foam insulation applied directly to the under side of the roof is going to 'bounce back' the heat before it can build up. The foam does not provide any real thermal mass to add to the heat build up.
It would be interesting to see a study done in the field where temperature sensors were applied to the surface of the shingles and the the under side of the roof sheathing and a few inches away from the underside and then had foam applied to encapsulate the interior sensors.
Also have air exchanger sensors in a vented and un vented attic to see real world results of how much the passive ventilation helps cool a roof deck.
Some could argue that they allow cool moisture laden evening air into the attic space to cause potential damage.
And here I thought the problem was warm moist air entering a cool area and moisture condensing....
The only time that usually happens is in the winter in homes that are not properly sealed. But in the summer that hot humid air brought into the attic during the day can condense in the attic as it cools in the evening. Not likely but not impossible.
Reinvent,
I'm not sure I follow your question. What is dissipating the heat is not a "one inch air barrier", it is a one inch ventilation cavity, open at the bottom through soffit vents and at the top through a ridge vent. Like any gravity system, as the air in that cavity heats up (which it will when the sun strikes the roofing material), it rises sucking cooler air through the soffit vent to replace it. The result is continuous air flow through that ventilation cavity (which also carries away any moisture condensation in traditional roof system design).
Andrew
CapnMac put it in better words than I did (see the post before this one). That was what I was trying to convey.