Hello All,
I would like to add a layer of rigid insulation board to the exterior of my CDX sheeted house to limit thermal bridging and will be installing galvanized (or Hardie panel) siding over this with some kind of rain screen venting. I plan to insulate between the studs on the interior with fiberglass batts and install a 6 mil poly vapor retarder on the heated side of the insulation.
Does anyone have a recommendation for this application? Since I will be installing an internal vapor barrier, the exterior insulation will have to pass water vapor, I presume. I read a recent article in FH about this. It sounds as though the white non-faced foam board would work. I am having a hard time understanding how rigid foam insulation can breathe enough in this application. I have heard about rigid fiberglass insulation board that breathes well but haven’t seen it for sale locally.
Thoughts on this?
Replies
what climate?
Aprox how many heating degree days?
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I'm sorry, I should have explained that. I will need to look up the heating degree data. This is in the Northwest corner of Washington state on Puget Sound. Pacific Northwest temperate coastal climate. Much rain and humidity with occasional snows and sub freezing weather in Winter. Summers are usually mild with temperature peaks sometimes as high as the 90's.
The reason Piffen wants to know the heating/cooling degree days is to understand what your primary need is, heating or cooling ?
Typically a primary heating climate places the vp on the warm side of the wall (inside), while a primary cooling climate it is still on the warm side, but that warm side is now outside.. Mixed can be either way, but not both. It all depends on the direction of the vapor drive.
Thanks, Yes the need is definitely for heating during winter months. wouldn't this always require a vapor barrier on the heated side of the wall? There was a recent FH article about the different permeability ratings for rigid foam and how you can use it on the external side of your sheeting to enhance the thermal break. I have so many solid posts and uninsulated headers in my wall framing that I thought I should incorporate that thinking into my design. My fear of course is trapping moisture between barriers.
>> I am having a hard time understanding how rigid foam insulation can breathe enough in this application. << Me too. I'll be interested to see what you come up with. I think normally when rigid foam insulation is used on the exterior no vapor barrier is used on the inside.
>> I have heard about rigid fiberglass insulation board that breathes well but haven't seen it for sale locally. << if you check with a store that sells commercial roofing materials they will have it.
Here is another thought - what if you installed the rigid foam on the inside? Then it would act as both your interior vapor barrier and your thermal bridging abatement system.
Also, try searching for "Mooney wall" here at BT. That is a wall construction method specifically designed to defeat thermal bridging, whereby firing of some kind is used on the interior.
It's too bad that BuildingScience.com hasn't done a "designs that work" for your climate. Still you might want to spend some time at that web site reading their "information" section and see what they say.
All: BTW - whatever happened to Tim Mooney? Does he still post here? Does he have a new handle?
Thank you Matt. I hadn't thought of installing the foam on the interior. I that case I suppose that foil faced would be the best. I like it. I wonder if the extra thickness would be a problem with installing outlet and switch boxes. I think I will still see if I can locate the rigid fiberglass.
The foil face rigid foam is generally used outside. With a air space between the foil face and the siding, such as a rain screen wall, you get the most benefit of the foil as a radiant barrier. Inside with dw sandwiched over it, it just becomes an expensive vp.
Your concern about trapping moisture in the wall is a valid one. There are several people here frm the PNW that can hopefully add to this discussion. Just knowing from which direction (inside or outside) the moisture is driven is the leading factor in deciding where to place the vp, I think.
Dick Russel should come along and help you out .
I, from the NE, should come along and help him out in the PNW? As someone else noted, it would be nice if BSC had a "design that works" for the PNW area. We sort of have to read between their lines to at least avoid things that might cause problems. But I'll throw out a few thoughts.Building on the ideas expressed elsewhere by Dr.John Straube on moisture movement, I follow BSC's general rules on order of priority.1. Don't worry about water movement into a cavity by diffusion or convection if you don't keep liquid water out (rain leakage).2. Don't worry about water vapor movement into a cavity by diffusion if you don't prevent convective transport (air leakage, cavity not sealed).Assuming rain leaks are prevented (rain screen in the PNW) and the cavity is well-sealed against air infiltration, I'm inclined to think that management of water vapor transport by diffusion then should(a) slow down the movement into the cavity from inside during cold weather periods, and(b) allow drying back to the interior at times (most of the time in the PNW?) when water vapor isn't going to diffuse the other way.The framing and sheathing, and insulation if cellulose, have a lot of water capacity between the highest considered "dry," perhaps 19%, and the lowest it likely will see at other times at the location. If the rate of intrusion by diffusion is slowed down enough, the wall materials will absorb the water safely. This can't go on forever,so at times water must be able to diffuse out again.For these reasons, I would certainly avoid use of a vapor BARRIER in the PNW, in the form of poly film or a foil-facing on anything. I would expect that an inside vapor retarder of a few perms (latex paint), and not as low as the 1 perm that would serve the NE well, would be appropriate for the PNW. That would slow down diffusion enough so that the wall assembly could absorb safely what little moisture might move into it from inside during the occasional cold spell, while allowing drying back to the interior at other times (most of the year?)I hope I haven't stepped in anything here. My calcs and thinking about moisture issues have all been aimed at NE climate. Here we don't worry much about moisture diffusing into the cavity from outside. But even here I'm inclined to think: don't prevent water diffusion with a barrier if all you need to do is slow it down enough with a retarder. It's OK to let the wall moisture content cycle with the seasons, as long as it never gets too high for too long.
Thanks Dick.
I'll claim a senior moment on your location, and lazeyness for not looking it up :).
I follow you logic and use it myself in my mixed climate, but was hesitant to suggest it for the PNW. I'm also not as concise or eloquent as you.
That is a very thoughtful approach. The only thing I wonder about is the idea of drying to the inside in the PNW. Rainscreen construction affords the opportunity of providing a permeable exterior sheathing without allowing water infiltration. The mild climate also means that even in inclement weather many people have their windows open and keep the interior of their houses much colder and damper than they do in the East. A good seal of the interior - with or without poly vb - and a highly permeable exterior seem to work well.
I'm just thinking out loud here about the direction of movement of water vapor through the wall in the PNW. As a starting point, I'll assume comfortable inside air at 70 F, 40% RH, which would put the dew point in the upper 40s (F). Further, being the PNW, I'll assume the weather outside is raining all the time (they must have a lot of suicides).Water diffusion always is from higher to lower concentration; dew point is a good indicator of concentration.Suppose the outside temperature is low 40s or cooler. There would be a slight drive of water vapor from inside toward outside, more as the outside temperature is lower (and thus the outside dew point also is lower). Water vapor concentration and temperature both drop continuously across the wall toward the outside, if the cavity is indeed closed. Some retarding of diffusion from inside is appropriate, to keep the concentration at any point below what the temperature there will support without condensation.Now suppose the outside temperature is 50s or warmer. Dew point also is up there (it's raining, RH near 100%). There would be a slight drive of water vapor from outside toward the inside. Until the outside temperature hits 70 F, the temperature at every point within the wall will be warmer than outside, so there will be no condensation. Without dehumidification, the air within the house will become more humid over time (quickly with a window open), with the dew point rising toward whatever the outside dew point is, or even higher due to inside human activity (use of water, breathing). Wood will absorb moisture and swell.Air change due to ventilation and stack effect leakage will moderate the effects of human activity on inside humidity, always in the direction of whatever outside water vapor concentration is, by dilution.The use of air conditioning to make the inside air cooler than outside can cause a problem in high humidity areas, since it creates the possibility of a place in the wall where the temperature is below the outside dew point. If the moisture diffuses inward, and it can't continue through the drywall to the inside, the concentration within the cavity will continue to rise, possibly to the point where condensation occurs. This is most likely if a poly vapor barrier is placed behind the drywall, and that would be the coldest surface in the wall cavity in this scenario. With poly there, there won't ever be any significant movement of moisture from the cavity into the house.
Edited 11/5/2009 12:09 pm ET by DickRussell
Thanks for the great information information. It helps me to get my collective thoughts together for my insulation scheme. When I built my last house in the 80's the accepted thinking was to create an impermeable moisture barrier on the warm side of the wall (inside in this case) so that water vapor couldn't condense in the insulation. I remember going so far (as was recommended to me at the time) as to duct tape off all of the outlet boxes and seams in the poly. I don't think that the construction industry was necessarily making this a standard practice at the time.I'll look further into your recommendations. I'm sure the thinking has changed over the years as we learn more about the physics of insulating heated structures. Thanks again for your thoughts on this.
"When I built my last house in the 80's the accepted thinking was to create an impermeable moisture barrier on the warm side of the wall (inside in this case) so that water vapor couldn't condense in the insulation. I remember going so far (as was recommended to me at the time) as to duct tape off all of the outlet boxes and seams in the poly."
If you want to get the insulation in a new house in Canada approved today you would have to do exactly that. For northern climates it is still the standard although alternate approaches are gaining some acceptance.
I'm guessing that something other than duct is used?
Ever use your teeth to tear that red tape? It is almost impossible to get off. My wife finds it very funny.
Are you talking about that red building tape that is a lot like house wrap tape?
Yeah the stuff that cost about 25 bucks a roll.
Here is a link regarding the IRC zones and a FH article on vapor barriers:
IRC Zones: http://www.energycodes.gov/news/pdfs/2009_IRCvsIECC_ARRA_23Sep09.pdf
FH Article:
https://www.finehomebuilding.com/item/5090/whats-the-difference-vapor-barriers-and-vapor-retarders
You may or may not have already found these. Just my 2 cents worth.
Lott
Thanks. My feeling is that we should design building envelopes that are very forgiving and will work over time as the building is lived in and modified. That is why I am so wary about approaches that rely on either the interior or exterior being perfectly sealed to work properly.
It seems like a good assumption to make that moisture is at some point going to find its way into the cavity, and the better the cavities' ability to dry itself, the less chance of damage.
I tend to agree. I also agree with the article's suggestion that a barrier's permeability as well as its position in, or on as the case may be, the envelope should be based on the climate the residence is located in. I am about to do an extensive exterior remodel of my current residence and have been trying to research some of the current thinking on insulation, vapor barriers, etc. Several years ago, when I was building houses (I live in zone 2) the thinking was a Class 1 vapor barrier (visqueen over kraft faced insulation) on the heated side of the envelope in the walls and ceiling with that same barrier (when insulating floor joists on conventional foundations) on the outside of the heated area.
Lott
Edited 11/8/2009 6:50 pm ET by Lott
True. It's good that the one size fits all approach is disappearing. I don't know much about building in your area at all. A well insulated house in the Northeast needs much more attention to air movement from the interior into the wall than we do here. In the PNW damage to the envelope comes almost entirely from water penetrating walls from the exterior. As long as there is an opportunity for the cavities to dry, air movement from the interior doesn't seem much of a concern.
Here, upon urging from the Energy Star people (our local building scientists), we have gone to using little or no vapor barriers in walls and ceilings other than unintentional vapor barriers like plywood, OSB and paint. NC code enforcement is backing us on this as well. What we have found is that allowing wall assemblies to dry is more important than worrying about moisture drive through wall and ceiling assemblies. If you stop the airflow, you stop a large part of the vapor flow as well.
The old uninsulated houses around here let water in from both the interior and exterior like sieves but, as you say, they dried easily and have remarkably little rot.
Are you saying there is a connection between IRC zones and the need for vapor barriers?
Not necessarily. However, based on the article, there is a need to adjust the permeabliity and perhaps the position of the barrier dependent on the climate the residence is in.
Lott
Check out this JLC article.
Thanks, That is very interesting. Let me digest that.
Wow, that is quite the retrofit. I see that there isn't an interior moisture barrier due to the impermeable exterior barrier created by the two layers of foam. There was a comment posted after the article regarding the increase in moisture levels in the house:"Great job. I hope someone monitors this homes energy performance and reports back. Problems I have encountered envolve excessive moisture levels inside the home since the HVAC units didnt have to run as much as they did before the upgrades. Good Luck to everyone that swings hammer for a dollar.
Posted by: J.T.N Inc. on November 9th 2009" It makes me think that it may not work in my climate conditions with only hydronic floor heating, although I imagine installing only a 2" or less thickness of unfaced foam on the exterior side of the sheeting.
Hate to say it but I didn't read the article carefully. I just referenced it because I saw it pertained to your situation.
Re your quote:
>> "Great job. I hope someone monitors this homes energy performance and reports back. Problems I have encountered evolve excessive moisture levels inside the home since the HVAC units didnt have to run as much as they did before the upgrades. <<
It sounds like he is talking about another house (houses?) not the subject house, however, I think what the guy is saying happened is that the house's AC system is now oversized resulting in shorter run times and inadequate dehumidification. So, I guess envelope upgrades can be a 2 edged sword. Now a new(?) (smaller) AC unit is needed. I can think of worse problems to have. :-)
I build new, not remodels, including a few Energy Star homes - when we can find a home buyer who is willing to pay for that. One of the "sales bullets" that Energy Star proponents use on builders is that with the right envelope upgrades, you can buy less tons of cooling and less BTUs of heating thereby offsetting the cost of the envelope upgrades.
Edited 11/13/2009 6:55 am ET by Matt
I was about to let you know that I rediscovered the article in FH regarding rigid foam retrofits. It explains vapor permeability ratings of the three types of foam and interior vapor barriers vs exterior, in the November 2009 issue (#206) on pages 34 and 35.
Then I received the most recent issue of FH (Jan 2010) Page 68. Amazingly timely, It closely follows your thinking about not relying on vapor barriers to keep water out of structural cavities but allowing the cavities to dry to the inside or outside depending on the individual systems.
Anyway, I'll shut up here and let you read for yourself, if you haven't already. Thank you for posting your thoughts on this.
Actually - I don't get the mag.
Barnes and Noble is your friend!>G<
<<<<<<<<<<<
Actually - I don't get the mag.>>>>>>>>>>
:-)
Just a correction in my last post:
It was your thinking, Dick, about "drying to the inside" that I was getting at. I was loosing track of who said what with all of the great collective brain power here.
BTW - I just ran across this article
I think it is worth a read, if for no other reason that it was written by people in your direct geography.
If you seal a house up tight, you need ventilation: period. so "with only" a radiant hydronic system you're right. you'll need a ventilation system as well.
I would talk to the REMOTE wall guys up in alaska. Southwestern alaska isn't all that different than you guys in terms of climate and they do some interesting stuff there. we are using a modified REMOTE system on our current shop project, 6" plus of EPS foam on the outside of the zipwall (vapor barrier) sheathing, no insulation at all in the stud cavity to make sure the dewpoint is hit outside the vapor barrier. we could add some insulation, but we found that the costs of insulating it to take us from an R25+ up to anything significantly higher just didnt make any sense.. we are saving the insulation AND the drywall, just for fun, and we'll paint the stud cavities direct as our "finish wall". extra money we'll put into solar or other fun energy reducing goodies for easily double the ROI of going from R25+ to R40+ walls.
http://www.cchrc.org: while they are about "cold climate" they do build in that wet, mild section of alaska as well.
-=Northeast Radiant Technology=-
Radiant Design, Consultation, Parts Supply
http://www.NRTradiant.com
Thanks for that information and those links. Amazing stuff!