*
I’ve read a bunch on DP cels and also on icynene, trying to decide which to use to retro-insulate the bays in my old house. These are some of the things i’ve read:
* Do not use a vapor barrier, bec the cels are a big sponge and it’s easier somehow for the water to get in via little tears than it is to get out.
i Why is it easier for water to get in than out?
* Somewhere, somebody wrote that three coats of latex paint is the same as a layer of visqueen behind the drywall. OK, i get that it’s on the front side of the drywall, but isn’t the principle the same, that
i you’ve got a de facto ad hoc VB if you paint your walls?
* Then there’s the matter of felting the sheating boards, plus i had thought of putting up an aux layer of 1″ XPS on the exterior of my 3-1/2″ walls over this (can’t do on the interior side), but
i will the walls be breathing or asphixiating if i do that?
Replies
*
Dear SG,
Years ago, I responded to a request for bids to insulate an old town rec hall. The inside was entirely sheathed with t&g. One of town fathers tried to get my goat by blurting out: you can't insulate an old building. I told him he was right. We had to modernize the building before it would be prudent to insulate it. I went on to explain why they should pay me half again as much as the rest of the pack to moisture seal the crawl space, fix the ducts, and DP everything else to provide some air resistance in the framing. The old guy was smarter than he looked.
To your comments: Icynene seems to be a fine product for open applications. It has a very short track record there. Results from installations into closed bays have been tragic. I have witnessed this directly and spoken to a bankrupt franchise owner with the same story.
1. Water enters framing cavities almost entirely by convection. It leaves almost entirely by diffusion. Plastic foils (you'll have trouble with the term VB) are very effective at suppressing diffusion. Since it is extraordinarily difficult to site-build a bag that will capture pressurized gas, plastic is not a reliable way to control convection.
2. There's that VB terminology trouble already. Paint layers will build an effective diffusion barrier. Drywall makes a fine pressure barrier if the penetrations and edges are sealed.
3. Gaseous water passes freely through tar paper. And to a lesser extent through XPS. Aside from respiratory allusions, walls must have some way to dry by diffusion. Walls become unreliable when both surfaces are sealed with diffusion barriers.
Exterior foam sheathing is a big retrofit project. We have never had to resort to that measure trying to make old buildings exceed the performance expectations of new construction.
Evasively, Fred
*Fred,I'll cut to the chase: the house has rock-lath with plaster and thirty-eleven coats of paint. Exterior sheathing is board with lots of spaces, so i want to mess that up instead. When i originally looked into cels last year, it seemed smart, but now i've read about its "near-liquid" flow. I thought of icynene bec i thought i could see it ooze bet the boards and thereby know whether the bay was filled or not. I'm now wondering if the cels would blow out the 1/2" and smaller spaces bet the boards. I saw The Blob in Issue #130 and i became afraid. If i used cels (or is it "cells"?), would i need to drill holes in the sheating boards instead of just pulling a board off? (Boards run about 8" wide, not your 3" gap in the drywall you shot through in the article.) In the Killing-two-birds-with-one-stone Dept. (skinning cats?...gotta get me some new visuals): I wanted to pull a board so that i could fish out the extant 1-1/2" FG batts, or would i have any luck blowing cels if i left them in situ?Are you saying it's OK to have one diffusion barrier (the painted side) as long as cels can "breathe" (i need my allusions)to the other side?Will this--without foamboard--be sufficient for my 3600' elevation in Montana? (I start my dog-walk where Vermont stops.) It's snowing and 20* out there.I've already bought the foamboard. Would i get into condensation problems if i instead used it bet the bays in my wood basement?Gas passes through XPS?!?! This bodes ill for under-slab insulation, methinks.
*sg,I gotta clarify a few statements made above:"1. Water enters framing cavities almost entirely by convection." This is not true, unless by convection he means pressure driven air flow and a thermal gradient. "It leaves almost entirely by diffusion." This is true."2. Paint layers will build an effective diffusion barrier." This is true of some paint, not latex based. Latex paint does not retard vapor diffusion.
*splints... if it has rock lath.. how or when did it get the fiberglass batts ?were they installed from the outside ?is it really 1.5 inch batts ?if they were installed from the outside.. were the walls opened right up.. ie: are tehy bunched up in teh top or bottom of the stud bay.. if they are uniformly installed.. you can pull a row of the horizontal sheathing boards at the top & bottom.. and either pull the batts out.. ORslide the hose into the bay .. between the sheathing and the batt.. the cells will flatten the batt against the rocklath...OR pull the batts out and blow the bays....you'll get better flow around electrical devices and other things in the bays...i'd go for the second method and forgeddabout... the foam...old studs.. you'll have 3.75" of DP... 15# felt on the exterior & siding of your choiceput the foam in your basement walls...
*Ya know splint, you can do what you want. It's your home, but you should know that in spite of what the guru says, pu foams have been used for the past 30 years and is the only system that will seal your home.It's the only product that I have found to do what it says it will do. period.I don't sell it, my companies don't handle it but we have a huge amount of it installed and it works.If it will seal and insulate 30 storey condos for 34 years without a problem, it will do a great job on your home.
*Tim: This house is at least 60 years old, so i'm assuming it has a mix of both oil and latex, plus i will skim coat and seal it yet again. I wanted to be sure one diffusion side would be sufficient in case it is impermeable to vapor.Mike: Attached is a picture of the light yellow-colored FG batts exposed in the kitchen where i removed the lath and plaster. They look similar on the other side; the front and back layers of kraft paper are glued together at the ears, best described as kind of a pillow going from compressed edges to a poofier center of 1.5 inches or so.The stapling shows they were installed before the rock lath went up. I remember now reading about inside stapling leaving channels for convection, so i guess removing them would be best anyway. They are pretty flimsy, so it shouldn't be too hard to rake them out from the sheathing side, though they aren't loose. I demo'd the kitchen and bath back to the studs so i could do my electrical and plumbing with no hassles, but the rest of the interior walls are intact.To clarify your advice to forget about foam: i'm assuming you mean the foamboard? In addition to getting a few more R's, i'm also thinking of thermal bridging, weighing the likelihood of telegraphing on my Hardieplank against the cost and work of installing XPS. You install a lot of fiber-cement--can you comment on house it shows up the studs with and without a thermal break? This is where my liberal arts education falls apart, where i try to make sense of the seemingly disparate figures i've seen for whole-wall performance.One more thing: any preference where to start? Middle of bay, from top, or from bottom?I just realized i could have called this thread Bay-Watch...damn!Gabe:The foam has an inherent appeal due to no mold issues and that someone else would have to guarantee it. I did some quick calculations, however, and i got over $2000 for just the walls...one of the pitfalls of it not being a DIY gig.
*The photo:
*well, hmmm, OTOH....how do you intend to nail your fibercement ?..if you use 1" foam... you will eliminate some thermal bridging, but you have the problem of a good nail base...depends on the detailing... door casing,window casing, corner boards, frieze board, watertable... to get a good nail base for your trim you need soild blocking.. so if you use 1" foam you need 1" blocking...2" foam... 2" blocking..it becomes another set of problems... one of the main reasons (among others ) we stopped putting foam boards on the exterior....even if you vertically furr teh exterior over your foam.. this will form a rainscreen plane... but the furring will try to crush also .. and you still need blocking...as to wether the studs show up or not... yes , they do.. but only when conditions are right..when the area at the stud gets to below the dew point and the rest of the siding is still above the dew point... now, i know this happens with cedar clapb'd, but i THINK it will not be as big a problem with fibercement , because fibercement sucks as far as insulation qualities.. ie. it will have a more uniform temperature and all of it's surface will tend to be ambient temp...so.. depending on conditions... if ALL of the surface is below the dewpoint.. the studs won't show..since you have colder winters, my GUESS is thermal photographing will not be a big problem...occasionally you will see it... and think of me (sigh)...so , to say it again differently... if you furr , the furr will photograph.. just like the studs...if you don't furr , you still have to block.. and the blocking will photographbut the fibercement should photograph less than wood..b NOWi'll say it again... i have no knowledge of icynene as to it's vermin resisting qualities... but all the other foams , xps, eps,polyiso, Styro-SM, Foamular... all have vermin infestation problems...so , the only one we use is Performguard EPS...b but hey, whadda i no ?
*Shop around, you should be able to do better. The payback is that the foam is at least twice as good as any other form of insulation. The house will be much more comfortable and draft free. You will have a healthy saving on your energy bills.Tony Helps
*"all have vermin infestation problems" What kind of vermin are we talking about? Mammals/rodents? Insects? Both?
*Mike:I had thought to install the Hardieplank on 2x2 fur strips bet the foam, or strips over the foam rainscreen-style, though i'm not too worried about 15" a year of precip. I'll blind nail on 16" centers, using 8" plank. The foam compression/wobbliness part worried me, so i'm glad you waved me off that. However, it gave me a new idea, that of installing fur strips over 1/4" or 3/8" fanfold, the theory being the fanfold, taped, since it diffuses, could eliminate the felt and provide a thermal break, but still be a water barrier and provide maybe an increase of 1.5 R, yet be thin enough i could nail blocking for trim over it for more thermal break area. And i thought the vermin could find better cover elsewhere, too!Additional benefit would be if i could make my wall prefectly plane by compressing it here and there instead of the shimming i was thinking i would have to do on the board wall. Effects on windows and trim and such are not important--this will all be new and i'm making them on site. So the questions are:* is fanfold a substitute for felt or do i still need both? I ask bec i see fanfold applied directly over siding before re-siding on vinyl jobs.* is fanfold compressible? Do people "shim" in this fashion?* what do you think of the new scheme?Gabe: I found only one icynene guy from Googling and the Yellow Pages, and only one (or two) more PU guys (they're related) in the Yellow Pages. I saw more benefits to icynene than poly, so i went with that type for estimates. I could be wrong, but in any case, the cost is quite high and i'm not sure the very next owner in two years will reimburse me enough.
Splints... i'm still trying to figger this dumb board out... reminds me of JLC.. which i hate...
before, i could tell at a glance if something new had been posted.. or a response had been made.. this is not nearly as intuitive as the old one...
besides .. isn't this a lot more important than insulation & vb / non-vb ?Rhode Island: Design / Build / Repair / Restore
Dear SG,
-cut to the chase
I thank you. My springy keyboard thanks you. The local ISP and power company also send their regards.
-could see it ooze bet the boards
Nope, I saw it react too fast to even fill wide gaps
-became afraid.
Me too. All the time. Girls screaming in the bathroom like Psycho when it creeps out from under the vanity. Solid closets. Buried beds. Etc. But Andy laughed.
-would i need to drill holes
My motto: any access in a storm. With your 8" boards, I'd be tempted to pull a course or rip them and pull half a course. (Where'd the siding go in all this excitement?)
-would i have any luck blowing cels if i left them in situ?
I have had that luck over ten of thousands of sf. But the batts are a contaminant. And they make the work of reaching the far ends of the cavities much more strenuous. I drag out all the fg trash I can with a back scratcher thing or naily board.
-Are you saying it's OK
No, I would say that the condition you describe is optimum. Houses similar to this in Saskatuan (12,000 HDD climate) with damp cellars suffered no moisture damage after 50 years of cels insulation. Loose boards for exterior sheathing are far superior to plywood for promoting moisture tolerance in the assembly.
-Will this--without foamboard--be sufficient
More insulation is always better. (Remember, I sell the stuff.) If you had siding in place, I would never pull it just to add foam sheathing. With the sheathing exposed, you have to look at the effort involved with detailing the new siding. Unless we're talking vinyl, you've got a chore to place clapboard over foam and get the trim looking pretty. A layer of foam, even fan fold will smooth out the sheathing. Don't expect the wall to get flatter b/c the foam will not compress enough.
More important: Deal with the air pressure on the wall. Then you will be happy. This means attic sealing, duct repair, and exterior felt.
-condensation problems if i instead used it bet the bays in my wood basement?
I don't understand you basement. Generally speaking, foam panels work poorly inside bays unless they are sealed with gun foam all around. And don't use it as a diffusion barrier.
-Gas passes through XPS?!?!
"passes" is too weak of a term to describe the process. Foam will block convective air flows perfectly. Individual water molecules will diffuse through readily. Remember, a water molecule is much smaller and faster than an oxygen molecule. (This point has proven to be a controversial one at Bt in the past.)
-This bodes ill for under-slab insulation
Get out the nerf balls. Liquid water is a completely different chemical than gaseous water. I had to slap Joe L. silly to get him to understand this.
Better late than never, Fred
Edited 3/20/2002 1:56:25 PM ET by FRED_LUGANO
OK, disregard the shimming qualities of fanfold. What do you think of using the 1/4" fanfold in place of the felt, if both are water repellent and vapor diffusers? And the thermal break part sounds good, eh?
If paint itself [eventually] forms a VB, why did i hear moaning over the Lentz article in JLC, where he used plastic on the interior walls? I can see why not with FG, but it seems like a belt-and-braces approach with cellulose.
Dear SG,
Hello up there.
The thermal and moisture performance of fanfold (non-foil faced variety) and felt are practically the same. Felt is easier to detail since you can just lap it. Foam doesn't tape well and this could lead to a funnel effect at the horizontal joints. Instead of trying to seal the faces of adjoining sheets together, I would use tape as a Z flashing.
With half inch gaps b/t your sheathing boards, you will get plenty of blow back from the insulation process. There isn't that much waste, but taking it in the face for a few days is annoying. Several years ago I did a house from the 1640's with very open sheathing/no siding, and ate serious cels for a day. Of course cels is compatible with a veggy diet, and I'm known for my good taste in insulation, but I had a helper chink the remaining gaps with empty bale wrappers to keep the peace with my snippy hairdresser and relentless makeup artist.
You might do better by installing your foam, whatever thickness, before packing the walls. With fanfold, you'd need to nail it well with caps or discs on one side of each sheathing joint to prevent bulging. One inch foam boards are rigid enough to allow for more casual nailing. This is the only way you could get any insulation value from your exterior foam. The cels will fill the sheathing gaps and bond the foam into the assembly. Otherwise, cold air would sneak in behind the foam and render it useless.
Even though the editorial offices are down the road from us, I don't read or write to JLC, so I don't take your ref. Generally speaking I hate the belt-and-[suspenders] approach to building. It usually means the pants fall down b/c neither gets done well. In poverty housing rehab, there's no money for doing dumb stuff twice. Imagine our glee when we learned we could build faster and better by eliminating plastic wall components.
Lastly, moisture protection is far more critical in moist climates. I have to be much more concerned about every project I tackle in Dampshire, Vermont than you will ever need be in Mule Bones, Montana. You just aren't going to have a problem with wet walls no matter how badly you screw this up.
Wondering if "braces" are some girly thing, Fred
[email protected]
splints... is fanfold really a water repellent covering ?
the article in JLC had the guy taping 15 # felt over bottles and inverting them
if you taped fanfold over a bottle of water and inverted it , would the water seep out or not ?..
if it would seep out , tehn it is not performing teh same rainscreen function that teh 15# felt does.... naturally i have no fookin idea wether it does or it doesn't...
but i am wary of the qualities of fanfold in general, even though we used it for some jobs..
Mike Smith
Rhode Island : Design / Build / Repair / Restore
Fred,
I apologize for the tone and the "ad hominem". Open mouth, insert foot. I appreciate your lack of same. I will argue points with you, but on this one I think your right about the size. I don't buy the speed issue, yet. The way I understand things, on a molecular level (and I do understand the diatomic gasses, though 3 hours ago, brain not fully engaged) the velocity of gaseous molecules is related directly to temperature and inversely to pressure, though not to molecular weight or size.
Dear Tim,
I knew we'd be buddies and I'll never turn aside your challenges. The only way to get deep into these matters is through cooperative dialog instead blow by blow confrontation. I'm not in the mood for that anyway.
The difference between convection and diffusion is key here. As carps, we don't generally need to concern ourselves with the molecular weight of gases involved in convective air movement. That's all about group behavior and air pressure is the interesting thing.
Diffusion is different. Here we deal with the behavior of individual particles. Size, mass, and temp are most important for us. For example, I see housewrap as a net. The anchovy get through, the tuna don't. Pressure doesn't change that (too much).
Starting to like you, Fred
[email protected]
Fred,
So d'you look at my wall in the other post, "VB in the middle"? I am not a constructer but I read everything I can find (on the engineering side of things) especially when it come to humidity, moisture and IAQ control. I am basically rebuilding my home. Soon to have a complete shell. I have been building, what, I later found out from reading Gene's posts, is an envelope wall.
In my area of the world, (northwest Winnebago County, IL, with an average of 6970 HDD, and about 2800 CDD) we have a mixed climate, mostly heating but still mixed. Most wall construction would not allow for a vapor barrier in the middle, though it seems like the best way to go in a mixed climate.
Having seen first hand in demolition of the interior wall fininshes of exterior walls in my house and seeing the mold in the fg batts, concentrated on the inside surface next to the poly VB, I WAS convinced that a vb in a mixed climate was a guaranteed way to grow mold and degrade wood construction.
Edited 3/22/02 7:47:20 AM ET by Tim
Fred,
In any of the projects/houses buildings/experiments that you have seen, read about or heard of, has any one ever tried to eliminate/mitigate air (and along with the, the moisture it carries) inleakage/inflitration/out-lekage by menas of controlling the pressure in the structure?
I believe that in the absence of air migration through walls and ceilings, moisture problems would be minimal. I understand that vapor diffusion would cause moisture to migrate from an area of higher concentration to lower concentration, but that amount of moisture would be significant only in areas near point sources, such as a poorly vented bathroom.
tim , controlling the pressure inside a structure is like trying to bail the ocean with a clamshell, except the consequences are so much more dangerous....
too much pressure will drive any moisture into the structure following the path of least resistance... and there is always a path of least resistance...
get it wrong in the other direction , inducing a negative pressure , will suck rain, runoff, and moisture into the structure from the outside with even worse consequences...
some hvac systems try to give a positive pressure, but this is in addition to balancing the supply and return systems from roo-to-room... it fast becomes an exercise in frustaration and expense.. and is a sure violation of the kiss principle...
but hey, whadda i no ?Mike Smith
Rhode Island : Design / Build / Repair / Restore
I found out that NO ONE who sprays foam in this area--and i found several more who do it--will put foam in enclosed bays. I thought one guy did, but he doesn't anymore. The fanfold would cost me 2/3rds again what the cels will--not a great bang for the buck there. I'm going to offset the furring strips from the studs for my next best attempt at a thermal break.
Dear SG,
I don't know much about your house, but I bet we could figure out a few ways to get more for your money than the thermal break provided by furring.
Just guessing, Fred
[email protected]
Fred,
Well, here was my furry thinking: Something has to hold that tarpaper in place whilst i blow cels in the bays. I can use a million cap nails or i can make a mere quarter million furring strips pre-marked for each course of siding and screw them on mid-bay after lining up the first course mark with my laser level (thinking ahead). This screwing them on mid-bay would best help the felt resist bowing out from the cels and provide a barrier so i don't have to plug each space between boards, yet leave enough room for my access hole(s) to be drilled. I could shim them to compensate for the board sheathing and they would be offset from the studs for a bit of a thermal break so not to telegraph the studs. Oh, and a rainscreen, too; it's windy here.
C.
PS: I was told by one contractor that he would drill two holes, one mid-height and one at the top bec you can't blow cels up. I seem to recall blowing-up from your article, however--is this a blower discrepancy?
Dear SG,
That all sounds very reasonable. But you get no thermal break from the furring. As a siding process, I would never have come to that solution. But every builder need to play her trumps. Congrats for knowing what works for you.
The Two Hole Technique is archaic. In yesteryear, the installer would drill the 1 inch holes and press a nozzle to the siding. This results in low density, poor coverage, and settling. On one job, we pulled siding off a Civil War Era home to find a row of nozzle holes. I was excited thinking this would save me some product expense. Turned out that the old stinker drilled his holes right on top of the studs. His entire insulation project consisted of thimble sized pockets of cels. Talk about saving product!
Now we use a single large hole and insert a tube the length of the bay as you saw in Andy's home. This lets us probe the bay vertically for hidden blocks too. Even with a wimpy blower, cels will fly out of the hose at speeds up to 50-100 feet per second. So refusing to blow up hill is pretty silly.
Thinking you've got enough work to keep you out of trouble this summer, Fred
[email protected]
Mike,
I don't buy the clamshell bailing anology. I know that I can easily pressurize any space positively or negatively. Unless the structure is a sieve, it doesn't even take that much of a fan or supply/exhaust differential. HVAC system-controlled pressure differentials are a key feature of almost all controlled environments in nuclear power plants, hospitals, clinincs, emergency and operating rooms and clean rooms, to think of out my personal design experience. Say a house of 3000 sf, with 8 foot ceilings is of "unusually tight" construction and due to wind driven pressure differential, leaks 0.25 air changes per hour. This is 100 cfm. Add 50 cfm for the dryer and each bathroom fart fan and another 100 for the kitchen exhaust (if its decent). Pick a fan to meet this with a little margin, and a variable sped controller. A 500 cfm at 0.25" esp fan would do. Cost about $300.
If by being dangerous you mean that it would be dangerous for the average contractor to install a system with no engineering support or knowledge, I can't disagree with that. If you mean that to attemp control air movement by controlling pressure is dangerous, you have to explain that to me a little more.
I fully agree with the KISS principal and I am not advocating such a system be installed unless required. This is about "what if", not "why not".
But, if it you were to positively pressurize (very slightly, just enough to overcome the exhausts and vent induced "negatives") a house in the cooling season when a properly sized central air conditioning system has dehumidified the inside air so as to induce a little "outleakage" of dry air through the path of least resistance and negatively pressurize the same house in the heating season (though depending on the house, this may not be necessary since most house ventilation "systems" cause inleakage, especially during heating) to cause dry winter air to leak in through the path of least resistence, wouldn't this keep the "path" as dry as possible?
I know this can be done, I was just curious if the people out there in the "business" had ever seen such a setup and what the results were.
Dear Tim,
Thanks so much for posting. After printing my son's TechniColor book report and spending 2 hours on the phone with Stupid Teen Girl telling me her positive pregnancy test must be a fluke, I'm really looking forward to focusing on your comments.
Deliberate pressure control. Yes, we see it all the time. My institutional clients are very big on keeping all unconditioned air out of their buildings. They don't any pollen, road filth, disease, or temp/humidity swings. So they keep a constant positive pressure on the envelope. Any air coming in has to go through the machinery, sliced and diced to their specs. A museum, for example, can't get a traveling collection for display without this kind of control.
On the opposite side of that strategy, when I contribute to a natatorium, you can bet that I'll demand that the interior be depressurized with regard to the outdoors. As Mike says, there is always a path of least resistance. I make mine a pipe.
In houses, there are interesting applications as well. A number of manufactured homes I've seen had a furnace set up where a 2 inch duct was run from the attic into the return plenum. The idea was to supply ventilation by sucking outdoor in whenever the furnace fired. Bad idea for a house on a wet foundation.
Right now we're developing an article on cellar construction that has been long delayed by my illness. Part of that story will be about controlling foundation moisture by pressurizing the crawl space. This is a mechanical system that provides whole house ventilation by blowing indoor air out of a vented crawl space. I think Bower touched on this concept in his Understanding Ventilation. The appeal here is easy code approval, low cost equipment, great IAQ, total protection from soil gases and backdrafting combustion appliances. I like it even if I have to grit my teeth on the word "vent".
Yes, I concur with your thinking about the weak effects of diffusion. But never wanting to blow a chance to blow things out of proportion:
moisture migrate[s] from an area of higher concentration to lower concentration
No, don't count on that ever. Even well known academics have fallen for this. Vapor pressure is the key metric. Concentration is a component of vapor pressure, but so is temperature. Sometimes we'll face a scenario where we see moisture diffusing away from the relatively dry zone to accumulate in a very wet condensation spot. High temperature differentials will do that in a buildings with unlimited moisture sources. But you don't see it too often in real houses.
Feeling better now, Fred
[email protected]
Fred,
I recently designed the HVAC systems for an operating suite of a clinic that wished to upgrade and become certified as an Ambulatory Surgical Center. Positive pressure control is not an option in this environment. The OR is ++, the clean, sterile OR utility is +, the corridors are neutral, and the biohazard storage and med gas sorage are both -.
As far as a natatorium goes, I dehumidify and exhaust the space, introduce enough outside air to meet the code for ventilation for occupants, on the design day (worst case). The rest of the time, exchange relatively dry outside air for humid inside air, condition it as required. However, I do not maintain a pressurized environment in such a case.
I agree with the "path" concept, especially with tighter construction eliminating more of the paths. As we make tighter structures, we concentrate leakage in fewer paths and consequently create greater problems in localized "paths".
I misspoke (mistyped?)on the "concentration" statement. I agree that we're talking about vapor pressure of the moisture in air, and that temperature plays a more significant role than dewpoint (i.e. concentration).
"A number of manufactured homes I've seen had a furnace set up where a 2 inch duct was run from the attic into the return plenum. The idea
was to supply ventilation by sucking outdoor in whenever the furnace fired. Bad idea for a house on a wet foundation." Bad idea, period.
If you were to install an HVAC system in a residence based on the International Mechanical Code, you have have to bring in outside air at the rate of 0.35 air changes per hour or 15 cfm per occupant, whichever is greater. Most building codes do not require this, but do not "disallow" this.
What you saw, I beleive, is an attempt to comply. Outside air cannot come from an attic, vented or not. A decent installation would provide an insulated, ducted OA supply directly from the outside of the structure, located a minimum of 10 feet from any exhaust of vent, to the furnace return. A better installation would have the outside air passing through an energy recovery ventilator and providing an equal amount of exhaust. What I plan to do in my own home is use the ERV as my bath exhaust (except the main shower area) and get 2 birds stoned at one time.
Tim
tim & fred... when the goal is pressure control.. as in labs & biohazards.. then you can design, spec & meet the goal... at an expense... the ME's recognize this expense and do not willy-nilly call for any and all areas to be so controlled...
in a residential enviornment, where you don't always have control over who writes the specs, who executes the contract, and who controls teh after-installation wnviornment by or for the homeowner.. then you are flirting with danger .. this is why i call for KISS in residential work...if it can get screwed up.. assume that it will..
now an ERV sounds like the ideal solution.. it control the pressure and the ingress and egress of moisture..
we were much involved with moisture and HVAC in the '70's & '80's.. and i still spend a lot of time talking about the pitfalls with a friend in the hvac business..as he points out.. the ME doesn't control the job... so disasters do happen.. even in commercial work.. never mind in residential work, where the average level of sophistication is way below public building /commercial work...
i still find some of fred's phrasing open to misinterpretation.. but i THINK we are in agreement, that controlling moisture sources is more important than trying to deal with pressurizing/ depressurizing home enviornments as a strategy for dealing with moisture..
but hey, whadda i no ?
Mike Smith
Rhode Island : Design / Build / Repair / Restore
Mike,
I agree with you. I can attest to the sentiments of your friend as well. The HVAC Engineer is seldom if ever hired by the one paying the bills and is sedlom paid to supervise installation of the systems. When we do find the installer ignoring specifications and drawings and call someones attention to it (the owner, GC or the architecht) we are typically "just causing trouble". Nobody cares until the part load, high humidity day and the building is clammy and then it your fault.
OTH, I would NEVER design or specify any type of space pressure control unless absolutely necessary for the use of the building. Part of my question is: "Is it (pressure control)a useful approach?, and Has it been tried? To what effect?"
But the issue is control of moisture in walls, as I see it, unless I misunderstand what were discussing here, i.e vapor barriers. Some will claim that the moisture problems originate in the soil and some will say that is not a major source, but rather other sources within the building. Approaching all buildings/houses as if they all have the same problem, is not overly wise. Moisture in the space is not bad, per se, but as it migrates through the structure, by whatever means, and condenses and "wets" stuff, then it becomes a problem.
Whether or not control of the moisture source or control of the movement of it more importantant, is also part of the question I was asking.
BTW, the ERV is almost always the first victim of the budget.
Approaching all buildings/houses as if they all have the same problem, is not overly wise.
This is such a good point, and i wonder if it isn't relative with zones within a house as well. In my instance i've controlled ground moisture completely, but i don't like to hear bath fans during my two-hour soak/library sessions--and i hate to think of the heat i'd be losing--so i wonder if for my particular use of a bathroom i shouldn't use a VB as well as insulation in at least that room, i.e. the maligned belt-and-braces approach. This would allow time for the moisture to migrate out of the confined space at least, and hopefully disperse to manageable levels in the rest of the house.
Fred, i included the thermal break benefit as one of the reasons to use fur strips. I thought of another benefit and that would be not to risk interaction bet siding and house wrap materials. I understand the plastic types have had the most trouble, but i thought it would be one less thing to go wrong. Do you have alternative ideas for a still-open mind?
Edited 3/27/2002 3:56:08 PM ET by splintergroupie
Dear SG,
You know with an optimum envelop, dry basement, and dry climate, you're obsessing here. But for the benefit of others:
If you really are reading and not say, playing water polo, a two hour bath is not likely to produce more than a pound of water vapor. [a pint is a pound the world around] You could mitigate virtually all of it with a quiet bath fan after reading. Substitute a dehumidistat for the normal wall switch and the fan will run just long enough to do the job.
Or you could simply ignore the entire issue. To get substantial condensation and serious damage, a house has to be subjected to a lot of water vapor. Maybe 50 or 100 pounds a day. That can't happen in the house you describe.
I've never seen framing visibly telegraph through any type of siding. I have seen the well noted Tyvek deterioration behind cedar up close and personal. But I thought you were talking about tar paper and cement board siding. I would have no problem mounting the siding directly to the underlayment and Tyvek would be ok here.
Recognizing that we have different tools, manpower resources, etc., and my ways may be functional but inappropriate for you, here's how I would treat the walls. First I'd probe the bays through the sheathing boards to find any weird blocking. Then I'd smooth the sheathing to taste. The whole thing would get wrapped in Tyvek to shed water and I would install the Hardieplank right onto it. Key courses of siding would be left off. The insulation would be tubed through those slots, the holes taped, and the last planks hung. I might even get tricky and blow some areas first. Little spots under windows and such. That would keep the siding installation simpler.
I don't see any need for the rain screen set up with self ventilating siding such as vinyl or planks. They breathe on their own. Ugg. Installing the spacer/nailers to support vented siding takes a lot of effort. (((don't tell anybody, but I use coax cable and I want to keep it a secret so I can publish the process)))
Sugar snowed, Fred
[email protected]
Splinter,
A long hot bath doesn't generate that much of a problem, as compared to say, a long hot shower. I believe that ground moisture (and all other external sources of moisture) has little if anything to do with what happens in a bathroom. OTH, if you don't like the fan because it is noisy, you can get a quiet fan or a remote fan. Does the area in question have any forced air? In my house, I have a supply register in a soffit over the entrance for the shower, and I run the furnace fan when someone is in the shower. The air in the room gets changed at the rate of about 6 times an hour. The moisture laden air from the bathroom exit the area and is diluted, mixed with return air and distributed throughout the house. My ultimate plan is to utilize an ERV to exhaust bathrooms and provide fresh air to the house, saving as much of the exhaust energy as possible. When I first built this bathroom, I put a poly vapor barrier in place, but hadn't completed the insulation (in the ceiling). Water would condense on and in the drywall because it was colder than the dewpoint of the air. With the insulation completed, the problem went away. Moving air makes a great difference.
Hi, Tim. With the installation of an Aquastar in the new house, i'm looking forward to just enjoying the long, hot shower option!
I try to make houses as electricity-independent as possible bec the power goes out frequently here, and as uncomplicated as possible bec there are no competent repairfolk that i could find, only ones who can rip and replace. I also take a cue from the deregulation messes of last year for electricity costs. Low-tech, low-cost, owner-friendly DIY solutions suit my nature and my location. For this reason, i'm tending to dispersed, small direct-vent units (Orbis) for heating with propane and going lightly with integrated systems.
I have to study up on HRV and ERV systems, but i'm wondering if you know of any systems that recycle my own air, as through some filtering system, that are not of NASA quality and cost? Wood-burning is big in these parts so i'm not so crazy about pulling that air in rather than recirculating the intoxicating smells i already know.
Splintergroupie,
In your quest for electrical independence, something to think about (because I have this problem) is that a lot of direct vent appliances use combustion air fans that are electric. I have a power vented Aquastar and a condensing, sealed combustion furnace. Both are LP but neither will operate without elect, because no air flow, no gas flow, no heat (the furnace would have no blower operating anyway). W/o elect, my well pump doesn't operate so not having hot water that I can't move is not a real probelm.
There are air cleaners that recycle air as oppsoed to excahnging air. But, ventilation (and by that I mean specifically the introduction of outside air) is a good idea for air quality that filtering and cleaning cannot address. In an environment with heavy occupancy loads this is critical and mandated by code. In a residence, not so critical. The issues are dilution of air borne "contaminants" in the space, such CO, CO2, VOCs, off gassing of various materials, cleaners, etc.. The list is long, but again, not overly critical for a house. AprilAire makes an air cleaner, as do many others, but their primary function is the removal of particulates (and smoke) but will not affect gases.
Do you have forced air? If you do, get the best filter you can find. There are residential HEPA filters available as well, but I don't know the cost.
Tim,
I bought the pilot-light Aquastar to avoid electrical dependence; i was thinking to put it in the attic (insulated rafters so it wouldn't freeze) to simplify the chimney install, but i see you have the power-vented one and in the other thread on Aquastars s.o. recommended the power-vented one as well. Should i take this one back and negotiate for a power-vented one? As you say, they don't work well without water and i can live without hot water if necessary for a few days. Heat is more critical bec i'm gone for days at a time, so that's why the non-electrical, Orbis heaters. No forced air to filter; my anti-dusting gene hates that system.
I think i'll tighten the old house up a bit and see how it feels before throwing money at an HRV since i'll be selling in two years. I like the Aprilaire filter idea...wonder if it will remove "dawg"!
Thanks for all the leads to follow.
Colleen
SG,
If you have water supply separate from electricity and and easy means to vent without the power option, I don't see a reason to change. BTW, the electric ignition versions run on batteries that last about a year before change is needed. In my case, a water heater without electrics did me no good and I "needed" the power vent.