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Our architectural firm normally uses 1-1/2″ Dow SM rigid insulation as an exterior insulating sheathing over 2X6 stud walls filled with batt insulation in order to beaf-up the exterior wall R-Value. One of our contractors is concerned that putting SM on the exterior side of the wall and using a vapour barrier on the interior side of the wall will create a moisture trap. Is this true?
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Rob:
Yours is a question for Fred or Gene to answer, but in the mean time let me ask you this: What is the perm rating of the insulation sheathing board that you refer to?
*I am no expert, and I await Gene and Fred's answer. But as has been pointed out here before (I believe by Gene) the perm rating of plywood and OSB put it squarely into the "barrier" category. Oh yeah, then this brings up Fred's point that poly doesn't allow moisture transmission back into the house. This does sound like a moisture trap. But is the problem the foam or the poly? How effectivly is the foam sealed?-Rob
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Here's an ivory tower view. For most of the US, insulating sheathing makes for good moisture performance in cavities. Poly on the inside helps up north, hurts down south, and probably is neither very useful nor harmful in the big middle of the country.
The word "trap" is the killer here. We picture a building assembly where it is easy for water to get in and hard for it to get out. One "trap" that is common is letting rainwater in with no provision for drainage, like at a leaky window or a wall without overhangs. That meets the condition of easy-in, hard-out. Another "trap" would be a Gulf coast impermeable interior finish building under negative pressure.
But the two common natural moisture transfer processes of convection and diffusion move a cavity toward moisture equilibrium, not toward moisture accumulation. The same process that may lead to accumulation leads to correction. You can design a wall for easy-in, easy-out (no poly, fiberboard sheathing) or hard-in, hard-out (poly, foil-faced sheathing) but in either case, and it's not a "trap." I suggest that anyone who is tempted to say "trap" in a moisture discussion, take a few minutes (well, hours) and download and run the modeling program MOIST from the NIST website. Run the numbers. Moisture contents of materials will go up and down seasonally and smoothly without the trap jaws clamping shut. What we can learn from modeling is that most building wall assemblies using insulating sheathing work fine in most of the US, with or without poly. As long as the rain is kept out. Otherwise, SNAP!
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Rob. Plywood is a sometimes vapor barrier (sic). Doug Burch, now retired, of the National Institute of Standards and Technology(NIST) has done extensive research into the permeability of various building materials. He found, for example, that the permeability of plywood and other materials is relative humidity (rh) dependent.
Exterior grade plywood has a permeance of 1 perm when the rh is 25%. This qualifies it as a vapor diffusion retarder--1 perm or less.
However, when the rh reaches 90% the permeance increases 25 times: now plywood's perm rating is 25! and is no longer a VDR.
Locating rigid foam board insulation (RFBI) on the exterior of an exterior wall, raises the wall cavity temperature and limits potential condensation. Any penetrating moisture no longer sees a cold condensing saurface--typically the plywood or OSB sheathing. Also see Bill Rose's cpmments.
Let's welcome Bill Rose--University of Illinois--to Breaktime. GeneL.
*With 1.5" SM the wall cavity will be warm enough to prevent condensation in most cases. If it was only .5" then I would make sure that the poly is very well sealed to reduce the amount of moisture that gets into the cavity.
*Bill, I think the second to the last sentence "As long as the rain is kept out" is what has a lot of people worried and thus the word "trap" haunts us. Condensation based on values of temperature, permeability, etc can be calculated on a modeling program, but what about the one drop/per hour dripping at a nail penetration into a sealed wall? It's visions like this which make me shy away from exterior insul like dryvit.
*SM or Type III polystyrene has a very low perm rate (can't find my file with the exact #) -- much less than OSB -- If all the joints are taped and sealed (between sheets and at headers, frames; etc.) It is a VDR and ADR when fully sealed.With the SM and the interior wall's VDR we all know you are putting the house in a plastic sandwich which ain't good for anything: natural moisture in the framing members and any moisture accumulated in the insualtion upon installation will have no place to go.I like foam board as the sheathing when its structurally acceptable -- no ply or OSB. If you cross strap the studs and put the foam board horizontally, you have a place to atach exterior finish and save the cost of sheathing. Give me foam board for a non-structural sheathing any day -- little to no frosting/condensation so the insualtion and framing members are better protected. Foam is also a superior "bulk water drain plain" if any gets past your exterior finish.Often we see overbuilding -- structural sheathing in a location where its not needed, followed by SM foam board and topped off with Tyvek/Typar and then a structurally rated outside cover (like clapboards). Over-built, prone to moisture problems and much more expensive than it has to be.BUT WHY USE SM. With a 25% to 50% increase in the thickness of Type II or Type III foam board insulation, respectively, you can get the same level of performance (R value and low perm) at 1/2 to 1/3 the cost, respectively. But I guss you American are obsessed with SM -- DOW has done a great job of misinforming the marketplace.
*According to an old FHB article the perm rating of foam insulations were as follows: 1" urethane or styrofoam = 1.0, 1 " beadboard = 4.0.The perm rating of 1/2" plywood is listed as 0.5. It doesn't list OSB but it must be similiar. (6mil poly is 0.06)Back to the original question, the sheathing appears to be more of a vapor barrier than the foam insulation which should ease any concern.
*If that perm rating info is right, I will eat a sheet of SM. No one would be using foam board on roofs or exterior walls or for basements. Does anyone have the real perm ratings handy ... mine are in a file at another location from my PC.tedd
*I just looked up one manufacturer's perm rating and it ranges from 0.20 to 0.80. I might need some salt and pepper, depending on the Type.
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Building science related to moisture is very difficult, for reasons that are evident in your message. On the one hand there are ivory-tower researchers like myself who have historically made preposterous simplifying assumptions in order to get to where we can crank some numbers. On the other hand is the voodoo of moisture--it "haunts" us, and "condensation" is a pernicious invisible bogeyman.
I think we need to know where our discourse about moisture is actually scientific, and where it is protoscientific. Both streams of discourse will be with us for years to come until we sort this out.
Meanwhile, permeance measurements of building materials makes for a rather useless scholastic squabblefest. A decade or so ago, various research laboratories participated in a round-robin to measure using wet-cup and dry-cup the permeance of samples cut from the exact same piece. The results were wildly out of whack. To many of us, this was reassuring. The basic metric unit for permeance is nanograms per sq. meter-second-Pascal. How big is a nanogram? A nanogram is to a gram (a sugar cube, say) as a byte is to a gigabyte. The English units are grains per sf-hr-inch of mercury. How big is a grain? The unit "grain" arose in apothecary shops to measure--you guessed it--grains of wheat.
There are two different meanings for the term "condensation"--one is the formation of drops of moisture on a surface, the other is the crossing of lines on one of those charts. These two different meanings have little to do with one another. Imagine on a hot afternoon pulling a cold beer from the fridge. Condensation. Now pull a brick from the fridge. No condensation. Same lines on the chart. What's happening? Science these days has to undo some of the damage done by science back in the 40s.
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We're going to hold you to that, you know.
*You guys are all yapping, but no one is answering the question. or maybe you asnwered it, and it was so technical that a carpenter (me)couldn't understand it.Is putting foam over osb ok?blue
*OK, I'll say yes -- by itself. (what the heck) However, insulation / OSB / barrier sounds like trouble because the OSB could easily be enough below room temp to cause condensation. The "vapor barrier on the wrong side" problem for heating climates (you're in Canada?). I wouldn't count on the poly to keep the moisture at bay, but once it condenses the poly will help keep it around in little pockets of rotting wood.The more important precaution is whether the wall is AIR sealed. If the foam and poly and housewrap are installed as usual, "no," and that means the fg probably doesn't have all that much effect either. So the foam comes out OK.Welcome back B.E.D. Early release?
*You guys have me all confused and worried.I'm renovating a 1942 home.I spoke to people around here(Quebec,Canada)about increasing the R value in my exterior walls(2x4).Everyone says use 1.5" rigid Styrofoam and Tyvek.From inside it's like this...Drywall,6 mil poly,batting,10"x7/8" plank sheathing(existing),1.5" styro,Tyvek,1x3 ferring,shakes.Now is this bad or O.K?Should I pull off the Tyvek(shakes are not on yet).Why do Tyvek instructions say to put over sheathing or over insulation?What about air infiltration if I don't use Tyvek?Gaby
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I'm looking for a simple answer too. I am renovating a 1900 balloon frame stucco house in Ottawa (very cold winters, hot, humid summers). I have used a bondex paint-on product to seal my partially underground concrete basement walls. My plan is to only use a 6mm poly as vapour barrier over the framing and the Roxual (sp?). Some poeple have suggested also using a tar paper type product against the wall. Is this bad advice? Should I just stick with the poly? The basement is dry but can be humid in summer.
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Our architectural firm normally uses 1-1/2" Dow SM rigid insulation as an exterior insulating sheathing over 2X6 stud walls filled with batt insulation in order to beaf-up the exterior wall R-Value. One of our contractors is concerned that putting SM on the exterior side of the wall and using a vapour barrier on the interior side of the wall will create a moisture trap. Is this true?
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Imagine what will happen to your framing if the vapor barrier works as intended and traps moisture behind it. The fungi will like it, but it might kill you.
The humidity of the basement may or may not be due to moisture released by the masonry. Outside air entering the basement and cooling could account for the humidity -- maybe.