I have a PWF basement wall that is part below ground and part above ground. The below ground part has bituthene/foamboard on the outside (impermeable) The above ground part has regular siding w/typar. I want to insulate the interior with R-21 batts and a VB but am concerned that the wall will not be able to “breathe”, especially the part with the bituthene on the exterior. Any thoughts on the right way to deal with this situation?
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use unfaced
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Where r u ?
The good answer depends on your climate.
j
Southeast Alaska....Low winter temp is about -15, typically 0-10 in winter. Dewpoint is a definite consideration.
Joe Lstiburek recommends rigid foam on the outside with a continuous membrane top to bottom on a PWF wall, unfaced batts between studs and semi-permeable latex paint on drywall for the inside. In other words, to the extent it breathes, it breathes in. In my area, which is colder than you but probably not as humid, the practice that I see is blueskin or similar membrane top to footer on the outside, poly vapor barrier top to bottom on the inside. I know some builders who don't seal the vb to the bottom plate or floor so that in the event that there is a condensation problem in the wall it at least has somewhere to exit. In the bigger picture, I think this is one reason why people avoid PWF. That doesn't help you, though. j
Thanks for your comments. Last winter I put a couple scraps of non-faced batts in the wall bays to see what happened. No surprise, but it iced up at the wall surface when it got cold. I wonder if putting sheetrock over the insulation would have any different outcome?
By the way, what is "blueskin"---an air channel?
By the way, what is "blueskin"---an air channel?No, it's a peel and stick waterproofing membrane similar to ice & water like you would use on the roof, except no granules on it. If you have access to the outside of your basement, even from the siding down to 12" below grade or something like that, it's great to put a couple of inches of rigid foam on there. You can side over it or stucco it or whatever and then insulate on the inside with better results. What's your groundwater situation like and the damp-proofing on the outside of the basement and a perimeter drain? Is the sheathing sometimes wet or frosty? If so, the inside of an insulated wall will never really dry and you'll have problems one right after the other. I don't know any other way to deal with that than to excavate and seal the thing up before you think about the inside. j
what's a PWF?
what's a PWF?
"Pressure-treated Wood Foundation" unless I miss my guess.
P-T plywood over PT wood stud framing, sometimes one side only, sometimes double-faced to make a stress-ed skin sort of structure.Occupational hazard of my occupation not being around (sorry Bubba)
Why not used foamboard in the below-grade area, and FG batts above? You won't ever have to worry about breathing bec there's nothing below grade to mold, is there?
I filled the below-grade portion of my wood foundation with FB in case the waterproofing membrane - i used Dorken plastic dimple fabric - ever failed. It hasn't, the foam was cheap insurance. I drywalled the basement with no interior VB and haven't had any mold issues at all.