Shoving loose fitting foam board into 3/4″ air cavity
I have uninsulated CMU walls with stucco exterior, 3/4″ furring + 1/2 drywall interior. The 3/4″ air cavities are open to the vented attic. I plan to seal the cavities with canned foam. Is there any risk to shoving 1/2″ foil faced polyiso into the cavities first? Climate zone 4a.
I figure the walls are R4 now with R1 from the air cavity. The 1/2″ polyiso+ 1/4″ air gap should be ≥R3. It’s ugly, but R4 to R6 is the same savings as R10 to R60. So why not do it until I have a good excuse to rip off drywall and install proper insulation?
I’m assuming convective heat loss won’t be a problem once the cavities are sealed top and bottom, so the very loose fitting polyiso will still be effective. Are there fire or moisture risks?
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Your existing walls are probably no more than R-2, so doubling the R-value will cut heat loss or gain by half. It won't do much if it's loose-fitting though. For fire safety it's far better (and required on new construction) to seal any transitions from vertical spaces to horizontal spaces, though foam is not an approved material for that situation. I can't think of any moisture risks, unless perhaps your wall absorbs a lot of water that vents into your roof before reaching the interior.
Thanks for the advice! I'll look for a better fire blocking material.
I've seen a table of effective R values for well-sealed air gaps showing ~R3 with radiant barriers. That suggests to me that convection is fairly small for a 3/4" space.
@Michael Maines: "For fire safety it's far better (and required on new construction) to seal any transitions from vertical spaces to horizontal spaces, though foam is not an approved material for that situation."
Trying to make sure I understand this. Don't they allow small gaps (probably a max size) to be sealed with fire safing foam? Could the OP do that after sliding in the 1/2" polyiso into the gap?
(although admittedly I'm unsure how there is sufficient space to get the 1/2" polyiso into the gap from the unvented attic, unless the OP plans to only do this from the taller parts of the gable end, or to do a tremendous amount of cut & cobble...)
Well, I gave it a try and it was totally impractical. I can score the insulation every 8-12" to make the angle work, but the gap is too irregular, the furring strips don't always line up with the rafters, and a single stray nail blocks it.
I could pour in perlite or similar, but I don't think I want a capillary bridge from CMU to drywall.
My only other idea is to drill through the top plates to fill the cores with perlite. But that only seems worthwhile if I have medium weight or light weight CMU. The gain is so small for heavy weight CMU, and I have no way to tell. I'm also having trouble finding perlite in stock locally so I can trial and see if it's impossibly labor intensive or just very labor intensive.
Now I need excuses to replace drywall.
Foam is only allowed by the IRC if it meets ASTM E136 requirements. See R302.11 and R302.11.1: https://codes.iccsafe.org/content/IRC2018/chapter-3-building-planning#IRC2018_Pt03_Ch03_SecR302.11.