Interior air barrier with exterior rigid foam
I’m considering including a service cavity wall on the interior side of the exterior walls in my house construction plans. I’m doing this to create a more durable interior air barrier that will not be compromised by electrical boxes or other drywall penetrations. Seems easier than the ADA method. However, I’m concerned that I might be compromising the wall’s ability to dry to the interior as is necessary with exterior rigid foam, which I plan to include. I’m in far northwest Arkansas, on the southern end of climate zone 4A, mixed humid.
From exterior to interior, the wall assembly will consist of siding, rainscreen furring, 1-1.5″ rigid foam (depending on what’s available reclaimed), housewrap, 0.5″ plywood – taped/caulked, 2×6 studs with mineral wool batts, OSB (for the air barrier, taped/caulked), 2×3 studs for uninsulated service cavity, and then drywall with latex paint (I know to avoid polyethylene and vinyl wallpaper).
Based on permeance numbers online, I calculated the combined permeance of everything on the interior side of the plywood sheathing (my other air barrier) and came up with ~1.5 perms (I’m not a building scientist, so this could be way off).
Am I asking for moisture problems with the multiple interior layers (OSB and drywall)? I prefer OSB over smart vapor barrier because it is more durable, but would you recommend a smart barrier over OSB? Would I need to include some kind of ventilation to the interior for the service cavity through the drywall? If so, how?
Thanks for your comments!
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Replies
The half inch OSB you're detailing as an air barrier is also a "smart" vapor retarder. It's less than 1 perm when the proximate air is dry, but will be over 5 perms (more vapor open than interior latex paint on wallboard) if the insulated cavity ever reaches serious mold hazard levels.
https://buildingscience.com/sites/default/files/styles/panopoly_image_original/public/public/figure_6.jpg?itok=-c4omakd
OSB works well as a variable permeance air barrier even in extra-cold climates and at PassiveHouse R-levels.
KS,
Why not insulate the service cavity too?
Dana, great info! The online tables always describe material permeance as a static value. Interesting that plywood is much "smarter" than OSB at higher humidity, with OSB's reputation as a "leaky" material, in terms of air. Given that fact, would plywood be a better material to use for my interior air barrier, as it would better allow the wall to dry to the interior?
Malcolm, I might end up doing that. I already achieve my R25 goal without, but more insulation is always better.