6mil poly location in ceiling
Another forum reader and I were discussing a detail last night and he suggested I open it up to the board, so have at it.
Between a recent bump of Thorsten Chlupp’s JLC article and a recent thread on avoiding adhesive between plywood sheathing and wall studs, I’ve come to the conclusion that for the small building that’s underway in my yard, I’ll see good airsealing results from 6mil poly, adhered at edges and openings, around the walls and another layer lapped over, across the ceiling.
The walls seem to fall within the rule of thumb: inside to out, they’re R19 batts inside 2×6 studs, plywood sheathing, 6mil poly air barrier, 5 1/2″ of layered-and-taped polyiso, rainscreen cladding. (R19 fiberglass, 6mil poly, R35.75 polyiso.)
The question is at the ceiling. To avoid running the 6mil poly in a manner that forms the shape of a wading pool between layers, the assembly would be, inside to out: R19 batts, 4″ of polyiso, then 6mil poly, and 5″ more of layered-and-taped polyiso. (R19 fiberglass, R26 polyiso, 6mil poly, R32.5 polyiso.)
There will be an ERV.
Condensation problem in the ceiling? Or, am I in the safe zone?
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