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Understanding wall assemblies – vapour retarder confusion

Mill_house | Posted in General Questions on

Climate zone 7
Insulating kneewall – unconditioned unvented attic
If R10 rigid board is installed on the attic side of kneewall (EPS unfaced) with roxul batts in the 2×4 studs of kneewall, why would it be a bad idea to put poly (6mil) over it?  What’s the simple non-technical answer?
I ask because it’s just a given here that poly is put on everything.  

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  1. GBA Editor
    Martin Holladay | | #1

    I'm guessing that you live in Canada, where (as you correctly note) "it's just a given that poly is put on everything."

    Briefly, assemblies with rigid foam on the exterior can't dry out toward the exterior, and for that reason they are designed to dry toward the interior. You don't need interior polyethylene on a wall with exterior foam -- and in fact, the polyethylene can (a) prevent inward drying during the summer, and (b) in some rare cases, become a condensing surface for moisture during the air conditioning season, leading to puddles on your bottom plates.

    A wall with exterior foam should have an interior air barrier (usually, gypsum drywall) but no interior vapor barrier. If an ignorant Canadian building inspector doesn't understand the building science behind this reasoning, you may have to install interior MemBrain to satisfy the building inspector. This MemBrain is unnecessary but harmless.

    For more information on this issue, see "Calculating the Minimum Thickness of Rigid Foam Sheathing."

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