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Community and Q&A

Vapor barrier at interior high humidity locations?

RMaglad | Posted in General Questions on

Location: Ottawa, Ontario
Wall is 12″ thick R44:
2″ R8 EPS, zip system sheathing, 2×6 c/w R22 Rockwool, Intello+ interior smart vapour retarder, 2×4 c/w R14 Rockwool, interior finishing as specd

It is a limited vapour open wall in both directions.

I have a bathtub shower combo (acrylic base and acrylic surround), and a shower (acrylic base and tile over waterproof gypsum) against the exterior wall.

Given that these bathrooms will see elevated humidity should i add some 6 mil poly on the inside of the studs, between stud and the acrylic surround/waterpoof drywall?  In these 2 areas of the house the concentrated vapour/humidity will not be able to cross the poly?

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  1. GBA Editor
    Martin Holladay | | #1

    This is a judgment call. If I were you, I would probably use interior polyethylene at the shower.

  2. Jon_R | | #2

    My take is that an exhaust fan will create negative pressure and stop humid air/water ex-filtration (which can move a lot of moisture quickly). For the short (I assume) time that it's super humid, not much diffusion will occur - without poly, it can dry inward over many hours of normal humidity.

  3. RMaglad | | #3

    Jon, I am using a balanced erv, so it wont have the moisture moving potential of a dedicated direct exhaust fan.

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