Interior foam board vapor barrier help
Supplyz
| Posted in Green Building Techniques on
Trying to use interior foam board between sheetrock and fiberglass batt for bathroom ceiling renovation. Rafters only 3×4, 24+” spacing. Plan on using 2×2 furring strips over 3×4’s for sheetrock. Want to use 1.5″ foam board between furring strips. Dow recommended to use fiberglass batts WITH vapor barrier, and use unfoiled foam board between fiberglass/vaporbarrier and sheetrock, saying it would not be double vapor barrier as long as foam board is against vapor barrier. I’m not too comfortable with vapor barrier in middle of wall. Bath is high humidity. Any suggestions? Thank you
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Replies
G.J.,
First of all, in most cases it's not a good idea to give GBA readers any recommendations on the use of an interior vapor retarder or vapor barrier unless we know what climate zone we are talking about. Where is this house?
In your case, however, I feel fairly confident in asserting that your 1.5 inch thick rigid foam is a perfectly adequate vapor retarder. You don't need any interior polyethylene.
In all cases, it's important to pay attention to airtightness with this kind of assembly, especially since you have all those cracks between the rigid foam and the furring strips.
U.S. building codes do not require the use of an interior vapor barrier. In some climate zones, but not all, a vapor retarder -- a less stringent barrier -- is required. For more information on this issue, see Do I Need a Vapor Retarder?
Finally, if you live somewhere like interior Alaska, where a polyethylene vapor barrier is often used, you can install it adjacent to rigid foam without any worries about whether these two adjacent layers cause a "double vapor barrier." If, for example, you were to install a layer of foil-faced polyiso, you already have a vapor barrier. Adding a layer of polyethylene adjacent to the vapor barrier -- on either side -- doesn't change the vapor permeance of the assembly in any way, so it does no harm. There was never going to be any vapor moving through the polyiso by diffusion in any case.
Sorry, forgot to say northeast, metro NY...
I was originally going to use unfaced fiberglass with foilfaced foamboard, with foil tape over 2x2's for continuous vapor barrier directly under sheetrock. The local 1.5" isn't foilfaced, and Dow told me it isn't a vapor barrier, just 'retards' vapor, whatever that means... they recommended using vapor barrier fiberglass batt. I'm just afraid the foam will end up effectively making double barrier
G.J.,
I hope you get a chance to read my first answer.
1. The "double vapor barrier" issue is groundless. When two vapor-retarding layers are adjacent, they don't interfere with each other in any way.
2. You do not need a vapor barrier. It is not required by code. Nor it is required from a building science perspective.
3. Use whatever type of rigid foam you want, and stop worrying.