Poly or MemBrain?
In Maine, zone 6, zip code 04240. Finishing a large bonus room over a garage.
2×6 wall construction: vinyl siding, house wrap, osb, roxul; all in place.
Finishing the inside wall is the next step. T&G boards will be used, but have not yet decided if drywall will be installed first.
Need to decide on correct air barrier and/or vapor barrier. 6mil poly is the usual method, but with summertime AC in use, I am thinking this may not be the best choice. Will Certainteed MemBrain be a better choice for this application? Or should I just use the poly and not worry about it.
For a second related question, one wall of this room abuts the gable wall of the house, and there is an attic on the other side of the wall. It’s extra hot in summer, and just plain real cold in winter. Should this wall be handled as any other exterior wall, or is there a special concern?
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Replies
Tony,
The wall between conditioned space and an unconditioned attic should be detailed like any exterior wall.
MemBrain makes more sense than polyethylene for any wall in an air-conditioned house.
Although the manufacturer of MemBrain claims that MemBrain can be detailed as an air barrier, MemBrain is thin and fairly fragile. I strongly urge you to install drywall (with taped seams, of course) as an air barrier before you install your tongue-and-groove boards. Tongue-and-groove boards do not prevent air leakage. Remember as well that your electrical receptacles are potential air leakage points; ideally, you will specify airtight electrical boxes.
If drywall is used before T&G boards. And it is taped and primed and painted. And air-tight electrical boxes are used. Is it still recommended to use the MemBrain, or it a wasted expense? ie. does the drywall/paint then perform as both an air and vapor barrier?
Tony,
Building codes require an interior vapor retarder -- not a vapor barrier -- for walls in your climate zone.
Options for the vapor retarder layer include (a) the kraft facing on fiberglass batts -- not applicable in your case -- or (b) vapor-retarder paint, or (c) MemBrain. Take your pick.
If you tape the drywall, note that the tape doesn't have to be sanded. All you need is so-called "fire taping." In this type of wall, there is no reason to paint the drywall unless you intend to install vapor-retarder paint and no MemBrain.