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Community and Q&A

Flash and Batt – what type of drywall?

user-5594283 | Posted in Energy Efficiency and Durability on

Hi Martin:

I have just completed a Flash and Batt insulation in the basement of our home:
installed in 16″ 2×8 bays – 2″ Dow Corning Foamular and Roxul R22. All seams, foamed, caulked and taped.

Can I use regular drywall on top of this or do I have to use a special Paperless Drywall – I was under the impression that Regular Drywall is actually a vapor barrier and would have the same effect of retaining moisture as 6 mil poly???

Mark Kozlowski
Crawford Bay
British Columbia

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  1. GBA Editor
    Martin Holladay | | #1

    Regular drywall isn't a vapor barrier. On the contrary, it is very vapor-permeable.

    If your basement isn't subject to water entry or the risk of flooding, ordinary drywall can work. If you are worried about possible water entry or dampness, paperless drywall is a safer option (because it is less likely to support the growth of mold under damp conditions than ordinary drywall).

  2. Expert Member
    Dana Dorsett | | #2

    Standard drywall is NOT a vapor barrier, or even a significant vapor retarder. Standard interior latex paint brings the vapor permeance down to about 3-5 perms, which is still a fairly vapor open Class-III vapor retarder. Paperless drywall would not change that.

    Crawford Bay's climate is essentially a US zone 6B type of climate, and 2" of XPS would provide sufficient dew point control to keep the R22 fluff dry, but some Canadian inspectors would interpret NBC in such a way to demand an interior side vapor barrier. If you must, half-perm "vapor barrier" latex would work, but 2-mil nylon sheeting (Certainteed MemBrain) would be better, since if the cavities ever became damp enough to support mold the nylon becomes vapor open, allowing it to dry at 10x the rate it would through vapor barrier latex.

    Is the XPS on the exterior side of the 2x8 stud edges(hopefully yes), or do you have a ~2" air gap between the fluff and the wallboard (hopefully not, but one of those conditions must be true)?

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