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Community and Q&A

CertainTeed MemBrain

fall50 | Posted in General Questions on

Can the MemBrain go over kraft-faced fiberglass? Would this be a double vapor barrier? My wall assembly is aluminum siding, a shiny like paper under the aluminum, 30-lb felt, and then shiplap sheathing over 2×4’s . Zone 6A

All the major box stores only carry R-15 Kraft faced FG no unfaced.

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  1. STEPHEN SHEEHY | | #1

    Is this a new build?

    If you want unfaced fiberglass, check with local lumber yards. You may find them a better source than the big boxes, often with better prices, better quality and free delivery.

  2. Dana1 | | #2

    Neither the kraft facer nor MemBrain is true vapor retarder. Both are "smart" vapor retarders that become substantially more vapor open when the proximate air inside the cavity is at a high enough relativity to support mold growth. When the proximate cavity air at the facers/MemBrain is at a low RH (which happens when the sheathing is cold, adsorbing much of the moisture from the entrained air in the fiberglass) an asphalted kraft facer runs about a half-perm, as does the MemBrain, which slows the rate at which moisture diffuses from the conditioned space into the cavity to very low levels.

    But when the sheathing is much warmer, releasing it's moisture, both the kraft facer & MemBrain become vapor open (more than 5 perms, more vapor open than standard interior latex paint), and do not appreciably slow the rate of drying toward the interior.

    Thus, installing MemBrain as a continuous air barrier over a layer of kraft facers is just fine, not a problem, since it cannot trap moisture.

    The exterior side aluminum wrap may or may not be a vapor barrier. Some such products have micro-perforations to keep them vapor open. If it has been installed for decades and the sheathing hasn't developed moisture/mold/rot problems, it won't be a problem to insulate using an air-tight interior with smart vapor retarder approach.

  3. GBA Editor
    Martin Holladay | | #3

    I agree with Dana Dorsett that MemBrain plus kraft facing is not a problem.

    I assume that we are talking about an older house. If this were my house, I would investigate the layer you describe as "a shiny like paper." If that layer is a vapor-impermeable radiant barrier, this isn't a good assembly.

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