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Flash-and-Batt Walls in Cold Climate

brad17 | Posted in General Questions on

Hey everyone,
We are building a house in Suffolk County, NY, in climate zone 4A. Contractor is wanting to do a flash & batt wall. We are being asked to use the 2018 code, in which I only need to get to R20 in the cavity, but will we have a wet wall without any exterior continuous insulation here?

Thank you!

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  1. Expert Member
    PETER G ENGLE PE | | #1

    Not having continuous insulation does not mean that you will automatically have a wet wall. Continuous insulation can help to prevent wet walls, but it can also hurt. The biggest risk for a flash&batt system is having too little of the "flash" component of SPF. If the SPF is too thin, the inner surface of the foam can drop below the dewpoint of the interior air. If the interior walls are not airtight, moisture from interior air can condense on the foam that can soak the wall over the winter and lead to mold blooms when the wall warms up. IN CZ 4, the SPF should have more than about 1/3 of the R-value of the cavity insulation. Since the IRC also requires R-20 total in a wall without continuous insulation, this means that there must be at least R5 worth of SPF, and R15 of fluffy cavity insulation. You can get there with a 2x6 wall with 1" of SPF (about R6) and a standard 2x6 FG batt compressed into the remaining 4-1/2" cavity (about R16). With good installation of the insulation products and an interior Class II vapor retarder, this is considered a "safe" wall system from a moisture and condensation standpoint.

  2. brad17 | | #2

    Thanks Peter, I appreciate your feedback! Do you have any personal preferences for brand or type of Class II retarder?

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