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Flash-and-Batt Insulation and Condensation

brad17 | Posted in General Questions on

Dear GBA Team
For a new house in Maine we are considering a 2×6 flash & batt wall for budgetary reasons. Are there any concerns about interior wall condensation in this approach since it omits continuous exterior insulation?

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  1. Expert Member
    Michael Maines | | #1

    Brad, here's an article I wrote a long time ago but the principles still apply: You do need to worry about condensation, but if the foam and closed-cell and at least 35% of the overall R-value, using a conservative, aged value of R-5.6/in for the foam, and you have painted drywall at the interior, and you keep indoor relative humidity under 50% or so, you should be safe from condensation. (35% is for climate zone 6, which includes most of Maine. Other climate zones have different minimum ratios. It's always safer to have more R-value in the foam layer, but that's also more expensive and creates more carbon emissions.)

    That's a lot of "if's" and the reason why my article title was changed from its original, "Flash-and-batt vs. cellulose." Cellulose was the clear winner in every category so the editors dropped that part of the article and focused on how to think about flash-and-batt. The "makes sense" part is compared to using all foam insulation.

  2. brad17 | | #2

    Michael, thank you! I've read through the article, which is great, and have circled back through our details. This is super helpful advice and we appreciate you taking the time to reply!

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