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Flash & Batt Dew Point Question

brad17 | Posted in General Questions on

In this great article by Michael Maines, one of the arguments in favor of flash & batt is that:

“If water vapor can’t reach its dew point in the spray foam, condensation will not occur in the wall.” 

If this is the case, where exactly will the dew point be reached in this assembly? I’m not certain I fully understand. 
I ask because we are in a Zone 6 (with a 2×6 wall) with NO exterior insulation and want to pursue flash & batt as an option. My concern was that I would get condensation within the wall or on back of sheathing. Is this not the case as long as I get the ratios of spray foam to batt right?

Thank you!

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  1. Expert Member


    Yes you have it right. With thick enough foam, the dew point will be somewhere in that foam layer, but the water vapour can't get there to condense. The problems with too thin a Layer of foam is the inner-surface is too cold and the condensation occurs on it.

  2. brad17 | | #2

    Thanks, Malcolm, I appreciate you confirming this.

  3. Expert Member
    Michael Maines | | #3

    Brad, thanks for the kind words. That article is old but the concepts are still sound. I always regret the title the editors chose; I would have preferred, "If you insist on using foam, here's why flash-and-batt can be a good option."

    Like Malcolm said, the dewpoint is somewhere in the foam layer, at least most of the time. It might move to the fill layer for short periods of time but the idea is that if the other details are done properly then a bit of wetting shouldn't be a big problem. The more foam you add in proportion to the fill layer, the safer the assembly (and the larger your negative climate impact).

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