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Help me determine my wall’s dew point

user-958765 | Posted in General Questions on

I’ve spent hours researching this site (which is great by the way) to try and determine what the dew point would be in a proposed wall assembly-(flash and fill). I’ve read the blog mentioning Joe Lstiburek’s approach which seems to be based more so off a wall system with exterior insulation board. I also found a table stating in need 2” of closed cell insul. So I’m a little confused.

Here’s my proposed exterior wall system: Zone 6B; interior 5/8” dry wall, (no vapor barrier), 2×6 studs 16” oc, 3 ½” blown in fiberglass insulation, 2” closed cell spray foam insul, ½” plywood sheathing, tyvek drain wrap, cement board siding. (i’m aware of the thermal bridging).

What I’d like to know is this system adequate based off an avg mean temp of 19 deg (dec-feb) to prevent moisture from forming in the wall cavity (assuming a 70 deg 35% RH interior)?

If not; what would the interior max temp and RH be to be safe?


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  1. GBA Editor
    Martin Holladay | | #1

    Your proposed wall assembly (a standard flash-and-batt job with 2 inches of closed-cell foam in 2x6 walls in climate zone 6) will work fine.

    There are several articles on the GBA site that provide the answer to your question:

    In the article, Calculating the Minimum Thickness of Rigid Foam Sheathing, scroll down to the section that begins, "What about flash-and-batt jobs?"

    Another resource is the article by Michael Maines, Why Flash and Batt Makes Sense. In that article, Joe Lstiburek suggests that even 1.5 inch of closed cell spray foam would be adequate in your climate zone. (My own advice is to stick with the 2 inches, because spray foam is often installed in an uneven manner.) The article notes, "According to building scientist Joseph Lstiburek, flash-and-batt installations require at least 1 in. of spray foam in climate zone 5, 1 1⁄2 in. in zone 6, and 2 in. in zone 7." Unfortunately, the article failed to explain whether Joe was talking about 2x4 or 2x6 walls.

    In any case, I advise you to stick with 2 inches of spray foam.

  2. Expert Member
    Dana Dorsett | | #2

    The dew point of 70F 35% RH air is about 41F. (Try it yourself: )

    Assuming you went with 1.8lb Spider or something, the fg runs ~R4/inch, and you're looking at ~R14 of fiber to ~R13 of foam. When the first condensing surface (the interior surface of the foam) is 41F, that's (41F / R14=) 2.93 degrees per R, so through R13 foam it won't reach that temp until it's (2.93 x R13 =) 38F degrees under the 41F dew point, which would be a (41F- 38F=) +3F outdoor temp.

    So, if your mid-January mean temp is above +3F (which it is everywhere in zone 6, or even most of zone 7), you're good. Condensing events will occur, but not of sufficient duration to cause liquid water to build up or even temporarily reduce thermal performance- it'll be a slight haze, like fog on a mirror on the foam surface until it warms back up. Only with substantial air leakage into the wall cavity from the interior would it ever reach any kind of mold hazard or rot condition.

    Use the online psychrometric calculator to figure where the yellow hazard zone is with ever increasing interior RH & temp. The higher you raise the interior temp, the warmer the condensing surface, but the higher the dew point of that air, the higher the condensing temp. If your mid-winter average outdoor temp is 19F you have some margin to play with though.

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