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Community and Q&A

Closed-cell spray foam in conditioned crawl space, sill gaskets, windows, & air sealing

Michael Geoghegan | Posted in General Questions on

We have our house plans back from the designer and are getting ready to start building. I have a few separate questions:

1. Our house will be built on a conditioned crawl space. I spoke with a local subcontractor about it and he said they use a closed cell spray foam to insulate the foundation walls and rim joist. He said they leave a 2-3″ termite inspection gap, but he doesn’t leave a 2-3″ wicking gap at the bottom. Building America ( ) recommends using something at least as permeable as vapor semi impermeable to allow for interior drying. Will the closed cell spray foam be an issue? I wouldn’t think so, but if anyone has any experience I’d be interested to hear it. I’m also curious if anyone has any experience with spray foam in the rim joist and whether or not it causes issues with the termite inspection (I’ve asked my builder to check with whoever he uses for pest control).

2. The EPDM sill gaskets seem to be pretty well regarded around here, but they only make them large enough for a 2×6. Our sill plate will be wider (2×8 or 2×10, can’t remember). Can they be doubled up? Is a “regular” foam sill seal sufficient?

3. Our builder typically uses Anderson windows either 200 or 400 series, but will use whatever we want. It doesn’t look like there is any significant U-Factor difference between the 200 and 400 series, as long as you order them with the same glass. Whatever we get will meet energy star requirements. Are there any manufacturers you would recommend that compare to the 200 series? The 400 series?

4. I’ve read the “One Air Barrier or Two” article here on GBA, but I’m still not clear on this. Is an exterior air barrier better in a hot climate to keep the humid air from reaching the cool interior wall surface? Is an interior air barrier better in a cold climate to keep the warmer indoor air from reaching the cold exterior sheathing? Or does it not matter? If it does, would a mixed climate be in need or two air barriers? Or would an exterior air barrier stop enough air that the interior air won’t really reach the exterior sheathing anyways? Right now I plan on taping the sheathing for an exterior air barrier. I don’t want to have any moisture issues in the walls in the winter time by not having an interior air barrier. I plan on using supply only ventilation so the house would be slightly pressurized, or at least move the neutral pressure plane down.

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  1. GBA Editor
    Martin Holladay | | #1

    Lots of questions. I'll answer them one at a time.

    1. Concerning the use of closed-cell spray foam on the interior side of a concrete foundation wall: This is the best type of spray foam to use in this location. Joe Lstiburek of Building Science Corporation has backed away from his earlier assertion that a below-grade concrete wall needs to be able to dry to the interior. For more information on this issue, see Joe Lstiburek Discusses Basement Insulation and Vapor Retarders.

    For more information on crawl space details, see Building an Unvented Crawl Space.

  2. GBA Editor
    Martin Holladay | | #2

    2. If you really want to do a good job of air sealing, you'll probably want to use more than just foam sill seal. If you install foam sill seal, you should come back later (after the weight of the house has caused the mudsill to settle) and caulk the gap between the mudsill and the concrete on the interior with high quality caulk.

    The same approach can be used with EPDM sill seal -- a little redundancy never hurts. If you center a 5.5-inch-wide EPDM gasket under a 2x8 or 2x10 mudsill, I think you will be fine.

  3. GBA Editor
    Martin Holladay | | #3

    3. There is no answer to the question, "What window is best?"

    My advice:

    (a) Check the window specs carefully (using NFRC ratings for U-factor, SHGC, and in some cases VT).

    (b) Be wary of window distributors who don't understand technical questions about glazing specs.

    (c) Ask around -- choose a window distributor who has a good reputation for customer service, because you may be asking your window rep to solve a problem on the job site.

    (d) Before choosing a window, see it and touch it so you know what you are buying.

  4. GBA Editor
    Martin Holladay | | #4

    Concerning air barriers: In theory, a single air barrier, well installed, should prevent infiltration and exfiltration well enough that you don't have to worry about convective loops and wind washing. Once air leakage is down to a very low level -- say, 1.5 ach50 or lower -- you're good. This can be verified with a blower door (usually before the drywall is installed).

    The most important issues when it comes to air sealing are: (a) Choosing a method that is easy for builders -- and the consensus is, that means taping the exterior sheathing; (b) Paying close attention to areas where dissimilar materials meet, and paying close attention to all penetrations of the air barrier; (c) Having an airtightness goal understood by all workers on the site, and verifying that the goal has been achieved with a blower-door test.

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