Closed-cell spray foam in conditioned crawl space, sill gaskets, windows, & air sealing
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 ( http://www.buildingscience.com/documents/information-sheets/crawlspace-insulation ) 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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