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Community and Q&A

Sealing and insulating “crawl space”

J. Bishop | Posted in Green Building Techniques on

I live in northern Canada. I’m building a “super green” home with R58 double stud walls and R80 ceiling. No basement, first floor on grade. Ground level floor joists will rest on the footing with about an 8″ space between the ground surface and the bottom of the joist. I am looking for the most practical and effective technique to (a) create a radon/soil gas barrier and (b) insulate in the “crawl space” to a min R10 and possibly higher around the perimeter. The current plan is to use 2″ of 2lb spray foam on the ground and build up to 4″ around perimeter. This approach should be effective for insulating but I’m less clear whether the spray foam alone will be an adequate radon barrier. Locally some builders and “radon mitigation experts” are recommending first installing a sealed 6 mil poly layer, then applying the 2″ 2 lb spray foam on top of the poly. Others are suggesting the spray foam won’t adhere well to the poly and recommend spray foam alone. I’d really like to get the radon barrier right and not have to do post-construction mitigation (although I will plumb in below grade a 4″ perforated PVC pipe that can be connected to a radon fan later if required). Appreciate your comments, advice for sealing and insulating the crawl space from grade. jim

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  1. GBA Editor
    Martin Holladay | | #1

    J. Bishop,
    1. Why build a crawl space that is only 8 inches high? I suggest you consider a crawl space with at least 3 feet or 4 feet of clearance between the soil and the floor joists above.

    2. I don't know if you are building on permafrost or not, but do you anticipate that your crawl space floor will ever freeze? As long as the floor doesn't freeze, 2 inches of spray foam should be an air barrier. But it is relatively fragile and subject to physical abuse, so it would be best to cover the spray foam with at least 3 inches of concrete.

    3. The conventional approach would be to install 4 to 6 inches of crushed stone, then at least 4 inches of XPS or EPS rigid foam, and then a taped layer of poly (at least 6 mil thick) as a vapor barrier and radon barrier, followed by 3 to 4 inches of concrete. Any reason you don't want to do it that way?

  2. Robert Hronek | | #2

    I would say Martin is right. I would say build a 3-4 foot crawl space or the other option is an insulated slab.

    I do not really like the idea of spray foam under the slab. Rigid foam will be easier to work with and you will know the thickness will be consistent. Rigid foam will also be cheaper than spray foam. A layer of poly between the rigid foam and the concrete will provide the vapor/air/radon barrier. I don't think taping the seams would be necessary and small punctures are not a worry either.

  3. Bill Bradbury | | #3

    I agree with Martin that a deeper crawl space is better, as I remodel houses and the 8" crawls are no fun. I don't see any need for the extra expense of a concrete floor in the crawl. We retrofit vented crawl spaces with; Poly, XPS up the wall a foot or two and spray foam the walls. Extra sheet goods like plywood are put in the crawl as cat walks instead of thrown away.

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