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Community and Q&A

Conditioned crawlspace question

DavidLLarsen | Posted in Energy Efficiency and Durability on

I’m building a house in Asheville NC and my insulation contractor has told me that in order for my house to pass the res check, I’ll need insulation In the crawl space ceiling (cold floor) which is fine, however, I’m doing a conditioned crawl with r10 on the walls and wasn’t planning on having any insulation in the joist bays in the crawl. Do I really need insulation in both the floor and walls?

as a side note, it’s 3 stories with 2×6 walls with r19 batts and we’re spraying 1 1/2” of closed cell with 7 1/2” open cell on top for a total of r40


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  1. Expert Member
    Michael Maines | | #1

    Are you under the IRC for building codes? If so, your contractor is wrong--you can either insulate the walls OR the ceiling; you don't have to do both. You're in climate zone 4a, so you need at least R-10 on the crawlspace walls:

    Because the blowing agents in XPS foam (the blue, pink or green type) are some of the most potent greenhouse gasses in any construction material, it's much better to either find recycled insulation or to use insulation with lower up-front carbon emissions, such as EPS or polyiso. On a cost per actual R-value they don't cost any more than XPS. (XPS ages to R-4.2/in, not the R-5/in advertised.)

    1. DavidLLarsen | | #2

      Your referring to ridged foam for the crawlspace walls, correct? I may end up doing it myself (I’m the contractor building the house) as I haven’t been too impressed with the guys showing up to quote it.

      1. brad_rh | | #5

        Have you looked at Fox blocks for the foundation walls?

  2. Expert Member
    Peter Engle | | #3

    For the rigid foam on the walls, I generally specify Dow Thermax insulation. It is polyiso with a foil facing that allows it to qualify for exposed use without a fire barrier. It is a bit pricey and generally a special order at the local lumber yard, but not needing a separate thermal or ignition barrier layer generally makes it come out about even in final cost, with one less thing to install.

    If you can find recycled insulation locally, it's probably worth using that and adding a fire barrier.

  3. DavidLLarsen | | #4

    Thanks for all the helpful responses!

    Has anyone used 1”- 1 1/2” of closed cell with R30 cathedral batts in a vaulted ceiling before? One of the insulation contractors suggested that as a way to increase the R value but reducing some of the costs over a combo of open and closed cell

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