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Community and Q&A

Open + closed cell insultation in crawlspace

| Posted in GBA Pro Help on

I recently hired an insulation contractor to remove fiberglass batts from the subfloor in my crawlspace and replace with 3in of closed cell spray foam insulation on the foundation walls and subfloor, close 2 vents, add a 10mil vapor barrier and dehumidifier.  The crawlspace is is northern California, climate zone 3, has a concrete rat slab over 90% of the space with a dirt floor on the remaining 10%.

I’m in the middle of the project and have discovered that the contractor has installed 2in of open cell with a thin layer of closed cell over the top.  His explanation for the change was that the wall was too cold for the closed cell foam to stick without first applying open cell.  It was 50 F when he started work and 40 F the night before.

While he’s agreed to remedy the work and install the correct thickness of closed cell foam I’m concerned that having a layer of open cell foam on the subfloor with a layer of closed cell over the top of the open cell will create a scenario where moisture could become a problem.

I have 3 questions:
1) Is the 40-50 F degree temperature a valid explanation for spraying an initial layer of open cell foam?
2) Is open cell with closed cell on top a standard or typical practice for subfloor and foundation wall insulation?
3) Will this hybrid of foam types trap moisture against the subfloor and increase the risk of mold or rot?


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  1. Jon_M | | #1

    picture of work in progress

  2. frasca | | #2

    Quick question - I may have misread your post, but are you trying to insulate BOTH the subfloor (isolating the conditioned living space above from the crawl space) AND the crawlspace’s exterior/foundation wall (isolating the crawl space itself from the outside air?

  3. Expert Member
    Michael Maines | | #3

    I'm not a spray foam installer but from what I understand, with most types of closed cell foam the surfaces need to be at least 50°F or the foam won't cure properly. I've sprayed canned foam onto cold surfaces many times and watched it melt instead of expand (you'd think I'd learn) and though commercial 2-part foam has a strong exothermic reaction and the 50° limit is probably conservative, I wouldn't say your installer is lying about it.

    It is concerning that they would bait-and-switch on you, though; you contracted for about R-18 insulation and got about R-14, assuming 1" of closed cell foam. I'm not sure what California's building code requires but if it follows the IRC for climate zone 3 you should have at least R-19 in floors. R-18 is close enough but R-14 is not.

    I would not worry about moisture problems. There is typically not a lot of vapor drive through floors, and 1" of closed cell will allow vapor movement so the assembly can dry to the exterior. If you have extended periods where you are using air conditioning in the house you could get moisture accumulation in the floor but I believe Northern California dewpoints are low enough that this is probably not a concern. Maybe someone with more experience in that area will chime in.

    Edit to add: I did not read the original post closely enough; though I believe my advice is accurate for a fully vented crawlspace (as opposed to a partially vented crawlspace) I agree with DCContrarian below that sealing a potentially damp crawlspace is not a good idea unless it's fully waterproofed and has a system to control the humidity.

  4. Expert Member
    NICK KEENAN | | #4

    Joe Lstiburek has an article on crawlspaces:

    His position is that the floor should be vapor sealed on the exterior. Even with ventilation a crawlspace will tend toward 100% humidity. While the thermal drive of moisture is toward the outside, the difference in humidity is so great that the vapor drive will be toward the interior. In practice there will be very little drying in either direction. It's very similar to basements, where Lstiburek now recommends that you build with the assumption that no drying in either direction is possible.

  5. Jon_M | | #5

    Thanks for your responses and the article link. I don't think I'll get what I originally planned but will make sure that I get at least the right R value, that the 2 vents are adequately sealed along with the vapor barrier on the ground and a permanent dehumidifier in the crawlspace.

    I will end up with something a little strange as Maximilian T commented above - a crawlspace isolated from the living space above and from outside air; hopefully dry as well.

    1. Expert Member
      Deleted | | #6


  6. frasca | | #7

    >> I will end up with something a little strange as Maximilian T commented above

    Life’s a compromise, they all tend to end up a little strange. I would have skipped spraying the underside of the floor and put more in the walls and band joist. But you’ll be fine.

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