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Crawl space with closed-cell spray foam

JTuck | Posted in General Questions on

I hate crawl spaces but it seems like I have to. This is in NC on the southern edge of zone 4.  A raised slab is going to cost me at least $10k in compactable fill. Maybe I’m beating a dead horse but what is wrong with a vented crawlspace with closed cell foam sprayed to the underside of the floor and no ductwork in the crawlspace?
Thank you everyone.

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  1. Expert Member
    BILL WICHERS | | #1

    Closed cell spray foam isn’t cheap either, and spraying the underside of the floor will make any future wiring or plumbing work a real pain to do.

    Why not just put down poly, pour a thin rat slab, and insulate the perimeter walls of the crawlspace? This might not cost any more than closed cell spray foam, and it’s a better way to go. If you want to keep costs down even more, omit the rat slab and just build a sealed and insulated crawl space.


    1. JTuck | | #2

      I like the rat slab idea. I'll dig into some details on here for insulating the walls of the crawl space but I've never seen it done well and the ones I've seen are nowhere close to being air-tight (especially in between the floor joists). Send some links if you know of something off the top of your head.

      1. Expert Member
        BILL WICHERS | | #3

        Usually rigid foam is fastened to the inside of the crawl space walls, and a spray foam kit is used to seal the rim joist area. This method works pretty well. You can put some rigid foam down under the rat slab too if you want a fully insulated space.

        GBA has lots of articles about this. Try using google to search for “insulating crawlspace GBA”. Here is one link to get you started:


        1. JTuck | | #6

          Ha yeah that last section makes me want to spend the extra $10k on fill....and then just makes me go back to my original idea with vented crawl space (lots of vents) with closed cell foam on the floor. I've been going in circles for a while on this.

  2. MAinspector | | #4

    10K for fill seems that with labor to properly compact in lifts? How big is your crawlspace? If it's that big the cost of framing a floor and spraying all closed cell (including fire protection if needed) cant be much cheaper.

    1. JTuck | | #5

      That is for material and labor for the fill only. That does not include footings, walls, slab, subslab insulation etc. It's 1800 sq ft. about 1.5' deep on one house corner sloping to about 5' deep on the opposite corner.

  3. JTuck | | #7

    If I go the conditioned crawl space route...I’ve seen Martin say several times to not use the ERV to exhaust air from a conditioned crawl space but I don’t see why that is. Exhaust any nasty air from the crawl space and supply the crawl space with air from above with a register in the floor. Any reason not to do that?

    1. Expert Member
      Dana Dorsett | | #8

      >"Exhaust any nasty air from the crawl space and supply the crawl space with air from above with a register in the floor. Any reason not to do that?"

      That's perfectly legit, meets code (assuming you hit the minimum cfm for crawls), and guarantees that any potential outgassing of foam or other contaminants in the crawlspace goes outdoors.

  4. Expert Member
    Peter Yost | | #9

    Relying on an ERV system for whole-house ventilation in your climate will mean lots of hours when you will be bringing in moisture-laden air from outside. You are going to need significant latent load removal and the ERV of course can't do that. How well your AC system can handle that load depends on what the Sensible Heat Ratio (SHR) is of your AC system. The SHR for your climate should be a maximum of 0.70.


    1. JTuck | | #10

      I'm not super educated on the subject but I'll check with my mechanical designer. I'm thinking that you're saying the ERV is good for ventilating the tight house, but it will bring in too much moist air? Doesn't the ERV transfer most of the moisture to the outgoing air? There will be a dedicated dehumidifier in the house. Does that fix the concern?
      Thank you for this info.

      1. Expert Member
        Dana Dorsett | | #11

        >"Doesn't the ERV transfer most of the moisture to the outgoing air? "

        Simple answer: NO!

        It transfers SOME of the moisture to the outgoing air, but the latent heat transfer efficiency is nowhere near as high as the sensible heat transfer efficiency. Figure on about half, best case.

  5. JTuck | | #12

    So you guys don't like ERVs? What do you suggest?

    1. GBA Editor
      Martin Holladay | | #13

      If you want to depressurize a crawl space (providing conditioned makeup air via a floor register in the crawlspace "ceiling"), the usual approach is to simply install an exhaust fan through the rim joist. You don't need an ERV to depressurize a crawlspace.

      For more information on the cfm requirements for this type of exhaust fan, see this article: "Building an Unvented Crawl Space."

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