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Attaching radon/vapor barrier to Inside of ICF stem walls

brentwilson | Posted in General Questions on

So let’s say that I want to build a conditioned crawl space. The stem walls would be concrete filled ICF blocks. The floor could be 4″ of gravel, 2″ of reclaimed XPS, a radon/vapor barrier, then a 2″ concrete rat slab. If I run the radon/vapor barrier up the side of the ICF about 6″, and “glue” it to the ICF foam wall with some sort of sealant/adhesive, will that form enough of a seal to keep radon from getting past? Or could radon work its way up through the ICF foam and enter the conditioned crawl space?

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  1. Expert Member


    Radon reduction strategies don't rely on air-sealing alone. It's a first step, but at least here, venting the sub-slab is also required.

    1. brentwilson | | #2

      Yes, I intend to add in the vent stack as well. But I am specifically wondering if sealing the barrier to the foam face of the ICF is good, or if there is a better way to do this.

  2. 730d | | #3

    Be very careful with your sealant adhesive, test it to be sure it does not melt your walls.
    Those little barbed plastic push fasteners add some mechanical advantage and can make a big difference short and longer term.

    1. 730d | | #5

      I suggest using tape such as is used on exterior foam or wall sheathing. It should stick aggressively to both the poly and foam.

  3. GBA Editor
    Brian Pontolilo | | #4

    Hi Brent.

    What you describe, sealing the vapor retarder to the wall is common practice. You'll have to research the best product for the two materials, but a mastic or air sealing tape should do the trick for air sealing. Some builders use battens to secure the vapor retarder material to the walls. Have you read this: Building an Unvented Crawlspace? You may find it helpful.

  4. Expert Member
    Dana Dorsett | | #6

    >"Or could radon work its way up through the ICF foam and enter the conditioned crawl space?"

    ICF blocks don't have gas-tight seams, even if the EPS itself is fairly gas-tight at 2-2.5". With sub-slab depressurization even with the slab sealed to the ICF with polyurethane caulk at the slab edge it's usually possible to hear the hiss of myriad air leaks through the joints/seams of the EPS blocks.

    A broadsheet air barrier or a materials-compatible spray on WRB can air seal the ICF.

    1. brentwilson | | #7

      What about adding a piece of vapor barrier that runs under the footing and up onto the sub-slab foam (pink line in the attached image). The primary vapor barrier sheet (green line) could then overlap it (and be sealed to it), before extending a bit up the stem wall? It seems like this could potentially minimize the chance of radon getting past the barrier through the ICF block seams.

      1. Expert Member
        Dana Dorsett | | #8

        Sure! Installing a vapor barrier under the footing can also serve as a capillary break if it's a full wrap over the top as well.

        Like moisture, radon can still come through the soil backed up against the wall too, but usually the concrete is sufficiently gas tight to mitigate against that.

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