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Community and Q&A

Does polyiso need to be flush with CMU crawlspace wall?

agurkas | Posted in GBA Pro Help on

When state-sponsored program came to put in vapor barrier in my crawlspace (it is 5′ tall around perimeter) they used polyurethane foam to stick poly sheeting to about 15 inches up on the crawlspace CMU wall. They also sealed some cracks and gaps in that wall with foam.

Now I am ready to install vapor barrier to deal with radon. I am also going to put some recycled polysiso I got. Those panels came out of commercial building, so there is no reflective coating on them.

My two questsions:

1. Does poly iso need to be flush with CMU wall? My idea was to put 2X2s and put polyiso over that. THis way I don’t have to scrape off the foam off the walls and remove the sheeting I am going to cover up anyway with new layer.
2. Can new vapor barrier go over the polyiso, or does it have to be under it? If it has to be under it, same question as in 1st one – can I do then vapor barrier all the way to the rim joist and then do 2X2s and polyiso sheets over that?


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  1. Expert Member
    Dana Dorsett | | #1

    Below grade the vapor barrier has to go between the CMU & polyiso, or there is a risk of ground moisture accumulation in the polyiso.

    Installing 2x2 furring between the CMU & furring puts the furring at risk. A better approach is to clamp the polyiso to the CMU with 1x4 furring through-screwed to the CMU with TapCons (or similar). Be sure to use washers to keep from splitting the furring.

    If the bead of can-foam they sealed the ground vapor barrier with is too thick, stop the foam just above that bead, or trim it flush. You'll still have to seal the gap between the foam & CMU to prevent convection anyway.

  2. agurkas | | #2

    Would it be OK to do vapor barrier flush with CMU wall, then do furring strip then polyiso over it? There would be a gap between polyiso and vapor barrier.

  3. Expert Member
    Dana Dorsett | | #3

    No, not OK.

    The gap between the foam & vapor barrier creates a convective thermal bypass unless you managed to seal it up perfectly. But if you were so diligent as to seal that bypass up, you will have created a moisture trap around the furring. For that to work long-term requires that the moisture content of the furring be low on the day it was sealed, and that for the next 100 years no moisture ever gets in. In construction there is good, better, and best, but there is no such thing as perfect. Any assembly that requires perfection to work isn't a very good design.

  4. agurkas | | #4

    Point taken. I think I will just have to man up, scrape the excess foam off the CMU and make everything flush with the wall.

    I guess I only need to do it once.

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