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Community and Q&A

Fastening Rigid Foam to Cinder-Block Foundation Walls

Holt131 | Posted in General Questions on

I am in Ohio and I am finishing a basement in a house built in 1930. Is it OK if rigid foam board is loosely held against the cinder block walls by the wall framing without adhesive?  It is taped and sealed.  Or is it required that the foam board be fastened to the block wall with Tapcons or something similar. The framing is already up so I would rather not remove it. Thanks.

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  1. Expert Member
    Michael Maines | | #1

    If air can't get behind it, loose is fine. Is the entire perimeter of the foam sealed tightly to the wall?

  2. Expert Member
    BILL WICHERS | | #2

    Ideally you want a pretty tight fit against the surface so that there is nowhere for moisture to accumulate, or for critters to hide. You don't want air circulating back there, either. If everything is taped up so that there is no airflow, then you greatly lessen the issues with air circulation and critters, but could potentially still have moisture accumulate and run out under the foam.

    My preference for fastening rigid foam is to use the nylon insulation anchors made by Hilti and a few other places. All you do is drill through the foam into the block, then poke in one of these anchors and give it a whack or two with a mallet to set it. Quick and easy. They're a bit on the pricey side for what they are, but they work well.

    How big is the gap behind the foam right now? You mention it's "loosely held". If a few anchors would pull it tight to the wall, I'd put in the anchors. If there is something keeping the insulation held off the wall a bit, I'd probably just leave it alone, but I'd be extra sure of my air sealing between panels and around the perimeter.


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