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What is the proper way to install underslab insulation?

Andrew Homoly | Posted in Green Building Techniques on

I have been searching your site for underslab insulation details and don’t see much consensus on the proper technique. Many details show no underslab insulation at all. I am building in Kansas City, MO using a poured concrete wall basement (walkout) with 2″ exterior insulation/drainage board (Tuff N Dri system). For the basement slab, I was planning on using 2″ rigid foam, but I am debating where it should go in the assembly. Right now I am leaning towards gravel, then foam, then poly, then reiforced concrete directly on the poly layer. Do you agree? As a second question, the rigid foam seems to create “voids” under the slab because it does not sit 100% perfectly flat on a bed of gravel. Would a “blanket” type insulation be preferable?

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Replies

  1. Expert Member
    Armando Cobo | | #1

    Andrew,
    You have the right assembly. The voids should not be there if you spread and compact at least 4" of gravel correctly.

  2. GBA Editor
    Martin Holladay | | #2

    Andrew,
    Q. "I am leaning towards gravel, then foam, then poly, then reinforced concrete directly on the poly layer. Do you agree?"

    A. Yes.

    Q. "The rigid foam seems to create 'voids' under the slab because it does not sit 100% perfectly flat on a bed of gravel."

    A. Are you using gravel (with fines) or crushed stone? If you are using crushed stone, choose smaller stone; 3/4-inch stone is much easier to rake than 1 1/2-inch stone. Anyway, the solution to your problem is choose the right product and compact and rake it well.

    Q. "Would a 'blanket' type insulation be preferable?"

    A. No.

  3. John Klingel | | #3

    Foam on top, or poly? Debatable, from my conversation with a gent at CCHRC here in Frb, and a few other people. One engineer at CCHRC opted for poly under. The poly underneath "may" help protect the foam from radon and vapor damage (saturating it???), though as Martin pointed out in an earlier thread, if you get rain water all over the show poly underneath will hold a lot of water, and that can ruin concrete poured over it. Just a thought.

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