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Community and Q&A

Lightweight insulating (perlite) concrete footing

Jerry Liebler | Posted in Green Building Techniques on

I’m considering perlite concrete for my basement’s subslab insulation. The sub slab will need to be rather thick to get the level of insulation. If I use a 6 to 1 volume of perlite to cement and a 10 inch sub slab I’l get about r15 and have a compressive strength of 125 psi+. Can I simply enlarge the sub slab and pour the walls on it’s edge, in effect making the sub slab serve as footings for the wall?
The sub slab will be on top of 6inches of coarse rock covered by poly.

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Replies

  1. User avatar GBA Editor
    Martin Holladay | | #1

    Jerry,
    I have experience placing concrete that included expanded polystyrene beads in the mix. My recommendation: don't do it. You will never be able to finish the top of your slab in a way that looks normal.

    Put the insulation under your slab, and then pour a regular concrete mix on top.

  2. Jerry Liebler | | #2

    Martin,
    I'm not proposing that the insulating concrete be finished, all it's replacing is insulation. Their will still be a normal slab (3.5") floor.

  3. John Klingel | | #3

    Jerry: Is this cheaper and easier than just putting rigid foam under the slab? It sounds like a God-awful lot of work. What is the advantage? I am not familiar w/ this at all; just asking.

  4. User avatar GBA Editor
    Martin Holladay | | #4

    Jerry,
    The last time this question came up (click here), the question concerned loose perlite under a slab rather than perlite concrete. One builder who did this, Robert Hawthorne, posted this comment on a GBA page: "We paid about $6.50 per bag of perlite (4 cubic feet). You'd have to do your own calculations for cost per R-value but I figured it came out a bit cheaper than EPS. It is more labor intensive, though, and I'd only recommend it for the simplest of slab configurations. We still used gravel and sand as a base and had a hefty vapor barrier. We did not increase the reinforcement in the slab due to the perlite but I'd make sure to use rebar and not wire mesh."

    Anyway, as I started to write, the last time this question came up, here's what I wrote:

    Information on the use of perlite under slabs:

    GBA links:

    Perlite as insulation

    Trekhaus: A Passivhaus Duplex in Oregon (see comments below article)

    Other links:

    TrekHaus

    Perlite as underslab insulation

    Leap Frog House

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