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Is spray foam insulation OK under concrete in basement instead of XPS?

Anita Brosius-Scott | Posted in GBA Pro Help on

I’m having my existing basement waterproofed (hopefully!) and insulated, and having a concrete floor poured in place of the present moist gravel. My Energy Advisor / insulation guy suggests continuing the spray foam we’ll be applying to the walls, on down onto and over the prepared (and drained) gravel under the new concrete, and having that serve as both the waterproof and insulating layer. I haven’t read of anyone doing that. The alternative is blueboard (I’m thinking 1″ is plenty) and vapor barrier (Tenoarm poly with sealed seams). So, is it OK to use only spray foam under the concrete?

Also, if I use the blueboard (that’s XPS, right?), does the vapor barrier (Tenoarm) go below the blueboard or above it? My understanding is that the layers would be gravel, Tenoarm, blueboard, concrete. Please confirm!! Thanks.

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Replies

  1. David Meiland | | #1

    Dunno about the SPF under a slab... hardly seems preferable to me over XPS boardstock, which is pretty much the industry standard for slab insulation. 1" is probably not enough, code here in balmy coastal Washington requires R-10 and it's nowhere near as cold here as it is there. I'd put down as much as I could fit.

  2. Riversong | | #2

    If it's closed-cell (not open-cell) polyurethane spray foam, it can be used sub-slab but it's important to try to get a uniform layer, both for proper insulation and to keep the slab thickness uniform.

    It's best to lay the sub-slab vapor barrier immediately under the concrete, which also protects it from the sharp crushed rock that's often used as a capillary break. Tenoarm is much better than standard 6 mil poly, but it doesn't have the reinforcement necessary for a sub-slab application. I would suggest cross-laminated Tu-Tuf #4, which is specifically made to go under a slab.

    But it's also OK to lay the vapor barrier first, tape the seams, then the tongue and groove XPS (blue or pink doesn't matter) with taped seams, then steel mesh or rebar grid.

    If you're basement is a full 7' or so below grade, then 1" of foam should be sufficient. There is a lot more heat loss from the above grade portion of the foundation.

  3. Expert Member
    Michael Maines | | #3

    Anita,
    You might have to pay to view it, but here is an article about spraying foam under slabs: http://www.jlconline.com/cgi-bin/jlconline.storefront/4cb735cc123531b927170a32100a0642/Product/View/1001foam. The author says that he does it mostly for commercial buildings, where the installation cost compares favorably to rigid foam.

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