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Rigid foam insulation on top of a concrete slab

user-7062830 | Posted in Green Building Techniques on

Weather zone 4 C (on an island in Canada just north of Vancouver). new construction of small cabin. the plan is to pour a slab of concrete on top of a bed of gravel over sound subsoil, then lay stringers of 2×4 on end with rigid foam insulation between the stringers. Top with 3/4″ ply sub-floor and 1/4″ nail-through oak flooring.

Decided not to put insulation under the slab because of potential infestation by termites / carpenter ants.

Question is what type of foam insulation, how much (R value), what about vapour barrier? The cabin will be measure 11′ x 20′, be heated by a wood stove / heater.

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  1. GBA Editor
    Martin Holladay | | #1

    User 706-etc.,
    First, can you tell us your name?

    Second, is this structure subject to building codes or inspections? The type of foundation you describe -- "a slab of concrete on top of a bed of gravel over sound subsoil" -- isn't allowed for homes in a climate with freezing weather, unless I'm mistaken.

    Q. "What type of foam insulation?"

    A. Either XPS or EPS will work, but most green builders avoid the use of XPS, because XPS is manufactured with a blowing agent that has a high global warming potential.

    Q. "How much R-value?"

    A. I'm not sure what the Canadian code requires. The IRC requirements for slab insulation are among the worst-written section of the code, and aren't easy to decipher. For more information on this issue, see Insulating a slab on grade.

    Q. "What about vapor barrier?"

    A. A slab on grade should always have a vapor barrier. The best place for the vapor barrier (at a minimum, 6 mil polyethylene) is directly under the concrete.

  2. Expert Member
    Dana Dorsett | | #2

    The stringers are not necessary, and create both a thermal bridge and wicking path. Even with 2x4s on edge completely filled with 3.5" of EPS it's thermal performance would be less than 2.5" of continuous EPS. It's better to go with a continuous layer with the subfloor through-screwed to the slab.

    With 3/4" subfloor the deflection /firmness of a floor supported with a continuous layer of EPS will be stiffer than the same 3/4" subfloor on 16" o.c. joists. If the seams of the subfloor are lapped by at least a foot or so over those of the foam there will be no permanent compression of the foam. You may have to adjust the torque of the screws to keep the edges of the subfloor seams perfectly level , with no ledge between sheets. (The localized pressure that can be applied with a screw is many times what would be experienced in a residential floor.)

    In a marine zone 4C climate it's financially rational on a lifecyce basis to install a couple inches of EPS (R8-R8.5)

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