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Slab, vapor barrier, insulation

tQ2oEocGVC | Posted in General Questions on

Hi folks,

Steve from northern VT. I want to double-check things. We’re about to do a slab over 1 1/2 crushed bedding. We plan to use StegoWrap 15mil as the vapor barrier, with taped joints. And, 2″ blue rigid insulation under the slab. Slab will be 4″ 4,000 psi, reinforced, over 10×10 wire mesh.

Two questions:

1. the proposed layering (from bottom upward) is: crushed stone, StegoWrap, blueboard, wire mesh. Correct?

2. the inside foundation walls (above finished floor) will be insulated with 2″ blue foam board. Is there any benefit or downsides to running the StegoWrap up above the height of the floor, up the walls to the top of the sill (i.e., between the concrete wall and the concrete insulation. It strikes me to be a great wrap to completely wrap the inside of the basement.

thanks, Steve

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  1. HDGFHkB5cP | | #1

    We just did one in s. NH. Slab was similar to what you describe, but 2" blue board on the walls just barely meets code. We used 2" Thermax (foil faced iso) on the inside of the foundation walls which is slightly better in terms of R-value, but the foil facing is our vapor barrier (we taped the seams with foil tape) so no wrap on the walls. Thermax was friction fit to the top of the concrete walls, then we framed 2x4 walls inboard of the foam for wiring and such. Oh yeah, we added R-15 batts between the studs. Spray foamed the band joist, bringing the foam down to the top of the ridgid. Snug basement.

  2. jklingel | | #2

    Northern VT is cold. Have you considered more foam under the slab, like 4 to 6"? Is the edge of the slab insulated? Ditto for the foundation wall?

  3. GBA Editor
    Martin Holladay | | #3

    It's better to put the rigid foam under the vapor barrier. The vapor barrier should always be directly under the concrete, so that you don't trap water in the cracks between the foam.

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