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Question regarding rigid foam under slab (for Martin)

sxv201 | Posted in Green Building Techniques on

I’m building a home in Ottawa, On. (zone 6)

I’m laying 2 inches of EPS under the slab (R-8) and 4 inches of EPS on the interior foundation walls. (R-16)

I will be installing Rub-R-Wall and Geo Wrap over the exterior of the walls.

From reading previous posts, I will be adding my poly directly above the foam, directly below slab.

Question: Can I tape up all the seams with recommended foam tape? Will this help? just so we can walk over the foam while installing the foam on the walls.

I intend on first applying the foam on the ground, then applying an adhesive on the foam for the foundation walls…then taping up all the joints with the tape.????

I first thought about applying a fastening system for the walls, but was told the drilling time in concrete for 69 sheets of foam would be too extreme.

Any advise would be much appreciated!

Steph Vizzari

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Can I tape up all the seams with recommended foam tape? Will this help?"

    A.You can tape the seams of the horizontal rigid foam under your slab if you want, but you don't have to. Concrete is a pretty good air barrier, so you shouldn't have much air leakage through the slab. (Although it's possible to get air leakage at the slab perimeter, so it often makes sense to caulk the crack between the slab and the wall.)

    If you are worried about damaging the foam by walking on it, just temporarily put down two or three scrap pieces of plywood or OSB to step on.

    Concerning your question about fastening your wall foam: will you be installing a barrier or finish material on the interior side of the wall foam? Many building inspectors insist on a layer of gypsum wallboard as an ignition barrier or thermal barrier. Some building inspectors will allow Thermax to be installed in this location without further protection, but you should check with your local building department before proceeding. If you intend to install rigid foam without any further protection, I think it's a good idea to secure the foam with special plastic fasteners like Hilti IDP fasteners or Plast-Grip PMF fasteners from Rodenhouse.

  2. dickrussell | | #2

    Regarding air leakage at the joint between slab and wall, you ought to have a capillary break between footing and wall anyway, to retard moisture movement up the wall. In my house, that is a strip of poly wider than and laid over the footing before the wall was formed up and poured. Later, the inside edge of that strip was lapped over the big sheet of poly on top of the foam and the edges taped. That detailing gives a continuous air and moisture diffustion barrier underneath the whole wall/slab assembly.

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