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Community and Q&A

How to seal 4″ EPS foam at slab edge (radon concerns)?

NEzone5 | Posted in GBA Pro Help on

Basement concrete 4″ slab to be poured on top of 10mil poly, 4″ EPS (2 layers staggered seams), and gravel at bottom with perimeter drains connected to vertical radon vent pipe. Building in a high radon zone, will the poly be sufficient covering the 4″x4″ foam perimeter strips not covered by concrete at the slab edge? Should I be tapering the foam edge instead of keeping the full width of 4″x4″ strips?
Tips for poly location at edge detail during pour (ie. multiple creases going over the 4″x4″ foam perimeter strips or just keep the poly flat until it meets the foundation wall and put the 4″x4″ foam perimeter strips on top of the poly?) How do you recommend sealing the poly to the foundation wall?
Will eventually be covered with a thick (5 1/2″ – 7 1/4″ depending outside grade) EPS foam and drywall, so aesthetically not a concern. But worried that radon may leak through – what to use to seal 4″?

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  1. GBA Editor
    Martin Holladay | | #1

    Q. "Should I be tapering the foam edge instead of keeping the full width of 4"x4" strips?"

    A. Yes. The bevel cut will resemble the one shown in the image below. After the slab has been poured, the crack between the slab and the foundation wall should be sealed with caulk.


  2. NEzone5 | | #2

    Attached image

  3. NEzone5 | | #3

    I shared your suggestion with my builder, he's worried about the concrete cracking due to thinness at the bevel. What about a 45 angle as shown on the sketch? He already has the square perimeter strips and liked how they would sit directly below the inside wall foam without a decrease in coverage.
    And does the poly continue over the bevel to the wall? Was thinking the poly could continue at the horizontal surface and fold 90 degrees up the wall, then place the foam perimeter strip above.

    While I'm asking, do you recommend a non-permeable moisture barrier behind the wall foam nailing strips?

  4. GBA Editor
    Martin Holladay | | #4

    Why is the 45 degree bevel in your sketch any different than the 45 degree bevel in my sketch?

    Plenty of slabs have been poured with this detail at the perimeter, and the concrete hasn't been proved to be fragile.

    Don't obsess about where the polyethylene goes. The rigid foam is also a vapor retarder, just like the poly, and the concrete is an air barrier.

    The moisture barrier behind the furring strips on your wall is called "rigid foam" -- right?

  5. Kopper37 | | #5


    We don't bevel the foam at this location. The slab edge is responsible for a considerable portion of the total slab heat loss; beveling the foam further reduces the effectiveness of this thermal break.

    We install the slab edge foam before the concrete pour. Poly vapor barrier goes on top, lapping up the slab edge foam. The foam establishes the level point for the entire slab, acts a screed point when placing the concrete.

    When we are ready to insulate the basement walls we install hard plastic flashing to cover the slab edge foam. We bend the flashing to an L-shape, provide an inch or two of overlap. We seal this material to both the masonry wall (CMU or concrete) and the slab. Depending on the surface we use a combination of elastomeric sealant and acrylic tape.

    This method creates an good air barrier. We've proven this with blower door testing. The plastic flashing also acts as a termite shield---a physical barrier to entry into the walls (it's one of our primary reasons for using this method).

    You can find this product at most building suppliers, even the big box stores. It is usually stocked with the standard roll flashing products. Locally the roll sizes range from 10" - 14". You bend it on a sheet metal brake the same as you do metal flashing. Builders also use this product to flash against pressure treated wood (since the aluminum roll material is not suitable against the copper-based preservatives in pressure treated wood).

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