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Community and Q&A

Slab edge: Foam board bevel detailing

Bruce_Davis | Posted in Green Building Techniques on

See attached guidance from EPA about detailing slab edge. This shows how to cut beveled edge for non-basement slab.  What should be done for slab with the basement foundation walls already in place. Seems it would be labor intensive to bevel before installation. I’m doing 2″ XPS under slab. Thanks.

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  1. GBA Editor
    Martin Holladay | | #1

    Attached is a detail from the Building Science Corporation showing a typical basement slab with horizontal insulation under the slab.

  2. Bruce_Davis | | #2

    That was what I was thinking before I saw this EPA thing. My only concern is the 2" wide exposed XPS flush with the surface of the slab. Anything need to be detailed?

    I'm hoping to stack fire-resistant 2" polyiso on top of that. Is that OK? For the framed areas, finished or unfinished, is it OK exposed? Have I used up my allotment of questions for today? Thanks.

  3. Expert Member
    Michael Maines | | #3

    Bruce, whether or not the polyiso you choose can remain exposed is up to your AHJ (authority having jurisdiction, typically a building inspector). Some brands have a fire rating.

    Flatwork contractors I've worked with don't like to screed the slab to the top of the vertical foam, so I spec the perimeter foam to extend 2-3" above the slab so they can snap a line like they're used to doing. Then the polyiso can go above that. Technically I suppose it is a code violation to leave even a small amount of XPS exposed but it's a very small area, in a low-risk location. If you (or your AHJ) are concerned, you could cut the 2" foam flush to the top of the slab as Martin's image shows, and it should not be a problem, but it's up to the AHJ.

    One note about XPS--you may have read here already that it is generally avoided by conscientious builders, due to its blowing agents, which are potent, persistent greenhouse gases (aka carbon polluters). EPS is available in various compressive strengths and is much better from an environmental standpoint, and there is little thermal difference--the aged value of XPS is R-4.5/in or less, compared to ~R-4.0/in or more for denser grades of EPS.

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