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What are builders that use exterior rigid foam doing between joists and grade?

Steven Bumpus | Posted in Green Building Techniques on

With 4″ rigid foam and furring strips and siding, a building ends up with 5-6″ extending past the basement wall. What are builders doing about the overhang/shadow? Are some leaving the overhang? And does it cause any type of issues?

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  1. GBA Editor
    Martin Holladay | | #1

    There are several options:

    1. Before installing your rigid foam, install metal flashing (drip edge) at the base of your wall; this flashing should be bent on a brake, with the center section of the flashing having the same thickness as the foam. You can see this detail in a video:
    Superinsulating a Home With Rigid Foam -- Click on the second video in this 2-video series to see the detail I'm talking about.

    2. Insulate the exterior of your basement wall with foam having the same thickness as your above-grade wall foam. Run the foam from the concrete footing to the top of the wall.

  2. Robert Susz | | #2

    Personally, I like the idea of the heavier shadow line. But I did 4" above grade and 3" below grade. With a 5/4 water table. I end up with a 2" shadow that is almost completely obscured by a wraparound porch. The old portion of the house will have about a 3" shadow though.


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