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Community and Q&A

Exterior foam substitute above thermal envelope on attic wall

ranson | Posted in Green Building Techniques on


I’m planning 4″ of exterior rigid foam on my walls with a cellulose insulated attic floor. My WRB is outside my rigid foam. Because design uses a monoslope roof, there’s a large portion of attic wall that’s above the thermal envelope of the house. In this area, are there any good assemblies that I can substitute for the rigid foam? My hope would be to reduce labor and use less foam.


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  1. GBA Editor
    Martin Holladay | | #1

    The answer depends in part on your siding choice, and whether or not you intend to install vertical furring strips.

    If you are installing 1/2" plywood furring strips over your 4 inches of rigid foam, you could, in theory, install three layers of 2x4s as furring strips on your attic wall. That makes the furring strips co-planar.

    To me, this approach is costly, fussy, and not worth the trouble. I would just install 4 inches of rigid foam over everything. If you want to save money, buy recycled (reclaimed) rigid foam.

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