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Community and Q&A

Is exterior sheathing over rigid foam board okay?

brookhillBob | Posted in Green Products and Materials on

Articles regarding exterior rigid foam sheathing deal with it’s installation over the wall sheathing. What about the sheathing over the foam? I’m considering Huber’s R-Sheathing as a one step process to achieve both for a new home at the far north edge of zone 5. Their 1½” panel has 1″ of foam under their standard green Zip Panel with a total R-value of 6.6. While this addresses thermal transmission, won’t it also keep the sheathing colder? And doesn’t that cause issues of it’s own?

The R-Sheathing will go over a 2 x 4 wall with the intent of using blown-in cellulose in the wall cavities. Does this cause any issues? Thanks in advance for your feedback.

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  1. GBA Editor
    Martin Holladay | | #1

    Zip R sheathing works fine. While the OSB layer is cold, the foam layer limits outward diffusion of interior moisture during the winter. If you are worried about cold OSB taking on moisture during the winter, a rainscreen gap between the sheathing and the siding (to allow rapid ventilation drying) should keep your worries to a minimum.

  2. brookhillBob | | #2

    Thanks Martin. We'll be using Hardie-Plank for most of the siding with some Hardie-Shingle on parts of the gables. No issues then with sprayed cellulose against the foam panels?

  3. GBA Editor
    Martin Holladay | | #3

    Q. "No issues then with sprayed cellulose against the foam panels?"

    A. No, there are no issues -- as long as you follow the guidelines in this article: Calculating the Minimum Thickness of Rigid Foam Sheathing (which you are).

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