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Community and Q&A

Foam board between studs and exterior sheathing?

dvaut | Posted in Green Building Techniques on

Someone in my area which is located in a zone 5 recently built a small house and sandwiched used 2.5″ polyiso between the studs and the exterior wall sheathing. At first I was skeptical but still intrigued. It certainly simplifies window details compared to foam on the exterior. However I guess you would loose some structural rigidity. But would you really loose that much? If the wall sheathing is screwed on over the foam, the friction of the foam against the studs alone should help keep the wall from racking. Also the screw shanks from the studs to the exterior sheathing would also help prevent wall movement. Of course you would want to use the proper amount of foam thickness for your building zone. Would there be a moisture problem where the foam and sheathing interface if they were tight together? I haven’t really heard much on this technique so I am sure that there is something that I am totally missing and my friend already totally missed. What do other people think?

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  1. Expert Member

    "Of course you would want to use the proper amount of foam thickness for your building zone. "

    The amount of foam is calculated to make sure the exterior sheathing stays warm enough to dry. Putting it inside defeats the purpose.
    I suppose you might be find an engineer to sign off on the shear strength of sandwiching a very small amount of foam between the wall framing and sheathing, although it wouldn't fly under our building code - and certainly not 2 1/2".
    Once you have eliminated the main benefits of both the sheathing and foam, it's hard to justify the time and expense of using sheathing at all in that situation.

  2. Expert Member
    Dana Dorsett | | #2

    Malcom has it right. The sheathing on most studwalls is structural, and it may not meet it's structural requirements on it's own if it's 2.5" or more off the studs. (Huber Zip-R only puts about an inch between the stud and OSB.) Counting on the friction of the foam over time isn't going to cut it without a complete engineering analysis.

    Cut-in bracing, or steel strapping on the diagonal can work, as can shear-bracing panels attached directly to the studs at corners- there are foam-clad houses built with T-bracing and NO exterior sheathing.

    Having the sheathing exterior to the foam is fine from a moisture point of view. The foam itself is a vapor retarder against interior moisture drives, and the sheathing can dry toward the exterior. The drying capacity is enhanced (and the capillary draw wetting reduced) if you put a minimum of 1/4" of "rainscreen" gap between the sheathing and siding.

  3. dvaut | | #3

    The situation I referred to did have cut in diagonal bracing. The area is also not known for high winds.

  4. Expert Member
    Dana Dorsett | | #4

    With cut-in bracing the sheathing outside the foam can then be considered just a nail-base for the siding, not structural, even though it offers some additional rigidity.

  5. wjrobinson | | #5

    Just like the fact that drywall is never in the calculations for structural integrity even though it is doing something..

  6. Expert Member

    Our new seismic code mandates shear walls every so often depending on which zone you live in. In some situations, the internal ones can be sheathed in drywall both sides, acknowledging the stiffening it provides.

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