Minimum exterior foam sheathing thickness for a double wall design?
Myrtleboone
| Posted in Green Building Techniques on
I read the article: https://www.greenbuildingadvisor.com/blogs/dept/musings/calculating-minimum-thickness-rigid-foam-sheathing and am questioning what R-value of exterior polyiso is needed to satisfy the moisture conditions in a double wall assembly: 2, 2×4 walls with 3.5″ cavity in between. Zone 7/8. In a linear calc., I’d expect ~ R-30. Is this a correct assumption?
Thanks.
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Replies
Matthew,
Most builders who choose to build a double-stud wall don't use exterior rigid foam. In fact, the avoidance of rigid foam is one of the chief virtues of the double-stud approach.
Remember, a double stud wall like the one you describe already has an R-value of R-39 if it is insulated with cellulose -- even without any exterior foam. That's a lot. You don't really need an R-69 wall -- so why add R-30 of rigid foam on the outside of your R-39 wall?
In any case, to answer your original question: you are in the right ballpark. An R-39 double-stud wall has so much insulation that you would need a LOT of exterior rigid foam if you felt the need to install any. The best way to answer your question precisely is to perform a dew-point calculation. If you are serious about pursuing the issue, all the information you need to know to perform the necessary calculation can be found here: Are Dew-Point Calculations Really Necessary?