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I’m concerned about rigid exterior insulation being a vapor barrier

user-611330 | Posted in Green Building Techniques on

Hello, sorry to add yet another question about exterior rigid insulation, but I can’t find information on this elsewhere. I am working on designing a Habitat for Humanity home in a mixed-humid climate. Dow will give us insulation free, and I want to wrap the home in 1″ over the sheathing and building felt, and under cement-board cladding. I am concerned about it being a vapor barrier, however. I can not find perm information for Dow Stryofoam. Any thoughts?

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  1. GBA Editor
    Martin Holladay | | #1

    You can find the information you seek here:

    Dow Styrofoam (1 inch thickness) has a permeance of 1.5 perm.

    As you may know, wall systems can be designed to perform well, even with a low-perm exterior foam sheathing. The performance of the wall assembly depends on the climate and on all of its components -- that is, whether or not there is an interior vapor retarder, the thickness of the studs, and the type of insulation between the studs.

  2. Riversong | | #2

    The product that Martin linked to is Dow CladMate, a lower-density XPS foam board. Standard Dow Styrofoam (25 psi) has a perm of 1.1 @ 1". This makes it relatively vapor impermeable.

    But, if you're building in a humid, AC-dominated climate, then an exterior VB might be appropriate as long as the wall assembly can dry to the interior (i.e. no interior vapor barrier).

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