I’m concerned about rigid exterior insulation being a vapor barrier
David Quillin| Posted inGreen Building Techniqueson
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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Replies
David,
You can find the information you seek here:
http://www.dow.com/webapps/lit/litorder.asp?filepath=styrofoam/pdfs/noreg/178-00568.pdf&%3Bpdf=true
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.
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).