WRB Perm rating and Rigid Foam
Just a general question that I have. Is there any benefit to using a higher or lower perm rated WRB if you will be installing 3-4 inches of EPS or XPS over it? The way I figure it doesn’t really matter as the wall would only be able to dry to the inside anyway but I was hoping to get an expert to weigh in.
On the same note, would there be any benefit from a drying perspective if you used had a wall with the WRB, 1 inch of comfortboard, 3 inches of XPS over the comfortboard or would the XPS still block so much vapor that it would be no use.
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3" of XPS is pretty much a vapor barrier, there will be VERY little drying through it. If you wanted to use Comfortboard for drying potential, covering it with 3" of XPS will cancel that out.
3-4" of EPS is better for drying than 3-4" of XPS, but the total amount of moisture that can make it through either of those is so little that it doesn't really make any practical difference. I used 2" EPS on my rim joist to allow what I think of as a "little bit of drying", and it's just a very small amount, but better than XPS or closed cell spray foam would have been. Anything over 2" of EPS and there is so little moisture migration that the differences between foam types don't really make much practical difference.
3" of unfaced EPS is > 1 perms. This much outward drying is significant if you have a limited drying (like a Class I or non-smart Class II vapor retarder) on the interior side. But you probably don't.