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Community and Q&A

XPS foam board on the interior side of kraft-faced fiberglass

Bobconrad | Posted in Energy Efficiency and Durability on

I live in southern Maine, zone 6 and I have a 95 year old bungalow that we just built a 12 x12 dormer on. The plan was to use cathedral r30 unfaced fiberglass followed by 1/2 inch XPS and then strapping and drywall.
I came home to my builder using faced r30 (which was put on AFTER he strapped the ceiling) and running the xps in strips in between the strapping…
Obviously a miscommunication. My question is, will the two vapor retarders cause problems?


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  1. GBA Editor
    Martin Holladay | | #1

    No, these two adjacent vapor retarders won't cause any problems.

    The main problem with this roof assembly is its low R-value. In your climate zone, you should really be aiming for R-49. Most of the R-value of your roof assembly comes from the R-30 fiberglass, and the performance of that insulation is reduced by thermal bridging through the rafters. And 1/2-inch rigid foam has a very low R-value -- about R-2.5. Finally, the performance of the rigid foam has been greatly compromised by cutting it into thin strips and inserting it between the strapping.

  2. Expert Member
    Dana Dorsett | | #2

    Unless it has vapor retardent facers1/2" XPS isn't even a Class-II vapor retarder- it's only slightly more vapor tight than latex ceiling paint.

    If the R30 has a foil facer (unlikely) that would be a Class-I vapor retarder, limiting what you can do later. If it's a kraft facer, it's a "smart" vapor retarder, usually about 0.5 perms when dry, but 5 perms or higher when the humidity in the fiberglass is high.

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