GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter Instagram YouTube Icon Navigation Search Icon Main Search Icon Video Play Icon Plus Icon Minus Icon Picture icon Hamburger Icon Close Icon Sorted

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?

Thanks,
Bob

GBA Prime

Join the leading community of building science experts

Become a GBA Prime member and get instant access to the latest developments in green building, research, and reports from the field.

Replies

  1. GBA Editor
    Martin Holladay | | #1

    Bobby,
    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.

Log in or create an account to post an answer.

Community

Recent Questions and Replies

  • |
  • |
  • |
  • |