GBA Logo horizontal Facebook LinkedIn Email Pinterest Twitter X 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

Vapor barrier in unvented roof

drichfield | Posted in Green Building Techniques on

I have an unvented roof assembly in Zone 5 New York that doesn’t quite match the Pennsylvania Housing Research Center’s spec. I have attached their PDF doc that explains I should have R20 of closed cell rigid foam directly under the roof sheathing, then a vapor barrier, and then additional open cell insulation.

Unfortunately, I was not privy to this document when we built the roof, so what I ended up with was 7/16″ zip wall with 1″ rigid foam directly underneath it.  Then 10″ of open cell spray foam.  I assume the surface that could experience condensation is the underside of the sheathing, but it would have to get through the 1″ rigid closed cell for that to happen.  Maybe the underside of the 1″ rigid closed cell since it’s only around R7 and will be pretty cold? It seems to me that a vapor barrier will still be useful underneath the open cell insulation as it will prevent moisture from in the house ever getting into the roof assembly, thus mitigating the possibility for condensation on the cold surface.

I am using the Certaineed smart membrane so if any moisture makes it in during the summer, it will be able to dry into the house. Also, since both the rigid foam and the open cell foam are not dramatically affected by water, the only thing I’m really worried about is mold and rot on the wood.

I understand this set up may not be the most ideal, but I am trying to find out if it’s problematic or not. Thanks!

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.


Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |