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

Interior walls acting like exterior walls

Michael Brackett | Posted in Energy Efficiency and Durability on

New construction in zone 5. We have 3 interior walls that we have to build up still where the vaulted ceiling meets the flat(see pic). We are planning unvented on the vaulted side (3″ closed cell, and fill the rest with cellulose, but just blown in cellulose on the flat ceilings. How would you recommend I build and insulate this wall? I figure I need to treat it like an exterior wall, so drywall, studs filled with fluffy and 3″ of rigid foam?(that is what we are doing for our other exterior walls). Thank you in advance.

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.


  1. GBA Editor
    Martin Holladay | | #1

    Your attempt to attach a photo failed. It sounds like you are talking about an attic wall, with one side of the wall facing a vented unconditioned attic, and the other side of the wall facing indoor space (namely the top of a "great room" space with a high ceiling).

    Yes, this wall should be treated like an exterior wall. If you intend to insulate this wall with a combination of rigid foam and fluffy insulation, the rigid foam belongs on the attic side of the wall, and the fluffy insulation belongs on the "great room" side. Needless to say, the rigid foam needs to be installed with attention to airtightness, and the drywall on the interior side also need to be installed with attention to airtightness (especially if the wall has any electrical boxes).

    Check with your local building department to find out whether the rigid foam facing your attic needs to be protected with 1/2 inch drywall for fire safety.

  2. Michael Brackett | | #2

    Sorry, the pic was too large.

  3. Michael Brackett | | #3

    Thanks Martin!
    I wouldn't need OSB, just rigid insulation taped/air sealed?

  4. GBA Editor
    Martin Holladay | | #4

    This wall isn't structural, so OSB isn't necessary -- as long as the rigid foam is installed in an airtight manner.

  5. Michael Brackett | | #5

    I'm sorry, Martin, but I forgot to ask you if you suggest I use the fluffy without face/paper in my great room wall assy?

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |