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

Rigid Foam Insulation for Walls and Roof

cgmorgan | Posted in General Questions on

Hi all,

I have been playing around with a few ideas to adequately insulate my Cape Cod style home. I’m wondering if there’s an issue with having 1” polyiso on the interior of the walls (dense blown cellulose underneath), and polyiso and EPS above the sheathing for the roof (with cellulose batts between rafters)? I’ve read through as many of the pertinent articles as I could find and know both are doable independently, but haven’t seen anything about them used together. I just want to make sure there’s no issue with the walls drying to the exterior but the roof to the interior.



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. Expert Member


    It will work fine. Neither cares what the other's drying path is.

    1. cgmorgan | | #2

      Thank you for the confirmation Malcolm.

      If I may, a related follow up to that situation: winter is upon us, I have a completely gutted second floor (in Montreal zone 6 to boot) and all roofing contractors are solidly booked until next spring. I’d like to get all of the interior work done over the winter and redo the roof at the earliest possible moment, but that would leave me with about 6 months of the cellulose batts in the rafters in direct contact with the sheathing without exterior rigid foam. I know this is not ideal, but given cellulose’s handling of moisture and proper air sealing, is it doable?

      Thank you,


      1. Expert Member
        MALCOLM TAYLOR | | #3


        I don't know. My gut feeling is it will be fine - especially if you aren't generating a lot of moisture. But I haven't worked in a cold climate like Montreal for over 30 years - and when I did, I didn't know what I was doing. Hopefully someone with relevant experience will chime in.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |