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

Adding Ice and Water Shield Membrane to Existing Unvented Roof

ericdopierala | Posted in Energy Efficiency and Durability on


I would like to upgrade the roof insulation in my 1970’s bungalow with the roof needing to be replaced the timing is ideal.  With that, I will also be removing the drywall from the underside of the 2×6 rafters to assist with upgrading.

We will be living in the home during this project which leads to my questions.  I would like to remove the existing roof shingles and cover the entire roof in I&W.  This will help keep the house dry throughout the process.  Above the I&W I would like to install 4″ of polyiso, 1/2″ OSB and new shingles.  Below the existing roof deck I would like to fur down the rafters 1 3/4″ and add 7 1/4″ of mineral wool and then drywall.  This assembly will get me to about R- 53.  I understand this is short of the R-60 that is now required for zone-6 but it is the best I can do with the existing circumstances without losing too much headroom upstairs.

Do I need to worry about the I&W causing a moisture issue on the bottom of the existing roof deck?  Is there a more permeable product I should use since this is an unvented assembly?  Thank you for any input.

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. ericdopierala | | #1

    Any input would be greatly appreciated, I added a sketch of the assembly for clarity. Thanks

  2. MartinHolladay | | #2

    In Zone 6, you want at least 51% of the total R-value of the roof assembly to come from the exterior rigid foam layer. Looks like you hit that goal -- so your roof will be safe.

    More info here: "Combining Exterior Rigid Foam With Fluffy Insulation"

    1. ericdopierala | | #3

      Thanks Martin, I’m not sure I’m following how you got to 51%. From my research 4” of polyiso is R-23.4 and the 7 1/4” of mineral wool is R-30. Should I not fur down the rafters and install 5 1/2” of mineral wool instead (R-23)?

      1. MartinHolladay | | #5

        You're right, I guess -- the actual percentage depends on the R-value assumptions for the polyiso and the mineral wool; these R-values vary from brand to brand. Using your numbers, you need to adjust the ratio somewhat. That shouldn't be too hard -- you either want thicker polyiso or thinner mineral wool (or perhaps fiberglass batts instead of mineral wool).

  3. Expert Member
    Akos | | #4

    I would check your local code as most have R value expemptions for renovation. When it comes to expensive roof insulation, there is very little energy (and cost) saved with high R value roofs.

    Your best bet is to stick to code min (could be R30/R38 or "the most you can fit"). This would save you having to fur down the rafters and would reduce the amount of rigid above the roof deck.

Log in or create an account to post an answer.


Recent Questions and Replies

  • |
  • |
  • |
  • |