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Adding Ice and Water Shield Membrane to Existing Unvented Roof

ericdopierala | Posted in Energy Efficiency and Durability on

Hi,

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.

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Replies

  1. ericdopierala | | #1

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

  2. MartinHolladay | | #2

    Eric,
    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

        Eric,
        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.

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