Adding Ice and Water Shield Membrane to Existing Unvented Roof
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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Any input would be greatly appreciated, I added a sketch of the assembly for clarity. Thanks
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"
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)?
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).
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.