Adding Ice and Water Shield Membrane to Existing Unvented Roof
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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