Compact roof with cellulose — Minimum rigid foam
I’m considering a compact roof with dense pack cellulose in the rafter space, and rigid foam above the sheathing. I will be building in Zone 5, Rochester, NY. I’m shooting for a whole-roof R-60.
The 2016 NY code requires R-49 (cavity? whole roof?) and R-20 of rigid foam above the deck in a compact roof. Alternatively, the temperature of the sheathing must stay above 45F assuming an indoor temp of 68F and an outdoor temp of 26F (Average of Dec-Feb). I think that would put 45% of the R-value above the deck.
I’m assuming that the worst case location for condensation will be the center of the roof, where there’s the least framing and highest r-value. In that area, with 16″ centers, the framing factor should be 0.1. That means 2×12 rafters give an r-value of 35.5 to 39.5. This would mean an additional R-29 to R32 above the deck to prevent condensation.
That seems like a lot of foam. Did I miss anything in my calculations?
–John
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Replies
John,
Your questions are answered in my article, Combining Exterior Rigid Foam With Fluffy Insulation.
The results of my calculations are slightly different from yours. As shown in one of the tables I created for that article, in Zone 5 you'll need to have 41% of the roof assembly's R-value in the form of rigid foam above the roof sheathing. So you'll need about R-25 of rigid foam.
You can do that with about 6 inches of EPS. If you are using polyiso, the thickness will depend on how much you want to de-rate the polyiso's R-value to account for cold weather performance. If you assume that the polyiso has an R-value of R-4.5 per inch in cold weather (a conservative assumption), you would need about 5.5 inches of polyiso.
Thanks. That's a great new article.
--John