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EPS over polyiso on reroof?

So based on everything thing I am reading should we be using R3 R4 or R5 for each inch of Polyiso since it decreases in performance as it gets colder. I seem to be seeing some variation in Answers. I am in zone 5 and was planning on putting 5 inches of Polyiso.(thinking that was R 30 Roughly but if is is only R 15 when coldest that is a problem) Now it seems like I would be better served with three inches of polyiso with EPS on top. I have access to 1.5 or 2.75 inch EPS. Would I still be OK with 3 inches of polyiso and 2.75 inch of EPS on top?

Asked by terry grube
Posted Aug 29, 2014 6:33 PM ET
Edited Aug 30, 2014 6:33 AM ET

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Terry,
Q. "Would I still be OK with 3 inches of polyiso and 2.75 inch of EPS on top?"

A. Yes. The EPS will warm up the polyiso and improve its performance.

In your climate zone, an unvented hybrid cathedral ceiling only needs R-20 of rigid foam insulation above the roof sheathing. Your plan -- about R-18 of polyiso capped by R-11 of EPS -- will give you about R-29 of rigid foam.

Answered by Martin Holladay, GBA Advisor
Posted Aug 30, 2014 6:40 AM ET

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