0 Helpful?

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 5:33 PM ET
Edited Aug 30, 2014 5:33 AM ET

Tags:

1 Answer

Select your preferred way to display the comments and click "Save settings" to activate your changes.
1.
0 Helpful?

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 5:40 AM ET

Other Questions in Energy efficiency and durability

Foil-faced batt insulation effect on wall construction?

In GBA Pro help | Asked by Michael Kalustov | Feb 27, 15

Insulating a low-slope EPDM roof from the ceiling below

In General questions | Asked by joseph moore | Feb 26, 15

Best way to have a wood fireplace?

In Energy efficiency and durability | Asked by Clara Kim | Feb 22, 15

Converting fiberglass+plastic basement insulation to rigid foam - worth it?

In Energy efficiency and durability | Asked by Jeff Watson | Feb 28, 15

Dense packing cellulose - height of cavity

In General questions | Asked by Graham Fisher | Feb 28, 15
Register for a free account and join the conversation


Get a free account and join the conversation!
Become a GBA PRO!