I am working on the details for the roofing assemblies of two houses which are modeled in the PHPP to meet the Passivhaus Standard. We are in the Sierra foothills of California.

For the roofs, I am planning on metal roofing, over 3/4 furring strips, over 2 layers of EPS with staggered and taped joints, over Grace Ice and Water shield on plywood roof decking. The rafter cavities are insulated with cellulose and the assembly will dry to the inside.

Recently, I ran across some information that has me wondering about the durability of the EPS under high temperatures. This is from the EPS Molders Association (http://www.epsmolders.org/4-ftc.html):

“In roofing, Type I EPS material provides the dimensional stability and compressive strength necessary to withstand light roof traffic and equipment weight at reasonably high surface temperatures. EPS foam insulation may experience dimensional and property changes when it is exposed to extreme temperatures greater than 167°F. However, low density EPS not subjected to load will show no noticeable loss of dimensional stability at temperatures up to 184°F. Duration of temperature, external load conditions and density are the variables affecting foam insulation at elevated temperatures. EPS should be adequately protected from temperatures greater than 165°F during installation and may require the use of cover boards, reflective ballast, or a light colored membrane depending upon the roof covering system involved. “

My question is if I use foil faced EPS on the top layer pointing up into the airspace am I providing enough of a temperature break to protect the foam in the long term? Temps of 180 seem quite possible in the air gap.

Thanks very much for any feedback.