EPS vs Polyiso for roof top insulation over cathedral ceiling
I know the cathedral ceiling topic has been beat to death but I am conflicted as to a specific area in a new build and which material is necessary. The roof in question is 3/12 pitch framed with 2×10 rafters. The roof was sheathed with zip panels and tied into the wall sheathing before overhangs are framed to allow the air barrier to be continuous. I am in climate zone 4 in Canada (same as zone 4 in the US as far as I can tell?) and I am pursuing step code 5 so the insulation used in the rafters will be R40 which is more than required. The ratio provided in the relevant code is 0.20 outboard to inboard. So the minimum required roof top insulation is R8. It is my understanding that it can be safer to use more outboard insulation so I had intended to use 3” type 2 EPS R12 because it is made locally and cheap. Is there any considerations for the use of EPS vs Polyiso? I have seen Polyiso used in this application much more commonly but didn’t know if there was a specific reason for that. The type 2 EPS at 3” has a very low permeability at around 1 I believe. Any insight is much appreciated!
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User-**1153,
Sounds like you've done your homework and are building a very nice home.
I think Polyiso or EPS would work just fine. I wouldn't worry about vapor permeance so long as you are well above the minimum foam-thickness requirements for your climate zone (which sounds like you are). Thinner foam is probably easier to work with and screws will be cheaper and easier to install. Also, thicker foam may complicate your overhang/trim detail so make sure you have a plan for that before you opt for thicker foam.
You're good either way. It's nice to see somebody building a 'hot roof' the right way for once! Well done.
With EPS and shingles, you might get .3 perms of drying to the exterior. Not much, but more than 0.0 perms of foil faced polyiso. If you nicely exceed the R value minimum, I wouldn't worry about it. IRC R806.5 says R10 and R15 for zones 4C and 4A/B.
Derate polyiso to ~R5, which typically makes it non cost effective.