Choosing Exterior Rigid Foam Insulation
Hello, long-time reader of this forum – hoping someone can help alleviate my confusion re-exterior rigid foam insulation.
I’m about to embark on building my own single-story family cottage in Muskoka, Ontario, Canada. Climate zone 4a.
My wall construction is as follows:
drywall > vapor barrier > 2×6 stud wall w/fibreglass R-20 batts (unfaced) > 7/16″ osb > house wrap > 1.5″-2″ rigid foam > 3/4″ strapping > siding
I’ve read plenty, and watched all the videos i can find, read manufacturers spec sheets etc etc – I’m still in a muddle about which exterior rigid foam I should be installing. Everything I see in big box stores is faced, with very low perm – (I’m thinking I want the wall to be able to dry outwards so I don’t really want the vapour ‘barrier’ element of the foam). Price, sadly, is a consideration – otherwise I’d probably use rockwool comfort board.
Whats the best bet? I’m looking at something like Durospan GPS R10 as the price vs R-value is good – this appears to have a ‘peelable’ face – is the best bet simply to peel both sides and/or not tape the seams in the hope of not trapping moisture?
Any help or suggestions very much appreciated, I’m just going around in circles reading myself into more rabbit holes 🙂
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Replies
If you're looking for a vapor open rigid foam, your options are unfaced EPS, unfaced GPS, or "roofing" polyiso with a fiberglass mat or kraft facer. Foil or poly faced rigid foam is out regardless of the type, as is XPS since that one is a pretty much a vapor barrier by itself in these thicknesses.
Note that at 2" thickness, any of these rigid foam materials isn't really going to allow for much drying. I would probably use polyiso myself for the best R per inch, just be careful not to get the foil faced version if drying is a concern.
Bill
Thanks for the response Bill, and the suggestions.
So the GPS I mentioned, assuming I peeled the poly faces off front and back (it appears peelable), sounds like it could work.
Is also not taping seams sensible, to allow some air movement, or bad idea? Thanks again.
Peeling off the facer might damage the foam. The facer does not always come off cleanly. I wouldn't count on that working.
I would tape the seams. I would prioritize keeping bulk water and critters out over trying to allow for some small amount of additional drying.
Bill
Thanks again for the response. So essentially, stop worrying about potential drying through 2" of foam, and just install/tape it up?
Im fully on board with this answer as it's the most straightforward! :)
Building this for myself I just don't want to mess it up or make a problem for myself down the line.
Yeah, pretty much. I'd put your 2" EPS around 1-2 perms or so, which is pretty close to a Class II vapor retarder. You'll get a small amount of drying, but nothing like what you'd get with something really vapor open like rigid mineral wool. I'd tape it up to keep out critters and bulk water and call it good.
Regarding the interior poly that I know is required by code in much (all?) of Canada, we have Canadian members here who have said that these assemblies in Canada are generally pretty robust, and don't cause problems, so you should be OK.
Bill