Rigid foam, OSB, spray foam: Can I have my cake and eat it, too?
I’m in the process of designing a home in West Central Minnesota (Zone 6), which we will start building this spring (2017). I’m evaluating SIPS vs Stick frame for my walls but to do that I need to determine what my typical stick frame wall section will consist of.
After many hours of research (many here on GBA, thanks guys), I have settled on some “ideal” design parameters:
– Traditional OSB sheathing. We can have high winds in this area as well as tornados so this will give me some peace of mind.
– A thermal break for the studs and top and bottom plates. Rigid foam on the exterior seems the best way to acheive this, but it also introduces other problems. I’m thinking of only having 1.5″ to 2″ though.
– Closed cell spray foam in 2×6 wall cavities.
Now, the recommendation for installing rigid foam on the exterior in zone 6 is R11.25 (to keep the OSB warm enough to stop “condensation”), and the recommendation is to no vapour barrier on the interior when rigid foam is installed.
I don’t really see the need of installing 2.5″+ of rigid foam to create the thermal break…so if i did install say 1.5″ then the OSB will be too cold, right? But if I’m using spray foam on the inside, then won’t that create an airtight and vapour impermeable barrier? Or is that wishful thinking? If did create that barrier, then the only moisture coming into contact with the OSB is from the exterior, and given that the rigid foam is so thin, then can that moisture dry to the exterior?
One more question – I’m putting a lot of effort into stopping thermal bridging across the studs, but does anyone have a figure on how much energy is lost through TB? I’ve seen the infrared pictures but haven’t been able to track down how much efficiency this will gain.
Thanks in advance for any feedback and I’m looking forward to hearing what the experts have to say.
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