Exterior EPS Rigid Foam with Open-Cell Spray Foam Cavity Insulation
Project Located 8200′ in mountains of CO in climate zone 7B. Cold and Dry. For thermal bridging I’d like to put foam board insulation outside of the 7/16″ OSB sheathing. Walls will be 2×6 24″ spacing with 16″ spacing on tall walls. Cavity insulation will be 4″-5″ of 3/4lb open cell foam. I’d like to add 2″ of EPS on the exterior sheathing behind a wood cladding (lap/vertical shiplap/board and batten). I believe this will work based on the open cell foam being vapor permeable and 2″ of EPS is rated at 1.75 perms so this should still allow some drying to the outside. The reason for the question is this wall would not have 45% of R Value outside of the sheathing that Lstiburek recommends (would be closer to 25%).
If we do go with 2″ EPS foam I’d like to strap it horizontally so I have something to nail my vertical cladding to. We could run a circular saw channel periodically to still allow some top to bottom airflow between foam and cladding.
I looked into Zip R and between polyiso not performing as well in the cold and looking at the 2.5″ insulation and the fastening that would require, I didn’t think that was a good option.
Rockwool comfort board seems hard to get and very expensive right now. Nobody is stocking it locally.
Thanks!
GBA Detail Library
A collection of one thousand construction details organized by climate and house part
Replies
If you're going to cheat the R-ratio that badly, DO use a well-detailed interior side "smart" vapor retarder/air-barrier(when detailed) such as MemBrain or Intello on the interior side, and make the sheet rock as air tight as possible. With multiple electrical & plumbing penetrations the air-sealing details add up, reducing the long-term air tightness reliability.
With board & batten siding there is no need to add details to promote vertical air flow. It's inherently air-leaky enough that the spaces between siding & foam will dry just fine toward the exterior.
Using cellulose rather than open cell foam in the cavities would do a lot toward mitigating wintertime moisture accumulation in the sheathing, wicking and redistributing any damp-ish spots. If you can't dense pack it with a blower, use cellulose (or hemp) batts, sculpting & installing meticulously for a full compression fit. For better air tightness when using batts, put a bead of polyurethane caulk (or the purpose-made low expansion foams marketed by fiberglass batt companies) to seal all of the framing to the sheathing along the perimeter of EVERY stud bay, and use housewrap tape on any horizontal seams between sheets of sheathing. Open cell foam has effectively zero hygric buffering capacity, where as cellulose or hemp (or even cotton) insulation has quite a bit, sharing the seasonal moisture load with the moisture susceptible wood. Cellulose reduces the peak & average seasonal moisture content of the structural wood, and it's borate fire retardents help avoid fungus & insect damage getting started.