Hello All,

Building contractor here trying to up his game thermally and using a personal project as a “guinea pig”. In anticipation of a soon-to-be move to a family property on the North Shore of Lake Superior in Minnesota where we will be building a new house, I am building a new woodshop addition onto the garage up there so I have it available for both my business and my house build. (Zone 8, but often warmer along the lake shore, heating, no cooling)

I will be using a combination of rigid foam and mineral wool insulation. I feel pretty good about the vaulted ceiling…a slightly modified detail based on one of the assemblies in the GBA article “5 cathedral ceilings that work”…2 x 10 rafters at 2′ o.c. with added 2 x 4s to the underside to create a 12 3/4″ cavity, 1 3/4″ ventilation channel using air sealed smart baffles, 2 layers R-23 mineral wool batts, 1 1/2” polyiso with taped seams on the underside, 1 x 4 strapping running opposite the rafter direction, and then rough sawn 1 x 10s running plate to ridge for the ceiling finish. Should be R 56ish with minimal thermal bridging.

My question is on the walls. I know the recommendation for foam thickness on the outside of a 2 x 6 exterior wall (R 23 mineral wool batts) in my zone is R-15, but with EPS foam, a rain screen, and the siding thickness, my windows (installed at sheathing and WRB plane) will be really far back . Since it’s a wood shop with no cooking, bathing, washing, etc., I’m anticipating pretty low winter time humidity levels (just some occasional human respiration). I’m fairly  confident in my air sealing details on the walls, but would I be inviting disaster with only 3″ EPS on the exterior (2 layers 1 1/2, seams offset, R 8)?

Theory says I would be better off with only 2 x 4 walls and less insulation in them to keep the backside of sheathing temps higher, but that seems to be going backwards. It takes the same amount of time to build 2 x 6 walls and install thicker batts, so it’s only a slight increase in material costs for an extra R7. The interior wall finish will be 1 x 6 pine nickel gap siding. Since I will have what I consider to be a pretty bomb proof exterior wall air barrier, the “leaky” interior wall finish doesn’t seem problematic to me. Should I use a Membrain or Intello on the inside, or skip it for maximum drying potential?