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Wood shop insulating question

averagejoe | Posted in General Questions on

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?

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Replies

  1. Expert Member
    Michael Maines | | #1

    Joe, EPS is fairly vapor-permeable so if you have to skimp on exterior insulation, it's the best option of the foams. It would be better to keep at or above the IRC-prescribed ratios, or to use an even more vapor-open exterior insulation such as wood fiber or mineral wool. But since you won't have a lot of interior moisture sources, you will probably be ok. You should still meet the IRC requirement of a class 3 vapor retarder at the interior. Painted drywall would work, but variable permeance membranes are easier. Because it's a shop and not a house, you might be ok without one, but I wouldn't risk it.

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