What to do with wall facing garage
I’m building a small living space(800sq ft) into a building that has a heated shop to one side of the living area. The living space is basically a rectangle with 3 sides to the exterior outdoors and one side facing the heated shop. The walls facing the outside have steel siding, wrb, sheathing 5/8″, 2×6 16oc packed with roxul, 2″ polyiso, 1/2″ plywood, 2×4 service cavity with roxul and finally 1/2″ drywall. All layers taped and caulk appropriately. Ceiling will have plywood on truss bottom cord and service cavity and drywall then cellulose r60 loose filled to vented attic. Slab is insulated with vapor barrier. This is in zone 6a.
Now my question is do I have to bring the wall facing the heated shop to the same r value as the outdoor facing walls? It’s currently just a 2×4 16″ oc bare stud wall. The shop will be heated to 55-60 degrees. I was thinking of putting plywood on the side facing the heated shop to tie in with the plywood air barrier in the other walls and ceiling then some polyiso 2″ foil faced for the vapor barrier(if a vapor barrier is needed on this wall) then steel panels for the shop interior face. I guess a layer of drywall is also needed between living area and garage for code. Would the interior layer of drywall count or does the drywall have to be on the garage side?
If this wall has to be same r value as the outdoor facing walls how can I get to that r value?
There is only about 6 inches of space towards the heated shop area to get this r value and 2×4 wall already takes 3.5″. My guess is the 3 exterior walls r values is somewhere around 25-30.
Thanks for any help.
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If the shop is heated it's conditioned space, and the wall separating it from the living quarters does not have to be insulated unless you need good to excellent zone isolation. Even cheap R11s in a 2x4 studwall is plenty of zone isolation. Presumably the shop walls are insulated?
You don't need a vapor barrier in that wall either. If the shop is 55F and the living quarters is 70F, the relative humidity in the living quarters would have to be over 60% for moisture transfer of any significance to be happening. During the summer the indoor RH might go that high, but during the summer the shop will be well over 55F (possibly warmer than the more fully conditioned living quarters.
Thanks for your help Dana. So it sounds like all I have to worry about is getting a good air seal on the wall and the fire code sheetrock.