Zone 7 (Saskatchewan) — new four-season garage / workshop
Apologies if this has been asked before, I have read a lot of similar designs but have not found someone using this same assembly in a very cold climate. Here is the plan:
Exterior to interior:
5/8″ Fiber cement board siding (Hardie Plank)
3/4″ (1×4) wood furring strips (to hold insulation and act as vented rain screen)
3″ Mineral wool (2 layers of 1.5″ roxul comfort board 110, staggered seams). R=4.2*3=12.6
1/2″ Fiberglass gypsum (GP DensElement, with PROSOCO fast flash to act as ARB and WRB). R=0.5
5.5″ (2×6) wood studs with DP cellulose in cavity. R=18
1/2″ OSB with MemBrain (Vapor retarder and second ARB). R=0.6
1.5″ (2×2) wood furring strips (for electrical chase, this will be open without insulation)
1/2″ Drywall with latex paint
Total thickness is 12-7/8″ and an R value of 31.7. Sound transmission should be very low considering the mineral wool, DP cellulose, and drywall attached to furring strips. Did I mention this is a workshop in a residential area and I plan to make a lot of noise? Sound transmission is important to me which is one of the big reasons I want to avoid foam insulation, I need mass to dampen the sound. I don’t trust the durability of EPS or polyiso and XPS is not “green” enough for my liking.
-Solid construction with 2×6 frame and OSB shear panels. OSB installed on warm side to prevent moisture and mold issues
-Good R value for northern climate and done with “green” building materials (I hate the over use of that word but you know what I mean).
-Wall can vent to the outside and inside (or is this a con in climate zone 7? We typically only vent to the outside)
-Exterior fiberglass gypsum is moisture and mold resistant and should be OK if it gets wet, can dry to the outside through the mineral wool.
-Wall is fairly thick but I believe most energy efficient walls are.
-Likely more expensive than the REMOTE wall or double stud.
-Ratio of exterior to cavity insulation is low (41% exterior insulation), but does this even matter if the assembly can dry to the outside? I have read that 70 to 75% exterior insulation is ideal for foam insulation for climate zone 7, but this shouldn’t apply to mineral wool.
1. What are your thoughts for the durability of this wall?
2. Would the MemBrain be required or would taping the seams of the OSB do the same thing for a air barrier / vapor retarder?
3. Any changes you would make to increase durability?
4. Any changes you would make to reduce cost?
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