Double stud wall vs 2X8 with strapping
I am planning to build a slab on foundation “pretty good house” in northeastern Oregon (Climate Zone 5). I have considered using a double stud wall (outside wall is load bearing) to yield ~R-40 center of cavity using R-4/inch mineral wool batts. The design from outside to inside would be fiber cement cladding, drainage plane house wrap, plywood sheathing, 2X4 wall, 3.5″ space, smart vapor barrier/airtight layer, 2×4 wall, sheet rock. This provides 10.5″ for insulation.
The concern I have is that having the vapor barrier/airtight layer on the outside of the inside (non-load bearing) wall makes it difficult to conduct air sealing after the sheathing is in place and requires install of batt insulation before the sheathing is installed. I would rather have the walls and roof constructed and then take my time to insulate and air seal after dry-in.
I am considering 2X8″ walls with strapping to provide 9-3/4″ of space for insulation. The design from outside to inside would be fiber cement cladding, drainage plane house wrap, plywood sheathing, 2X8 wall, smart vapor barrier/airtight layer, 1″ polyiso spacers between studs and 2X3″ horizontal strapping, sheet rock. My thought is to use 2X4″ studs, either staggered or in-line with a 1/4″ space between for a thermal break. The staggered studs would provide more of a thermal break, but would make it harder to use batt insulation. Another insulation option would be something like Spider Plus blow in fiberglass insulation. Dense-Pack cellulose is probably not an option because there are no local installers.
The 2X8 wall will use more lumber, have a lower total wall R-value, and require more labor. Assuming labor after dry-in is free and time is not an issue, does the 2X8 option make any sense? Is there a better way of providing an air tight layer/vapor barrier with a double stud wall that can be installed after sheathing and dry-in?
I would appreciate any thoughts.
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