Air barrier location in a double stud wall
I’m considering an R-40 double stud wall for Rochester, NY, zone 5, with the following construction:
1×3 furring (rainscreen),
Loadbearing 2x4s or 2x6s filled with cellulose,
Plywood (air barrier),
Gap filled with cellulose,
Non-loadbearing 2x4s filled with cellulose,
[Smart membrane (maybe)],
Paint (Class III vapor retarder)
I’ve seen a number of designs that put the plywood on the exterior surface of the inner wall. Located instead on the interior surface of the outer wall, will it be insulated enough to be protected from moisture buildup?
Here are my thoughts: Code lets you use a Class 3 vapor retarder if approximately 1/3 of the whole wall R-value is exterior rigid foam. Now, assuming an R-14 (whole wall) 2×6 exterior wall, my design also has 1/3 of the R-value outside of the plywood. Unlike rigid foam, my wall can additionally dry to the exterior. So it seems to me that 2x6s on the exterior should be safe.
However, it’s not clear to me if the 2x4s would be okay. The plywood isn’t quite as well insulated, but it can dry to the exterior.
Any thoughts on this?