Currently designing a 2×6 wall assembly for a home in Minnesota – Climate zone 7.

The owner wants to use R-9 zip-r sheathing with fiberglass or rockwool batts for the 2×6 cavity. The reasoning for using R-9 is to get the advantage of exterior insulation but still thin enough to not warrant buying specialty tools or change the building process too much.

Here is his desired assembly:
-LP smartside lap siding
-3/8 plywood battens
-R-9 zip-R
– 2×6 wall with R21 fiberglass or mineral wool
– Smart Membrain vapor retarder
– 1/2″ drywall

Although this assembly may be bullet-proof in other climate zones – I’m worried this could be a risky assembly in climate zone 7.

From my understanding – in climate zone 7 you would need R-15 exterior insulation to eliminate the risk of condensation in the 2×6 wall cavity.  Is this still a large concern using zip-r where your sheathing will be nowhere near the condensation point in the wall?  what if you were to use a hydrophobic cavity insulation such as rockwool?  Would this still be a worrisome assembly if the wall could dry to the inside?

What if we were to step up to the R-12 zip sheathing?

We love the idea and simplicity of zip-r but making it work in climate zone 7 is a challenge.