Talking to a friend about a personal project.

He is converting an older detached 1 car garage to a small art studio for his mother in law.

CZ 4C (Portland OR)

He’s asking about insulating the structure. The wood clapboard siding is nailed directly to the studs. The roof is OSB with asphalt shingles. The garage door is getting deleted and turned into French doors. A spray foam contractor proposed spray foaming the walls and bottom of the roof deck to 2.5-3 in thickness. I am hesitant on that idea of the walls because that would put the foam in direct contact with the wood clapboards and be a recipe for rot or paint failure every few years. I agree with spraying the bottom of the roof deck.

I proposed either
placing 1/4” or 3/8” plywood in each stud cavity and then either spray foaming or rockwool in the cavities
or
using a material like 30lb felt or commercial house wrap or a more exotic WRB gapped off the back of the clapboards 1/4” and then insulating. I’m not sure it would be a good idea to sprayfoam to a fabric like product?
or
just filling the cavities with mineral wool and moving on with life. 

The goal is not airtightness or high insulation value but rather just keeping the space kinda conditioned as this will not be living space. 
Ideas? Feedback?