Any reason that I couldn’t/shouldn’t cut & cobble 1″ foil-faced polyiso into rafter bays and then spray foam (closed cell) over it? The thinking is that if I ever have to replace the roof sheathing, I won’t also have to rip out the ccsf.

I’m remodeling a mudroom that was exterior space. A previous homeowner enclosed it, used fiberglass batts between 2×4 rafters up against the 1/2″ plywood roof sheathing, then 1/2″ drywall. I’ve gutted everything, no sign of sheathing rot, which is great because that portion of roof was not vented. But at ACH 17… pretty sure there was enough air moving to dry things out. No feasible way to vent this portion of roof, I can’t afford to reroof and insulate on top. Climate Zone 6, southwest Montana.

It’s a fairly small area at 13’x10′ floor space, and it’s prohibitively expensive to have someone come out and spray foam the underside of the sheathing, but I can FrothPak it for less than a thousand bucks.