Hello

See the attached detail. The garage will be heated (approx 10 C) and most of the garage has living space above, except the front 6 foot portion, which has a balcony with vinyl deck above to keep the water out. We are considering spray foaming R-20 to the underside of the roof deck (PWF plywood) – enough to avoid the underside of the deck condensing and treating it as a hot roof. We are located in Winnipeg, Canada. My concern is the joists and any thermal bridging – would this lead to condensation buildup in the cavity? We could put a couple of vents in the drywall ceiling at either end, bore some holes in the centre of the joists for cross ventilation if this was a concern. 

Our other idea is to strap this down with 2×4’s, use a 6 mil poly vapour barrier and put R-28 batts, then drill cross-ventilation channels through the joists and out each end using soffit vents. This would essentially created a cold roof scenario, but less risk of condensation.

I would appreciate any thoughts on this. Thank you in advance.