I have a customer who wants to insulate the ceiling of a 1930's era lake cabin from the exterior so he doesn't cover the exposed wood on the inside (2x6 t&g and log beams). I would like to get as much r value as is practical for a northern MN cabin and also create an air space to keep the roof cool. My idea was to lay felt on the existing roof deck, then nail 2x4s +half inch plywood (2in total height) flat over log beams (4ft oc), then lay 2in foam between nailers parallel to foof pitch. Then lay another layer of foam (1in or 2in) perpendicular to the roof pitch with seams ofset. Then add a second set of 2x4 nailers over the first set with plywood on top to create air space. Then a sophit and ridge vent? What do ya think? any better ideas?