I'm looking at insulating the ceiling of a large gothic barn, in the process of converting it to a house. The problem is trying to preserve the shape, whilst getting sufficient insulation.
The rafters are laminated curves (like a glue-lam, but nailed and screwed, not glued), and the cavities between are only 4" deep (real, not nominal). Beyond that, there's thin lath, then plywood sheathing, then an asphalt shingle roof.
So far, the plan is closed cell foam in the cavities, then a 2" high density foam board over that (will hopefully bend to conform smoothly), and screw drywall or wood finish with 4" screws to penetrate it back into the rafters. 6" x R6.6 = R39.6 However, it's about 7500sqft of coverage, so the closed cell foam is going to be about $30K, plus another $7.5k for the foam board plus install of that, so ~ $40K. Ouch.
Anyone have a better way? I think building anything out with wood to match the curves is going to be very difficult.
I've seen some people spray foam round the edges of rigid foam board. The board seems to be about half the price of the spray foam per volume. Does it really give good tight air-sealing, etc? I guess another possibility is to mount foam-board in the outer 2", then spray foam the next 2, then board again. There's a bunch of roofing nails sticking through, but maybe it'd just press in OK.