Hello,

I am hoping for advice on my historic home. The house is a 150 year old italianate home in climate zone 6a, with a low-hipped roof and no true top “ridge.” The roof assembly is unvented, and accessible from the unfinished 3rd floor space, which is a large, open 40×40 ft. room, The rafters are full dimension 2×8 boards.
I want to use this large space, and need to insulate the rafters. I have read multiple posts here, and understand that foam board insulation on the exterior of the sheathing would be best, followed by closed cell spray foam on the interior as second best. However, the roof does not need to be replaced anytime soon, and I am looking for options to avoid interior spray foam for health/environmental concerns. Are there any other options? Would installing full size foam board insulation on the surface of the rafters (not cut and cobble) work, followed by tongue and groove interior board siding? How thick of foam boards? If so, would I leave the spaces between the rafters empty?

Also, the rafters come down onto a 3 foot hip wall of 3 courses brick (which is the exterior shell of the house). Would the foam board work over this as well, leaving a space between bricks and foam board that communicates with the wide overhanging eaves?

Thanks in advance for any advice.