I recently acquired several 3 3/4" x 4' x 8' closed cell poly iso foam core faced with heavy black fiber-reinforced organic felt (non-asphaltic) on both sides. The brand name is AC Foam II. This material is apparently used to insulate commercial buildings with 'hot asphalt or coal tar BUR, modified bitumen, and single-ply membrane systems'. I was wondering if I can used it to insulate my pitched roof by putting it between the sheathing and roof shingles instead of cutting it to size and putting it on the inside between the rafters, which would lower an already low headspace in the attic even more. I know two big issues are getting fasteners long enough to reach the sheathing and covering up the exposed ends. Can this work?