Cathedral ceiling reinsulation
I am in zone 8. One cathedral ceiling I have to rebuild consists of 2″x6″ rafters with 2 layers of 1.5″ of white expanded polystyrene sandwiching a layer of aluminum foil paper attached to the underside ,or interior side.Old pest damaged fiberglass insulation is between the rafters. Plywood and shingle is on the exterior of the rafters. There is no other structure to the interior of the rafters other than the polystyrene.
The plan is to remove the shingles, plywood,old fiberglass insulation and reinsulate.
I assume that placing 6″ of polyisocyanurate above 5.5″ of newly placed fiberglass between the rafters would create a double vapour barrier , and therefore be problematic.
Alternately I could spray 6″ of closed cell urethane between the rafters, directly in contact with the most exterior layer of polystyrene, with an air space above; then plywood and shingle, and arrive at close to r-50. Would this work ?
GBA Detail Library
A collection of one thousand construction details organized by climate and house part