old hip roof with finished attic dilemna
I have an ca 1928 36' by 28' two-story frame home with about an 8/12 pitch hip roof located in eastern Wisconsin about three blocks from Lake Michigan. There is a finished attic, but no insulation. The wall sheathing and cedar shake siding come up between the rafters to meet the roof decking. The sub floor in the attic continues past the knee walls and likewise continues between the rafters to meet the roof decking. The combination of wall sheathing plus attic sub-flooring make it totally impractical to create vents under the eaves. The obstruction to venting is compounded by the structure of the eaves, for the main roof rafters, which are 2x6 on 16 inch centers, do not continue beyond the wall. The eave rafters are decorative exposed rafters on 24 inch centers and are tied into the roof by resting on the top plate of the wall and by tying into blocking every other roof rafter bay about two feet in. On the top the interior finish on the hip roof does not permit any venting to flow to the top except in about the center 8 feet of the building, and that for only the front and rear panels. The finished side of the roof creates pockets everywhere the adjacent panels of the roof meet. The knee walls are only about 30 inches tall, so most of the roof is above the finished space.
I want to insulate the attic so this finished space is truly usable, but cannot determine what is the best course. As it is, it is too hot in summer (deforms plastic models) and too cold in winter.
Both fiberglass and cellulose (batts or blown) require venting and, I am told, so do asphalt shingles, lest they get too hot. (Mine are charcoal gray.) Some web sites say that sprayed in foam (or flow in slow-rise) can be used to completely fill the space between the rafters. I understand that the closed cell foam creates its own moisture barrier, but it would seem to be a problem for the shingles. The fact that both wall sheathing and attic sub-floor both extend to the roof decking creates an inaccessible space at the side of the house between the attic floor and the second floor ceiling, which cannot easily be filled with anything.
Does anyone know how to insulate in this case?
Last edited by Rand; 07-15-2009 at 11:50 PM.
Reason: typo and clarification