Our house in Northern Ohio was partly built in the early 1800's with timber frame construction. The 'newer' portion has balloon framing. Both roofs use what are more or less 3"x4" rafters 32" oc -- the rafters and structure remain surprisingly straight. The upstairs rooms have knee walls and a small flat ceiling, but it primarily amounts to fairly low cathedral ceilings. There was no insulation, and I have removed the old plaster and now need to figure out how best to insulate. Headroom is an issue, so adding a great deal of additional framing is a challenge. Here are my issues and ideas for solutions:
1) In the Timber Frame portion of the home, there is old 5/4 pine sheathing for the exterior and then studs installed below the beam to create a flat wall panel. This leaves a 5" unconditioned space between the knee wall. I can insulate this knee wall, or the area behind the knee wall. If I use fiberglass between the studs, should I install something like a Tyvek barrier behind the insulation, or against the exterior sheathing, or both?
2) The second area of this is on the opposite side where the roofline continues down to a lower level room, thus leaving a triangle shaped attic space with the downstairs ceiling below the attic floor, and the knee wall separating the finished area of the room. What is a reasonable solution for insulating this area? I would plan to install fiberglass insulation in the knee wall backed by tyvek, and insulate the floor cavity.
3) The cathedral style ceiling (in both portions) has no ventilation and little space for insulation -- headroom is a premium, so taking more space than needed is a concern. My proposed solution is to install 2x6 joists sistered against the existing 3x4's and spaced to leave 6" from the roof deck. Then to fill this space with XPS foam board sealed at any joints/edges (alternate spray foam). Then I will install a solid panel of 1" XPS foam board over the joists covered by the wallboard. This should give me a R35 insulation rating and a very well sealed cavity, although with no ventilation. I loose 3" of headroom by doing this. Is an R35 viable for such a roof structure, or are there other ideas. Thanks for your thoughts.