Steve, they recommend a lot of insulation for attics and ceilings in our area-like R50 or R60. You will have a real tough time achieving that with a vaulted ceiling. I'm not sure but I think it would be like 18-24" of fiberglass. Our first house was a bungalow and I finished the attic. The roof rafters were only 2x6s as I remember and were already sagging quite badly. I solved both problems (no room for insulation and the sag) by reinforced them by screwing I think 12" high 1/2" plywood alongside them with a 2x2 screwed to the bottom for something to screw the drywall to. I started on each end then strung lines to fill the rafters in so the ceiling would come out in a nice plane. I couldn't go any more because there was only so much headroom.You have to be patient and meticulous with the insulation. With the shingles, plywood, fiberglass, visqueen VB, and drywall I'm sure I probably only got R35 or 40 but it was always comfortable and our heating bills went way down because there was very little original insulation in the floor.