My wife and I want to finish the attic in our 85 year old bungalow and turn the attic into a guest room or bedroom. First we need to insulate the slopping roof. We had a new roof put up about 3 years ago with 6 roof vents along the south side of the roof near the ridge. Underneath the new shingles, felt and plywood are the old 1x7 horizontal runners. Underneath those are the old 2x4 roof rafters spaced 24" oc. The rafters actually measure 1 5/8" x 3 3/4". There isn't much room for insulation with rafters like these. I want to use 2" thick polyisocyanurate rigid foam boards with aluminum foil face on both sides, in between the rafters. This type of isulation has the highest R value per inch that I know of. There should be plenty of air circulation between the iso and the horizontal runners/plywood and vents. Then we want to finish with wallboard, either 1/2'' or 5/8''. I have read many messages on this board about insulating the attic but no one has mentioned using rigid foam boards between the roof rafters. We reside in Portland Oregon where the climate is fairly mild year 'round in relation to other parts of the country. I was wondering if there is a good reason for not using this approach. Any response is appreciated. thank you