I am remodeling a victorian house in Nebraska. While removing damaged plaster and lathe, I found that the roof joists were 2 1/2" x 4 1/4" with 'no' insulation. (No wonder I had $750 fuel bills.) What should I do to get the most 'R' factor and what should that 'R' factor be ??? I was figuring insulation, vapor barrier, and drywall. Any suggestions would be greatly appreciated.