Hi!
I have a 1.5 story house as well. When I recently renovated one upstairs room, I put 1/2" thick by 1" wide spacers against the roof rafters for an air gap, and then filled the space with layers of 1 1/2" isoboard. That gave me R27 in the rafters. On the small ceiling portion I have about R52 of rock wool insulation. The knee walls in that room have R20 but the end wall ended up being only R14 (before I came to this site).
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TomS
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