My house is similar to yours. Mine was built in 1951 and has 2x6 rafters. I put in 6" of iso-board in the sloped ceiling, R50 in the ceilings and R14 in the knee walls. In one of the other rooms upstairs, I had leftover R20 so it went into the knee wall. All of this insulation was Roxul. I sealed off the iso-board with low expansion spray foam.
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TomS
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