My son purchased a 1940's home and we plan on making the attic space a bedroom. The roof rafters are 2x6 on 24" centers, we would like to add as much R value insulation as possible. Our thought is to extend the depth of the 2x6 by adding or attaching a 2x4 to the rafter, making it now a 2x10. Our thought was to glue and screw the 2x4, then add 9.5" batts for an R value of 30. Then, attached to the added 2x4, we would cover it with 1/2" drywall. There would be a ceiling about 8' tall and a 4' knee wall. Is this doable?