[SiberianDVM]

I just finished cleaning out 50 years worth of junk from my attic. The house is 1300 sq ft, built in 1939, with 2x6 joists on 16" centers. Only a small area around the stairs is floored, with 1x6s. I insulated between the joists years ago. It gets VERY hot up there in the summer even with a gable fan and eave vents, so I want to cool it and put down some more flooring for storage.

My questions are:

Flooring: 15/32 plywood OK? I will probably have to cut these to 2' x 4' or 8' to get them up the stairs by myself? OSB would be cheaper but would it be OK?

Insulation of roof between rafters (2x6) and end walls (2x4) : 3.5" roll fiberglass with vapor barrier to inside? Or no vapor barrier?

Thanks in advance.