I don't see a need for 12" of insulation in an attic... the insulation only should be as high as the joists, and should not have to be compressed to lay a floor of plywood. As long as the insulation is not being compressed, you don't have to worry about sacrificing insulating value for having an attic floor. Both homes I have lived in, we laid a plywood floor, just for as much storage space as we needed. I view an attic as valuable for storage, otherwise it is wasted space. :o) remember to wear the appropriate face mask while working up there around the insulation! good luck.