I am converting my walk up attic into liveable space and would like advice on the specifics of the subfloor. The floor joists are 2x12s spaced 16" OC and currently there is 1/2" plywood glued and screwed to the joists. To add structural stability to the floor, bring it to the right height to line up with the stairs, and reduce footfall sounds I want to put down a layer of 0.08" rubber underlayment (AbsorbaSound) and then another layer of 1/2" plywood followed by carpet (can't use any thicker or will be too high). I have two questions
Should the top layer of plywood be glued and screwed into the floor joists as well as the bottom layer of plywood? I know if I was tiling the recommendation is to just screw into the 1st layer of wood and specifically not the joists to help separate any movement and prevent tile crack. But my concern is just adding solidness and preventing squeaks. I am a little confused by the NC 2018 residential building code whether 1/2" alone is sufficient or 5/8" is required (and if my 1/2 + rubber + 1/2 is equivilant to 5/8). Code R503.1 says 5/8" is required for 16 OC but then right after Table R503.2.1.1(1) says 7/16" is ok.
Should the baseplate for the walls go on top of the wood/rubber/wood subfloor sandwich or screw only into the bottom layer of plywood (with the rest of the subfloor brought up next to it).