I'm putting in 12mm floating laminate with a 4mm felt underlayment in my living room and I noticed some bowing in the current subfloor to the point where I can see it while standing up. The high points are on the joists which are spaced 24 inches apart on center. The low points are of course in between those joists. I was wondering, could I use 4x8 foot by 1/2inch piece of plywood or osb on top of the current subfloor to make it more even? Which material is better for a second subfloor? Also I heard there is a paper of sorts that can prevent the two subfloors from squeaking together or is there a better way to install the second subfloor. Thank you all in advance.