Flooring – Can Tongue and Groove plywood be used if interior walls already in place

Use screws instead of nails to hold the plywood down. If you can, screw it into the joists, not just the existing floor. You can also put down some glue betwen the plywood & subfloor to prevent the plywood from moving at all. Use tongue-and-groove plywood if possible.

If that doesn't help (and you should be able to do that & check to see if you still have squeaks before putting down the hardwood) you might need to beef up the joists, or add additional bracing, from underneath. Not so hard if the ceiling below is open, a lot of work if it's finished.

The way we lay flooring is to use subfloor adhesive and lay a bead down the whole length of the groove. You wedge it together till the gap totally closes. Doing this in addition to using tongue and groove plywood/OSB accomplishes two things:

1. You have an airtight floor membrane. This gets rid of heat loss and also the intrusion of moist damp air possibly mold contaminated from the crawl space.

2. You have a much stronger floor that is seamless which can carry more weight and not have bounce because the edge is supported by and locked into the adjoining sheet.

Why you shouldn't use standard plywood -> if you just butt plywood up to create the subfloor, you get this neat sawing action from the unsupported edges that slowly fills the crawlspace with sawdust as the wood eats away at itself, causing the seams to open up. It also groans nicely if you have the right amount of frictional contact and leaves you with a spongy floor because there is no support from the mating plywood sheet along the length of the seam to prevent differential motion between the two sheets.