I am about to install fresh 3/8" plywood over the original 3/4" tongue & groove planking to give me a smooth and solid surface for the final lauan and sheet flooring in my 5' x 9' bathroom.

Here's my question:

In order to avoid any possible squeaking in the future would it be advisable to place a thin sheet of some material like landscaper paper or the rolled foam stuff that gets used under laminate flooring between the original planking and the new 3/8" ply? Is there a better material? I would think it would deaden and insulate the floor a bit as well or am I thinking about all this too deeply?

Also, what is used to fill the seams of the 3/8" subfloor and lauan before final flooring (if anything)?

Thanks.

I don't think a thin layer of anything is going to stop squeaks. I would rip the old plank stuff out. Add 2 x 4's to the joists if you need something to nail the plywood to. If you do leave the plank floor, dump talcum power in the cracks to take up the friction. Then solidly screw the plywood down using 2 1/2-3'' screws into the joists. Stager the joints in the luan and plywood. Screw or staple it down. Fit it tightly and I don't think you need to fill the joints.

Thanks for the good advice. I have to leave the planks and they are in good shape. I did renail a lot of them down again today as the 80 YO nailings werer working loose. Right now the planks are quiet. My concern is not from them but the new potential for friction between them and the new plywood which is why I was thinking about a membrane between the two to avoid future friction in that plane.

Thanks again.