Just what I was telling a guy the other day. The floor was rotton and he did a quick cheeeeeep fix on it. Foam will firm up any floor to make it feel good. And long enough to sell it to a sucker. I watched a guy dig out bad foam under a rotton floor, not even replacing the wood. He dumped new foam into the hole, shaved it off and covered it up with glass. Put down some new carpet and send it out to be sold. Chances are it had 3/4" ply and he cut it out, used a 2+4 to help hold it up. Dump in the foam, SHAZAM good as new.