Q: We have just added new felt over a 1/2 inch plywood subfloor and then put 5/8 inch plywood over that. If we now use hardwood flooring, rather than carpet, should we first place another layer of 15 pd felt on top of the 5/8 inch plywood, before laying the hardwood?
A: I don’t think I would have put the felt between the layers of plywood / felt between subfloor underlayment, but I doubt it will create any issues in the long run. It’s purpose is to retard the movement of moisture. By slowing the rate of transfer it gives the hardwood floor on top a better chance to absorb small amounts and discharge it without cupping or expansion issues. So, I would still put it down before installing the finished floor. If this is over a crawl space or damp area you might want to rethink this. You don’t want to create a situation where the first layer of plywood can’t release moisture and ends up rotting on you.