I've installed hardwood floors, over top of OSB subfloor with a layer of roofing felt in between. I copied this from the hardwood floor contractor who did a kitchen in my home once. He'd been in business for 20 years so I trusted him. I think he said it was to prevent squeaks.
Is this true?
Does roofing felt used in this way also provide a vapor barrier, and if so, why is that important under hardwood? I can understand why it would be useful on a roof. I can understand why it would be useful for housewrap. Under a floor?
I read elswhere that rosin paper was originally used to keep floors clean (protect from the dust from drywall finishing, for example), and make it easy to slide floorboards. That seems pretty minimally valuable. Do I care if my OSB gets dusty? I usually shop-vac the OSB before laying down the floorboards, so is this really a benefit? And using rosin paper to ease sliding boards during install? Really? I never noticed a problem sliding the boards. Is this because I'm really strong? (not likely). Maybe it's because I'm using OSB as a subfloor which is a pretty smooth surface? I can imagine using plywood as a subfloor, in the days before OSB, might/could cause hitches when sliding boards during installation.
Do either of these layers provide sound-deadening or anti-squeak qualities? I figured the lack of squeaks was from using a sound, level subfloor, and installing the T&G tightly.
This question gets asked a lot in various DIY or even contrator forums. The problem is, there doesn't seem to be a general convergence to a single answer.