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Installing 3/4 hardwood over crawl space

I am wanting to install 3/4" Pine, T&G hardwood flooring in my house. I will pull up the existing 99 cent a foot "laminate" that the prior owner installed 20 years ago. It was installed over OSB, 15# roofing felt, and with no moisture barrier or foam. That said, it is remarkably good after 20 years with some obvious squeaking from not having foam. There is no buckling evident, even after 20 years. I'm thinking the moisture in the crawl space below has been controlled well by the 6 mil plastic barrier on the crawl space floor. So I will pull up the existing floor, pull up the OSB, inspect the barrier, correct where necessary, inspect joists, etc. as was done 20 years ago, and put 3/4" plywood down as a sub-floor. On top will be installed the 3/4" T&G pine flooring. My question is, can I or should I do anything else other than upgrade the roofing felt that rests on top of the sub-floor to 30# weight? I thought about applying sealer to the bottom side of the plywood to retard any moisture. That said, it may not be smart if the moisture that naturally occurs needs to migrate through the sub-floor and through the hardwood. Also, will the varnish on the hardwood act as a 100% barrier to moisture and cause failure? I am thinking the assembly needs to breathe, but 4 coats of polyurethane will seal it.

The crawl is about 18", on an older house, and has no venting. Recently a trap door was cut in to it so plumbers could inspect, and it appears dry, and "normal" for a crawl, and the MVB seems to be working well. Joists look healthy and even the underside of the original OSB was looking good.

I always apply sealer to the underside of 3/4" flooring when installing but I have never installed over a crawl, only a basement.

Thanks for any advice!


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