I am building a small cabin. I have installed concrete posts, floor girders and floor joists made with pressure treated lumber. I am now to the underlayment stage for the floor. My question is what would be the best option to put on the underside of the subfloor to resist moisture and bug entry? Tyvek or tar paper?
I would not put anything on the underside (bottom) of the subfloor, it is just going to create a moisture trap, and a home for insects when gravity takes over and it sags down from the subfloor. If you are using t & g plywood for the subfloor, you can run a TINY bead of caulking in the joints as extra protection against small insects like ants. You should not have a moisture problem with the bottom of the subfloor as long as there is plenty of ventilation , and that should be no problem, as you do not sound like you intend to close in the crawl space. If you want peace of mind, you could roll a coat of porch and deck enamel on the bottom of the sheets before installing...can't hurt.