Hi--we're finally finishing out the white oak that is in part of our main floor--we've got a garage full of white oak ready to go. What underlayment would be the best insulator over a crawl space that gets cold in the winter (we're in Oregon)? We plan to insulate the joist bays, but I want to maximize the insulation in as many ways as possible.

Cork is one possibility I understand, but I also see things that have higher r-values and are less expensive. I will make sure that the nails are long enough to reach far enough into our subfloor (read about some nails that have tiny spines that prevent pullout)...

Any thoughts would be welcome re insulation/underlayment for a naildown. THanks!