I'm DIY'ing replacement floors in 1908 craftsman house in Southern California. The house sits on a cripple wall with a modest crawlspace, appears well ventilated & dry. The first floor has 3/4 fir or pine 3" T&G boards for actual subfloor. Some areas are patched with plywood. We're acclimating the new 3/4" hardwood oak (from Country Wood Flooring). I've got 30lb roofing felt as an underlayment.

My question is do we need a 6 mil poly vapor barrier between the subfloor and the tap paper? Just wondering if that will instead cause moisture differentials between the subfloor/joists and the actual flooring..

Thanks!

The 6 mil should be on the ground completly covering all exposed earth and run the walls about a foot. Do not put 6mil under the wood floor as a vapor retarder the felt will be adequate the floor needs to breath a little.

Thanks for the prompt reply! Is laying down plastic barrier in crawlspace common in Southern California, specifically Los Angeles? Reason I'm asking again is that I've seen a fair amount of crawlspaces in this area without it, and the corresponding floors are just fine. It is, after all, quite un-humid most of the time.

I would lay it down as cheap insurance, most of the time doesn't mean all of the time.