Is a moisture barrier required? 3/4" hardwood to be install over 5/8" sheathing in rooms located over crawl space. Crawl space floor is concrete. Crawl access is open to basement. Crawl appears to be dry. took readings one day on existing conditions. Outside temp=83 RH=70,
Rooms temp=76 RH=52, Crawl temp=68 RH=54, basement temp=69, RH=57.5, AC on in rooms. Location is Philadelphia PA. metro. Getting opposing opinions from installers.

I would cover the concrete "rat slab" with an 8 mil poly vapor barrier. Then moisture test the plywood subfloor. Or moisture test the wood subfloor first and if within 2% of the new wood flooring, you are good to go.