I've just completed extensive repairs on a post and beam floor (4X6 @ 48" on center) over an unconditioned crawl space. I don't want to reinsulate with fiberglass batts because in my experience these invariably sag in the middle when you have a 46" span. I am considering using 4.5" polyiso (rmax brand) which would give me R30 and gluing it directly to the plywood subfloor. I would fill any edge gaps with foam and tape seams as well. I would probably also support the sides at the beams with a 3/4" ledger. I can find no information about this kind of horizontal "suspended" type application. I don't want to have the adhesive failing in 20 years and the polyiso sagging under it's own weight. One thought I had was running a batt down the middle and using long screws to fasten this through the polyiso to the subfloor. Just curious if anyone has any experience/thoughts/ideas for me. Steve

A good glue isn't going to let go after 20 years. But consider eliminating thermal bridging and gaps by putting EPS on top of the floor - eg, OSB/foam (staggered layers)/plywood all glued together (effectively a SIP).

you are better off by conditioning the crawl space.