Originally Posted by HolyRoller
If you actually seal the underside of the subfloor you risk capturing the moisture indefinitely (vented or not).
This will guarantee rot that comes from the inside out.
I disagree. When sealing any sheet of wood, it's typical to seal both sides evenly, else any changes in humidity cause the unsealed side to expand or contract, which warps the wood. With the wood bolted to the frame, it probably wouldn't bend, but it might crack, which would defeat the seal and weaken the piece. I would suggest it's best to totally encase the wood if you have the opportunity. Just my opinion.
I went to some great lengths to avoid recurrence of rear floor rot. Sealed entire floor in west system epoxy, wrapped rear edge of plywood in a butyl window gasket called Protecto-wrap, added a piece of aluminum to deflect water, and re-engineered the bumper lid attachment to direct water through the bumper compartment to the ground rather than under the plywood floor.