I have a 12x12 screened in porch with a concrete floor. The floor is poured over a plywood sub-floor supported with beams. I was in the crawl space under the floor and noticed that the plywood is moist in places. I am concerned about the plywood rotting from the moisture. The crawl space does not have any vents.Can the moisture be condensation from the concrete due to lack of ventilation? Any ideas what else could be causing the moisture? Is the plywood sub-floor vital to support the concrete floor?