Yeah, I moved the fan over to the dirt areas yesterday. Tonight they're slightly dryer but still moist. Once I get it dry I'll move the fan again and see if it gets wet again.Did you try running fans where there was no plastic? I suspect you'll find the air movement is what caused the gravel to be dry, not the plastic......unless theres a water source you haven't discovered.
I would guess the floor is close to the temp of the room, but it's worth a check. I've noticed it stays about 10 degrees warmer inside than outside, and today my humidity meter wouldn't even give me a reading, it just flashed "HIGH". LOLIf for some reason the floor is exceptionally cooler than the air, it could be pulling moisture from the air being vented through, concentrating it in the stone and increasing the humidity overall. Pumping lots of air through could be making things worse.
Check the temperature differential between the floor and the air.