Romex size/type question

I am going to run some baseboard heater wire out the wall of my camp below the fuse panel in some plastic conduit, down to the crawlspace and under the camp floor, and up to the 2 rooms that i want to put baseboard heaters in. I will run one wire per baseboard and it will have a built in thermostat to save wiring. Question is, what guage and what kind of wire do i use in damP conditions under the camp. thx!