I am sure you will probably get varying opinions here, but I used 1/2" plywood on my 6' x 16' layout. Plywood was cut into 6 pieces, 2' wide by 8' long all butted as tightly together at the joints as I could get them. That was 5 years ago and no problems so far.
The layout is in an unfinished basement, but I have a couple of vents in the main ductwork to condition the space. The vents remain open year round. Humidity averages about 35% to 55% year round. It's 53% right now and our outdoor humidity was very high today, it's raining.