@jwildeboer Concrete slab. Fair amount of (relatively minor) cracking from over the years. No obvious stains, but can't see the entire surface due to (as is typical) lots of stuff stored in there. I have a hydrometer in there but haven't noticed drastic changes at least in the air. This time of year the temperature changes day/night aren't anywhere near dramatic as they will be soon when spring/summer hits my corner of L.A.