We have new construction, there was moderate moisture before the cement completely dried out. But I currently store 4 of my g/f's father's guns down there and no more rust than what was on them when I received them.
If no case, I'd recommend silicone impregnated gun socks... I keep them on MY guns in my gun cabinet upstairs. I'd love to buy a gun safe but just can't afford it right now, with some dessicant packs or a moisture stick.
Good luck, just keep your guns LIGHTLY oiled. I've heard people complaining about their XD's rusting but I keep mine in my room in a plastic case in this humid summer that we've been having it hasn't rusted. Just with a little bit of oil and TLC.