My gun safe sits in our unheated garage and, so far after 6 years, I haven't had any problems. Granted, Flagstaff, AZ doesn't get as cold as Iowa does in the winter time, but my ambient garage temp is typically right around freezing every night. Coming up from Tucson, where winter lows are still higher than Flagstaff's highs was a bit of a difference. I had to switch from a big dessicant box to one of those "golden rod" dehumidifiers. That seems to have helped quite a bit.
Getting the garage buttoned up to trap heat has been on my list of things to do so I can still work at my reloading bench and do other stuff in the winter time.