Seeing you are from Denver the humidity problem shouldn't be as big of an issue as other locations. Some form of heater, a golden rod or like one guy said a light bulb should work. I have two safes, one has a golden rod the other doesn't have anything & I haven't found any difference to my guns between the two, but I do keep them wiped down & use rig on the seldom used ones.
Being as we, both live in the same general area. I would be more concerned with the potential theft issue of having my guns stored in an un-occupied outbuilding. I do most of my gun work in my detached garage, but I sure wouldn't put my gun safe out there for a couple different reasons. One is it isn't climate-controlled year around even though I do have a gas fired heater in it for warming when necessary and two, is because it's unoccupied and it's just plain easier for someone to break into. My experience has been that it's not that big of a deal to carry a gun to the garage when needed while having the peace of mind that the rest of the time they are in a climate-controlled environment with people & dogs around most of the time.
