Some fun physics for ya to help you decide.
Air is gas. When gas heats up (by the sun in the case of air) the molecules start moving faster, running into each other and pushing each other away. As a result, the space between molecules increases allowing other molecules(such as water molecules) to get inside. This is why hot air is capable of holding a lot more moisture than cold air. when air cools, the opposite happens: molecules of air get closer to one another, leaving a lot less room for water molecules. As a result, water is expelled in the form of dew, rain, or snow.
A couple of conclusions can be derived from this:
- Cold air is actually better for firearms since it acts as a moisture blocker. when given a choice, cold air is always better than warm air in terms of rust prevention.
- Cooling is bad for firearms. As the air cools, the moisture that used to be suspended in the air settles on surfaces in the form of dew. This is why weather people use the term "dew point". The cooling of air through the dew point temperature will cause moisture to settle on gun surfaces.
It is the second reason that makes it a bad idea to keep firearms in unheated environments. Not all unheated environments though. If you can keep the temperature away from the dew point temp and cold enough that you don't get excess humidity, there is absolutely no reason why you couldn't keep the safe in an unheated area.
The reason why heating rods help is that it raises the air temperature of the safe, increasing the distance between the dew point temp and the air temp. the further they are apart, the less likely the moisture will form on a surface.
The reason desiccants sometimes work in safes is that desiccants use a chemical reaction to absorb moisture from the air. The less moisture in the air, the fewer water molecules there are to settle on the gun's metal surfaces. That said, desiccants only work if the safe is sealed from the air outside and only for a limited amount.