Being in the southwest, you are most likely in an arid enviroment but it's a good idea that you have something to remove any unwanted moisture that can into a safe. My safe's are in air conditioned rooms and I use the 18" model golden rods with out any problems. I think the 24"model might be overkillbut that will depend on your situation as you know better than anyone. With the safe being in the garage you are wise to get something in there to take care of moisture.