I prefer non-electric dehumidifiers
I'm an electrical engineer and usually want to use electrons for everything, but I prefer the non-electric dehumidifiers. You an buy silica gel in various sized containers. The larger the container, the more humidity it'll absorb and/or the longer it'll absorb it. There are other chemicals that absorb water, but the silica gel is very effective and probably some of the most cost effective as well.
Some drying agents have a built-in indicator to show when they're reaching their maximum absorbing capacity. If not, you can buy very inexpensive moisture indicator strips. They're little cardboard cards with a chemical indicator.
Silica gel can be renewed by baking in a 200 F oven for a few hours. Yes, that uses electricity too, but I do this in the winter when the heat is also used to heat my house. During the summer, the GoldenRod will be producing heat that you will need to pay for twice. The second time will be when you run the air conditioner to remove that heat from your house.
The GoldenRod works well, but if the safe has a decent door seal, there is no place for any moisture to go. The silica gel will absorb it fairly quickly.
I wouldn't advise leaving the door cracked to let out the moisture for a couple of reasons. It also lets IN the moisture, and a safe isn't very secure from fire or theft if the door is left ajar.
Probably the best deal on silica gel is in the cat litter aisle of your grocery store. You can buy silica gel that is intended to be mixed with cat litter as an absorbent. It's about $5 for a large jug, and it has indicator mixed in with it so you'll know when it's expended. You should be able to set the jug in the bottom of the safe and remove the lid. Every winter, dump it on a cookie sheet and bake it, let it cool, and return it to the jug and put it back in the safe. It uses less energy than a GoldenRod, and it's a LOT cheaper than a GoldenRod.
