my safes and gun cabinet are in my master bedroom walk in closet which has a door to the shower room. I also keep my cigar humidor in the same closet and all i can say is that the humidity is high enough (around 70% with my digital hygrometer) that I don't need to do much to maintain my humidors humidity levels. with that said I use desicant packs that i got from midway. they have a color indicator so that I can see when they are full and I can simply remove the desicant packs from the containers and then put them in the oven and bake the moisture out. also I keep my guns all lightly oiled with clp (a nice thin film that wipes off easily when i want to take them to the range). on the wood stocked guns I get the gun socks that are treated and that seems to help prevent any rust under the wood and the wood doesn't swell at all with the combo of gun socks and the desicant dehumidifier system.
digital hygrometers (like the ones for cigar humidors) are really cheap and really accurate (cigar afficianados are a bit of control freeks when it comes to humidity and accuracy is a must). good idea to put an analog or digital hygrometer in the safe or gun cabinet to monitor the humidity levels. analog ones can be had real cheap at lowes or homedepot for basic uses (in the heater/air conditioning section).
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