Locating a safe inside a controlled environment is best (house) but either way will work. Just make sure you use Silica Gel (Desiccant) inside the safe, this is a must no matter where you put it. The Silica Gel will absorb the excess moisture. These that I found work great in safes, plus you can reuse them:
http://www.silicagelpackets.com/dry-packs-dehumidifier/