Hugh,
Every small fire resistant safe I’ve found has a warning that the fireproofing material emits moisture, and they recommend putting papers in plastic to protect against mildew and rot. They also recommend airing the box out periodically, which might be difficult if it’s cached someplace. We use our Foodsaver to protect against this. Just wanted to mention it in case someone doesn’t read the instructions that come with the safes. Regards, – P.T.