1" XPS, plywood, and flooring with gun safe
Assuming your safe is 30" x 30", that is 900 square inches. Even 900 lbs is only one psi; trivia. Even the lightest XPS you can get, somewhere around 15 psi (5 psi to avoid creeping, ie, deformation) is overkill. If you are concerned, they make XPS up to 100 psi. I'd put plywood under the safe so you don't beat the snot out of the foam wrestling the gun safe in. BTW: EPS (white, "bead board") is cheaper, more Earth-friendly, and has nearly the same R-value. You can get it up to 60 psi.
Last edited by jklingel; 11-07-2012 at 11:48 PM.