Anyone ever use fiberglass on the underside of their deck?

[Oregon Brew]

The deck of my trailer is basically a sandwich - Plywood on the bottom, a 2" x 2" frame with solid insulation fitted between the crossmembers, and more plywood on the top to form the sleeping deck. I'm thinking of laying fiberglass over the whole thing. Seems like it would be plenty waterproof, rot-proof and so on. Anyone else take this approach?

[bonnie]

The BUB has a sandwich deck. It's fiberglassed. I'm actually finishing the trailer, so I will be protecting the fiberglass underneath. Haven't decided just how yet.