The dB headliner/wall stuff is noise insulation. It is nice stuff but ... Sound insulation is usually open cell foam as it kills sound much better than closed cell foam. Also, the polymer used to make the dB stuff (polyester) is known to absorb water. Maybe dB can tell you how quickly it will absorb water and how much it will hold. Clearly, it works in some applications.
Tucker told me it uses 1" thick, closed cell "sponge" foam with a sticky back that it gets from Barrier Corp. in Tigard Oregon. Tucker said it covers the foam with a marine-grade carpet-like material. Barrier is a wholesaler and doesn't sell small lots. The sales person I spoke with didn't know what material it is made from.
I've installed 1" thick closed-cell polyethylene foam that I covered with a marine grade headliner upholstery. Polyethylene closed-cell foam does not absorb water but is hard to bond to. I bought the foam with a pre-applied adhesive film - big mistake. Halfway through the first season the foam separated from the film and dropped off. Pain in the rear to remove the adhesive film from interior. 3M high-strength 90 is a spray-on adhesive compatible with polyethylene and works great. You have to spray both surfaces to get a good bond. We get lots of high humidity days in California winters and the foam is fine after three years with no mold or absorption.