Having done demo work on old houses, I can say that rockwool/fiberglass does hold moisture to an extent. It can get pretty soggy if it's left in the rain for a while. The fibers themselves might not soak up moisture, but they can hold it in place which would be bad against a metal wall in a van. Just be sure to seal off those trim attachment points on the side of the van, and vent well while you're cooking/sleeping in the van so condensation doesn't build up.You'd also have to find some that was the right thickness - or thinness actually. Stuffing a 4" thick piece into a 2" deep gap won't give you the advertised R value because you've reduced its capacity to retain air between the fibers.If you've never worked with it before, be prepared for the itch factor while you're installing it. That's those tiny particles getting loose and poking your skin. They're undoubtedly doing the same to your lungs. Like ECU says though, once it's in, so long as you don't have gaps the particles shouldn't get out into the living space of the van. We do sometimes get puffs of air from behind the walls when we close the rear doors, so there's potential in our build for the fibers to migrate. You may be able to seal the walls better than we did, but then you get into the "should I put a vapor barrier on the inside?" question which I won't even START to talk about here.Having said that, Sportsmobile, Winnebago and Outside Van have all at some point used fiberglass as their insulation of choice. It's cheap and effective.More here: http://sprintervanusa.com//06/25/comparing-van-insulation-options/#fiberglass
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