I looked at an Airstream B190 today that showed obvious signs of condensation seeping into and staining the wall coverings and headliner. I ran because to me that screams RUST and MOLD.
I'm going in the exact opposite direction with my build. Five layers of different types of insulation. Overkill? Maybe. But I really like the idea of driving a high speed quiet booth.
My first layer will be a gallon of this
. Total moisture barrier plus sound and temperature insulation. Seems like it should be easy to apply too. Maybe try that at least?
FYI, the guy over at Sound Deadener Showdown
has a super informative site. No way I can afford all his products, but if you understand the principles he explains, you can find much cheaper alternatives.