Hi folks,  having spent all my winters in cold climates, fiberglass house 
insulation is the way to go. The condensation problem that causes rust 
occurs with ANY insulation when you have higher temp and humidity inside 
than out. You must use a vapour barrier on the warm side to stop the moist 
air from getting into the insulation cold side where it condenses back to 
water. Check behind your door panels, there should be a plastic sheet stuck 
to the metal frame. This is not just preventing dust from entering, it's 
preventing the moist air from leaving. I've used fiberglass insul. in 6 
vans, from '71 on. the vapour barrier is crucial. Fit it in place, use 
decent thickness, not drycleaning bags, and seal ALL the seams etc. You want 
it airight. Tape or glue it to the metal wall frames, braces whatever. Then 
put your panels back on. If the stuff gets wet from underbody rust holes it 
holds the water well, so fix up all water/vapour entry points before you do 
this. Other insulation, styrofoam, neoprene will hold water to a lesser 
extent, rug underpadding is a good sponge. All insulation needs a vapour 
barrier, you can rot out a wooden house if you don't vapour seal, more 
costly than a Westy to fix.    regards  tim s.