I just found a roll of Tyvek Thermawrap 5.0 marked down at HD for $0.82/sqft (CAD). It resembles 2" Thinsulate bonded to Tyvek. Google says it's hydrophobic polyester. I only found a few vague references in RV forums and none negative. Google also says there was a class-action suit filed by homebuilders (etc) over false (exaggerated) advertising. That doesn't bother me since it wasn't the product's fault. I'm thinking it should be OK to treat it like Thinsulate, i.e., glue the scrim side (Tyvek) to the body metal (or foam board). Anybody know of any downsides?