Insulating a metal box is not a trivial task. It seems that everyone in the van community has their own theory and despite a plethora of options, there is no clear winner. Fiberglass is highly toxic. Spray foam is permanent and difficult to install. Cotton batts absorb water. And foam boards squeak like a thousand horny crickets! In the end I chose 3M Thinsulate SM600L as my bulk insulation. It’s non-toxic, easy to cut with scissors, and provides some sound isolation as a bonus. The only downside is that it doesn’t have a very high R value, meaning it’s not as good at retaining heat as say, fiberglass. But hey, at least it doesn’t squeak! That would probably drive me off a cliff in a matter of days.