WHat to use? What's the sticky aluminum foil thing? Use that and home depot foamular?
Reflectix, I've used it with great results.
If you're gonna be out in the cold with heat inside the van, you can cut down on the cold steel walls sweating by using the Reflectix directly on the outside walls as a moisture/condensate prevention. If the wall ain't cold then the moisture in the inside air can't condense on those walls. Van stays drier inside.
If you're gonna be in the sunshine, it's also used directly on roof decking (and walls) with no gap as a radiant barrier to help keep the sunshine from heating the interior. If the walls ain't hot they can't transfer that heat to the inside air.
So it'll help both ways. And it's cheap.
The foam board on top of it will increase the R value by a bunch.
I've always used fiberglass over it, well supported to prevent sagging.
There's other materials that say they are specialized, that they work better but they sure cost more.