The sprinter is a tin box on wheels, and when the sun hits it, the interior metal becomes hot enough to burn you, not to mention the interior turns into an oven. Insulation of at least the roof is critical.
For the roof I applied 2 layers of 3/4in foam. I used polyeurethane construction glue, and double sided carpet tape to hold it in place while the glue dried. The corners bends were made by scoring 3-4 times with one inch spacing between scores.
Insulating the side walls was much more time consuming than I anticipated. Maybe it’s just because I took my time to do it. I used the same 3/4 in foam, cut into small panels, and glued into place