Insulation and condensation control in a steel van

A van is a steel box that sweats. Every person exhales roughly a liter of water overnight, and if that vapor reaches cold sheet metal behind a sealed insulation layer it condenses and rusts the panel from the inside.

This is why closed-cell spray foam has fallen out of favor in DIY builds: it bonds to the panel, hides the metal permanently, and traps any moisture that does get behind it.

3M Thinsulate SM600L is the current default because it is hydrophobic, breathable, and drapes over the Transit's curved ribs without cutting. Wool batting is a comparable natural alternative. Both allow incidental moisture to migrate back out instead of pooling.

Ventilation does more work than R-value. A roof fan running on low all night moves the vapor out before it can condense. Insulate the walls and ceiling well, insulate the floor lightly, and never seal the van so tightly that it cannot breathe.