Don't use expanded polystyrene
Expanded polystyrene (compressed polystyrene beads) will hold water between the beads. Extruded will not absorb water. Also, I know for a fact that critters will attack polystyrene in any form. I have extruded closed cell polystyrene enveloping my entire house protected on the interior side with 2" T&G pine boards. At night I can hear the mice in the insulation. Luckily they've never penetrated the pine and gotten into the house. BTW, if the boards are white they are expanded polystyrene. If the boards are blue (Dow), pink (Owens Corning) or yellow (Certifoam) they are extruded closed cell polystyrene.
Doug's idea of using polyisocyanurate boards with an aluminum foil vapor barrier is commonly used in commercial roof insulation. They can, by code, be applied directly over a metal roof deck without a fire barrier. Polystyrenes require a gypsum board fire barrier. The foil faced type of boards can be used inside a building with the foil on the warm side. Polyisocyanurates are very similar to polyurethanes, are closed cell non water absorbing--and safer than polystyrene.
2015 Sprinter Class B Camper Van
(Former 1971 vintage Airstream Owner)