The styrofoam will absorb moisture. In house construction, the vapour barrier, is used to keep the warm moist air from the heated space away from the insulation. What are you going to use for flooring, and sub floor? The most important thing is to keep the humidity down. Here where we are, the worst time is of course the winter, when the dew point is reached in the bus, the will be condensation everywhere. Its best to keep a little heat in there all winter. A few hundred watts is all you'll need. Unless you are opening the door, and bringing in more moisture.
When is is minus 30, outside moisture is not a problem, as there isn't much humidity. This is the time though, when you need to keep the moisture from inside living, away from the cold underside of the truck.
For just condensation, a little dry heat is all you need, and some air changes.
