I have never added any insulation to my vans, but what frscke1 is saying would certainly qualify as the "prevailing theory" on this forum. So I'd certainly take that into consideration.
If I do anything more with my current van, it will be in the vein of a stick-on auto sound deadening with foil shield, or a roll-on product like a LIzard Skin or Eastwood coating.
Lizard Skin LinkEastwood LinkWhile many don't like fiberglass bat insulation in a van, for the reasons frscke1 mentioned, it was the "factory fill" on my '95 Chevy Shorty Sante Fe conversion, and is also used in my '97 Chevy Express Midwest Conversion. My dad had 3 other conversion vans before this Express, and I suspect (but do not know for a fact) that they all had fiberglass bats for insulation. Cheap, light, easy to work with and fairly effective, I think it was the default insulation for most conversions.
But if you have a window or body seam leak, it WILL hold water like a sponge. Just goes with the territory.
Closed cell. I have no hands on experience, though I am eyeing some poly-iso on Craigslist for possible use in my shop. Poly Iso is billed as having "low water absorption and low vapor transmission" according to one manufacturers site. So that sounds promising, but not perfect.