Fiberglass insulation releases fibers into the air. This is the last thing you want to breath in your confined space. Do you know what kind of chemical binders are being used in your insulation? Additionally, when Fiberglass insulation gets wet it slumps and becomes a breeding ground for mold.
Rigid foam may seem like a viable solution, but consider first that your van’s walls are typically curved. This means that installation will require adhesion with a toxic glue. Beyond toxicity, this medium is known to squeak when the van is in motion.