We insulated our van with self adhesive Armaflex Insulation. The normal panels and straight forward jobs were done with 19mm, the curbs and other special corners were done with 6mm. We also used the 6mm to overlap the seams of the 19mm ones. No need for a vapor barrier when using Armaflex insulation. Before going loose with the insulation be sure to attach sound-deadening panels to reduce noise from vibration when driving. It really makes a huge difference. You don't have to do the entire panel, small patches (50 x 50 cm) of sound-deadening panels do just fine.
The floor of our van is also done with 19mm of Armaflex insulation between the wooden slats. The slats were glued directly on the floor of the van. And then finished off with a 18mm multiplex floor board.
Wooden slats 40 x 20 mm
Soudal FixAll Silicone glue