Check out sprinterforum for some really great insulation posts. Googl Hein 3m, he seems to be everywhere.
I just insulated my van last weekend. Going from a bare steel cargo. I used a combo of the following:
Dynamat: 25% coverage on the large roof and wall panels. I have done testing, this is truly all you need. there is no thermal advantage to dynamat.
Armaflex AP PSA neoprene in 3/4" sheets for the flat surfaces. Designed for the HVAC industry to kill condensation and thermal transfer on boilers. has R value, and sound dampening qualities, and obviously moisture barrier. Used on ceiling, window areas of walls, and floor.
3M 600L thinsulate material: 1.5" thick, excellent noise reduction, thermal qualities.
I used it in all the horrible hand cutting areas that are impossible to get to. The 3M is super easy to work with. My wife fell asleep on the roll
Reflectix: Used it for its moisture barrier properties. I sheeted the inside walls of the van in reflectix. I had a little of the cheap stuff from home depot, and some of the lobocrud high dollar stuff from an old project. when you are gluing it to a surface, there is no gain one over the other. any real thermal qualities are lost. but it makes a great vapor barrier!
3M 90 spray adhesive seems to work well for all materials listed above.
Dont use the thinsulate anywhere what will possibly have standing water (like underfloor). it soaks it like a cotton towel and doesnt dry out.
When insulating floor, space up the valleys to allow air movement. 7/16 ply cut into 2.25" strips works great.
Dont use reflectix type material on floor. there is no thermal advantage, and it will got flat form the pressure.
If the cost is concerning, think of the money youve invested in your van. depending how its insulated, it will either destroy it or improve it.
Armaflex PSA is expensive. about $40 for a 3x4. if you had to choose one surface to ise it on, Id tell you the floor. I truly cant think of a more ideal product to use there.