Noico Sound Deadening mat is the best bang for the buck amongst all the sound attenuation material out there.
2 to 3 boxes will be enough for most van applications.
Aluminum backed, butyl rubber
80 mil = 2 mm
36 sqft bulk (9 sheet size 29.5 x 19.5 inch)
Weight 27 pounds
80 mil Sound deadener, attenuation mat 36 sqft.