I am in the same boat. I have a fiberglass top and ended up going with 1/2" foam with a foil on one side and some sort of plastic cover on the other. bought it from Home Depot and was cheap. not sure if I will add another layer before I cover it with some sort of carpet or paneling for looks. I am not done as winter is upon me and to cold for gluing it in is now on hold.
but when it comes to the van body I am thinking about this stuff. I have read that if you do foil backed bubble wrap or likes there of you have to secure it well so it dont rub on the metal and cause static or something like that over time.
http://www.homedepot.com/p/UltraTouch-4 ... lsekVPOSnQ
but not sure how well it will be on insulation in cold climats. yes it has the radiant barrier but i plan on ski trips so I want good insulation. it does help with sound deadoning and easy to install.
here is some reading on insulation that might help.
http://www.camperize.com/insulation.html
best of luck