Insulation: Bubble insulation
Bubble insulation generally consists of a layer of plastic bubble wrap sandwiched between two aluminum layers. The trapped air in the bubble wrap slows heat transfer, and the shiny aluminum reflects radiant heat.
Reflectix is popular for window shades - cut to size, and placed in the windows, it provides excellent insulation from both heat and cold. When you're parked in the sun, much of the heat that comes into your vehicle is radiant heat (up to 97%, according to the manufacturer) and can be reflected back out.
I don't think Reflectix would be a bad choice for whole-van insulation, either, with a few caveats. First, aluminum and steel don't generally play nicely together - see galvanic corrosion. While the paint on the van's steel will isolate the two metals, any blemishes will create an electrical current that may hasten corrosion of the steel*.
Second, the thermal and acoustic insulation is provided by the trapped air in the plastic bubbles. Sending any fasteners through the Reflectix will puncture the bubbles in that area, destroying insulative properties. This probably isn't a big deal, though, unless you're planning on extreme temperatures, in which case you'll likely be using an additional insulation layer with the Reflectix.
* this is an interesting debate, to which I haven't found an answer. Looking at each metal's properties, it would seem that an aluminum/steel pair would result is faster oxidation of the aluminum, with a protective effect on the steel. However, some theories propose that the steel will corrode more quickly, since aluminum is generally protected by a layer of aluminum oxide, changing its electrical potential enough to switch the corrosion potential.
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