Other than something that's on the Space Station (more on that later), the best there you can do is something made by "Mirco & Alex Magnani" and can be found at
www.micromagnani.it
It's best if you can speak (read) Italian, as they are an Italian company. But if not, select the British flag and then select the catalog, then look at the 'Insulating Baffler' section. The material is called "Thermo-Mat Isoflex".
This stuff is silver reflective, but far more than that. It's a really high tech 7-layer thermal barrier that lets
zero heat through. It comes cut to fit for the Sprinter and most European vans and campers, but you can order a do-it-yourself kit that comes with plenty of material and edging. On the Sprinter, it fits the windows and windshield perfectly, and lets no cold air get through in the winter, and no heat in the summer. It's pretty awesome, actually. I cannot recommend this stuff too highly.
When it's been extremely cold outside, I've had it, I dunno, 80, 90 degrees in here with the Espar heater going, then I'd take down the Thermo-Mats and there's a thick layer of frost on the
inside of the windows. If if wouldn't cost a bazillion dollars, I'd have used this stuff to insulate the van instead of styro, canned foam and silver bubble pack. Then again, looking back on it, the Thermo-Mat wouldn't have cost much more than the other stuff, and it would work far better. If I had known about the Thermo-Mat before I insulated the Sprinter with the other stuff, I would have given this stuff serious consideration as my primary insulating material.
The do-it-yourself kit includes 61 inches by 11.48 feet. So it's basically a roll that's 11 1/2 feet long by 5 feet high, and you cut it to fit your windows and windshield. It comes with edging to finish it off, and 30 suction cups that hold it in place on the window. The pre-cut panels for the Sprinter have 4 suction cups for the driver and passenger windows, and 6 for the windshield (of which I usually just use 4 of them, unless it's really cold out there), so the 30 cups will be way more than you need, probably.
It's $99.95 from
http://www.upscaleauto.com/window_insulation.htm (The Sprinter Store, great bunch of people to deal with)
and it's $89.95 from
http://secure.mycart.net/catalogs/catalog.asp?prodid=4757859&showprevnext=1 (Eurocampers, again, great to deal with, and it's where I got mine, actually).
Greg might be interested to know that the Thermo-Mat Isoflex material was developed by Micro-Magnani under a direct contract from NASA for the Space Shuttle, and is, in fact, used on the Space Station under contract from NASA and the European Space Agency. It is also the primary insulation material on the Mars Rovers, both of which have far exceeded their expected lifespans.
The Thermo-Mat Isoflex is in use in vehicles all over Europe, but for some reason here in the states (not sure about Canada) the best we have is a way, way over-priced 3-layer cheap knock off (CoverCraft) that costs almost as much just for the windshield covering, and gives little to no real thermal barrier. I've placed the Thermo-Mat in front of the Espar heater output, as well as in front of an electric heater and a fireplace, and while the front gets too hot to touch, the backside is cool, ambient temperature. Like I said, it's pretty awesome stuff.
Incidentally, to insulate an '06 or earlier long and tall Sprinter with the Thermo-Mat instead of the silver bubble pack stuff, requires six of the do-it-yourself kits (I've done the square footage math). About $600 versus about $200 for the other stuff, but the insulation R factor is far more than three times of the price difference.
Slow and steady, even in expediting, wins the race - Aesop