I guess I'm the only one who's tried this. I'll be sure to report my findings. Of course its fifty degrees today instead of ninety.
I took out all the curtains in my van and now use this stuff, the reflectix. It's useful for insulation (though thin), padding, and privacy, as noted. Keeps a bit more warmth in during cold weather too.I have done something similar but with water heater insulation from the hardware store. Had the rear sliding windows of my 80 series covered for a good year and have noticed on long trips the temps inside do stay a bit cooler but I don't have any scientific evidence to back that upI mounted mine using suction cups to the windows themselves.
Also like stated above, the privacy factor is nice too.
If you can put it on the outside, it seems to work a bit better as a sunblock. No hard evidence to offer on that though.