OK, here's the report on using Reflectix in the windows of my passenger van.
I have Reflectix cut to fit all windows except the windshield, where I have a jumbo bubble windshield screen (doesn't fit all that well).
A cloud layer kept temps in the low 30s each night rather than in the 20s. In the morning, about 7:00am, the temp inside the van was 3-4 degrees warmer than outside.
I was more than snug in my sleeping bag, but I do worry about electronics (camera, GPS, computers, iPod) when the temp inside the van drops below freezing.
1. Make curtains/shades with "warm windows" liner. This, with the Reflectix when necessary, should improve inside temps by a couple of degrees.
2. Make/get a better fitting insulated windshield cover.
3. Make a cover or foam insert for the Fantastic Roof fan.
4. Make a rug to fit the exposed vinyl floor.
5. Make a insulated curtain to hang between the cab and body.
6. (This is the hard one!) Figure out how to remove the interior wall trim, and put in insulation. Same for the doors.
If I do all of this, will I be able to stay warm in the winter evenings, when I'm reading or working on the computer? And will I (and more importantly, my electronics) be able to stay warm overnight if the temps drop to the 20s or teens?