Not a bad idea. Back when I was living up north, I haven't always been in Florida, a friend used scrap pieces of outdoor carpet to cover the windows on his car. He placed them pile side down outside the car. When frost and ice formed it formed on the carpet which made clearing the windows in the morning easy. He just pulled off the carpet pieces and stuffed them in the truck.

He didn't mention the effect but I suspect that the carpets greatly improved the insulation of the vehicle by limiting heat loss through the glass. Most late model cars have some insulation in the body and door panels but the large areas of glass remain entirely uninsulated.

Reflectix is pretty good stuff. It is light, easy to work with and fairly good insulation. A bit expensive for the R-value you get but it has its uses. If you can arrange for an air gap you get better performance. Ive used it to insulate electric water heaters, pipes, a pump house, and a hen house. The later two cases it worked out well because the stuff doesn't absorb water like fiberglass does. Which meant that the insides of both were easier to clean because they could could be sprayed with detergent and/or bleach and get hosed out.