I've been watching this thread with interest, and curiosity got the best of me today. I took my temp gun out to the bus at 4:00 this afternoon and measured the temps on the inside of the windshield and the outside. With a 93 degree outside actual temp, the windshield measured 138 degrees on the outside surface. I have the 1/4" foil sandwiched bubble wrap as an inside cover while the bus is parked. The temperature on the inside surface of the insulation measured 77 degrees with my front Coleman roof air running on low. I was amazed at the difference that thin Lowe's insulation made! I checked the temperature on a couple other interior surfaces, and they were within a couple degrees of the insulation temperature. So---- reflectivity really is an asset for keeping things cool in our buses.
Since you have the bubble foil and a temp gun, here would be an interesting test to see how sandwiching with other materials affects the performance. Cut two pieces of cardboard the same size (i.e. 2ft x 3ft) and sandwich the bubble foil with no air space between the cardboard and bubble foil on one side of the windshield. Do this early in the day and have the same AC setting. When the heat of the day is in full swing and the windshield is getting direct sun, measure each side, the foil only and the foil sandwich.
I have read statements on both side of the issue from people whose knowledge I highly respect. I think this would be a good way to test the impact of using bubble foil without air space (or glass on the radiant side) surrounding it.