It may not feel like it at times, but the roof glass seems to have impressive heat protection properties. I have recently fitted a Tesla sun shade for the front roof panel and thought that I would see how effective it was in first day of decent prolonged sun since fitting. Not a scorching day, but the images below are after nearly 2-1/2 hours of direct sun. The roof glass is very effective, keeping the seats around 20C (uncalibrated) cooler than the areas where sun is coming through the side window. The net result was that the area of seat would have passed the 'shorts test' (front edge of seat slightly cooler again). The area protected from just side window glass would definitely been a fail. The same result was had from the windscreen. Certainly back of legs would have had problems on front of seat not protected by roof panels. The sun shade would have made slight difference, but not anything significant. Unfortunately I forgot to take roof glass shots. The main reason for the sun shade was to put a barrier between my head and the significant heat that radiates off the glass. The jury is out on that at the moment. Based on this one days observation, windscreen and side glass are the ones most needing protection.