After some "reflection" I'm not too surprised that the space blankets didn't perform well in the heat. After all, they are marketed to be used mainly in the cold. They may have been designed to allow sunlight through as this could provide an extra advantage in some cold weather situations.

I wonder how the AMK Heatsheet would perform as a sunshade? With the orange backing it would be interesting to see how it compares to the trashbag.