Originally Posted by RCZ

gold is the best insulation, but its expensive. There are new, much cheaper alternatives that reflect about 95% as much as gold foil does. Unfortunately, its silver.

I wouldnt be surprised if NASA had come up with better stuff though. Probably some sort of layered mix of aluminum and fibers... I remember they came up with a crazy insulation for the space station and other missions, but that stuff is crazy expensive.