Tests we did in college in the desert showed that 3 feet of airspace was required between the dark shade cloth (black plastic
sheeting) and your head. The thinner the material, the more
heat (light) came through. Space blankets were not quite as good,
tho the heavy duty kind weren't bad. The thin ones did very
little as did clear plastic sheeting.

This was in temperature hot enough that we wore thick
wool socks cause the sand was so hot. If I remember right it was
about 115 degrees F. If you had the black plastic next to your
body you would cook. The airspace was crucial.