Heimhenge uses three lines of defense to reject the heat of the desert Sun …

BARRIER 1: The roof surface is white — the only sensible color in this climate. The immediate result of choosing this color is the reflection of up to 80% of the Sun’s radiant heat energy. I would have used mirrors, but they tend to blind airplane pilots. On a hot summer day I can put my hand on the roof and feel the coolness. Try that on a roof with red clay tile or asphalt shingles and you will burn your hand.
BARRIER 2: This is a cross section of the roof. There’s about 2 inches of hard polyurethane foam between the white surface coating and the plywood roof base. You can walk on it without causing damage. This is the same type of foam insulation used in picnic coolers. The 2-inch thickness is rated R-15 against heat conduction. Cost was $2500. It was sprayed directly onto the plywood base, and then coated with the white elastomeric surface layer. This layer needs to be reapplied every five years, or as needed due to wear from birds and blowing sand.
BARRIER 3: Underneath the plywood base, and inside the “attic,” is a layer of foil-coated cardboard (InsulFoil™ radiant barrier). It also acts as a reflector of IR radiation, and turns back whatever amount of heat penetrates the first two barriers. In the winter, when the interior is warmer than outside, it reflects heat trying to escape from the house back down. Cost do our entire attic was $300 (around 10¢ per square foot), plus a fair amount of time on a ladder using a staple gun. 

The total effective insulation value of these three barriers is R-35. The FHA recommends R-26 for our climate zone, so we’re well above average already. But when you add the effect of the solar-powered attic vent fans (only run during the summer), and the 6″ layer of fiberglass insulation above the interior ceilings, the vertical total works out to around R-75! Along with the earth sheltering, this is why Heimhenge is so easy to cool.