The difference between a house that is unbearably hot and one that is substantially cool lies in the color of the roof. A flat roof in the midday sun receives about 1,000 watts of sunlight per square meter. A dark will absorb most of this energy, heating up the underlying building as well as the surrounding air that air conditioners suck in. A white roof, on the other hand, reflects around 85 percent of the sunlight that hits and heats to just a few degrees warmer than the outside air temperature. With such a massive difference in heat, it’s no wonder the Greeks have been painting their roofs white for centuries.