Cool Roof

Swap a dark, absorbent roof surface for a reflective one and the roof does the same job but runs far cooler in direct sun. That is the whole idea behind a cool roof. It keeps the attic and the rooms below it cooler, so the air conditioner works less during a long Arizona summer.

Two properties make a roof "cool." The first is solar reflectance: how much sunlight the surface bounces away. White roofing reflects about 60% to 90% of sunlight, and even special "cool colored" products reflect roughly 30% to 60%, far more than a dark shingle. The second is thermal emittance: how readily the surface gives off the heat it does absorb. The U.S. Department of Energy notes a conventional roof can hit 150 degrees F or more on a sunny afternoon, while a reflective roof can stay more than 50 degrees F cooler in the same conditions.

For example, a dark tile roof in Casa Grande can radiate that 150-degree heat straight into the attic, where it heats the ductwork and pushes attic temperatures well past comfort. Swapping to a reflective tile, a light "cool" coating, or a reflective underlayment keeps the deck much cooler, so less heat ever reaches the ceiling and the AC has a lighter load through July and August.

A cool roof is not the same as attic insulation or a radiant barrier, though they all attack heat. The cool roof stops heat at the surface, insulation slows what gets through, and a radiant barrier reflects what radiates into the attic. On a desert custom home, the three work together. The cool roof is the first line of defense against the sun before the heat ever reaches the structure.