Want to do something cool this summer?

If you’re building a new roof or replacing your old one, you can conserve energy, help reduce the threat of electrical blackouts and help minimize your local urban heat island effect — all by installing a “cool roof.”

Cool roofs are white or light-colored, so they reflect sunlight rather than absorbing it and converting it into heat. Dark roofs absorb sunlight and get hot, transferring some of that heat into the buildings they cover and increasing the need for air conditioning.

So cool roofs can save energy and money. But how can they help prevent blackouts (or rolling brownouts, which are the electric company’s attempt to forestall blackouts)? It turns out that cool roofs do most of their work when they’re most needed.

Air conditioners are heavy energy users, and most blackouts and brownouts occur between mid-afternoon and early evening on hot summer days, when air conditioning demands are at their peak. That’s also when the high summer sun is beating down most intensely on roofs. By reflecting some of the sun’s peak radiation, cool roofs reduce electrical demand during those critical hours.

Air conditioning demand can also be increased by the “urban heat island” effect, the rise in air temperature over cities caused by replacing vegetation with heat-absorbing buildings and roads. The built environment heats up all day and radiates its accumulated heat into the night air. The resulting air pocket (heat island) can be 8 to 12 degrees warmer than the air in the surrounding countryside. Reflective roofs are an inexpensive, passive means of reducing this effect.

However, there’s more to cool roofs than their reflective capacity.

Even the most highly reflective surfaces allow some heat absorption, so cool-roof materials are rated by both their reflectance and their “infrared emittance” value — that is, how easily they cool down by emitting accumulated heat as infrared radiation. Reflective/emittance trade-offs can be understood by comparing two common roofing materials, galvanized steel roofing and asphalt shingles.

You might think that because metal is moderately to highly reflective, it would be an ideal cool-roof material. But while it’s true that bare galvanized metal has a reasonably high solar reflectance value (0.61 on a scale of 0.0 to 1.0), it has a very low infrared emittance (0.04 on a scale of 0.0 to 1.0). Does the phrase “cat on a hot tin roof” ring a bell? (One premium, factory-coated white roof has a somewhat higher reflectance value, 0.67, but it also has a much higher emittance value of 0.85 because the coating “hides” the underlying metal and keeps it cool.)

Common white asphalt shingles, by contrast, have a surprisingly low solar reflectance value of 0.21 (black shingles, at a dismal 0.05, transfer a lot of heat into buildings). But shingles have a very high infrared emmitance value (0.91), so they are not hot to the touch on sunny days and don’t store heat that warms the urban heat island at night.

Galvanized steel roofing and asphalt shingles are just two cool roof options. Others include lightweight cement shingles and fiberglass shingles treated with reflective surface material, and reflective elastomer membranes for “flat” (very low pitch) roofs. (Most houses have sloping roofs, but if yours has a dark, flat roof in good condition, consider applying a coat of special reflective paint to help preserve your roof and cool your building.)

Which cool-roof covering should you choose?

The most important factors in product selection are your climate and solar exposure. If you live in an intensely hot, exposed area, you should pick a material with the highest solar reflectance and infrared emittance combination you can afford (such as a premium metal roof). Even if your winters are cold, reducing your summer air-conditioning demand will more than offset your loss of solar heat in the winter, when the sun is low and mostly strikes your south wall, not your roof.

High reflectance and emittance are just two of many characteristics of a sustainable roofing material. Embodied energy, life cycle considerations and local availability are all factors in this complex decision. Seek advice from local contractors and architects who are knowledgeable and genuinely committed to green building — and to being cool at your ecological house.

Learn more:

Click here to visit Lawrence Berkeley National Laboratory's Cool Roofing Materials's Database.

Click here to see the Environmental Protection Agency's cool roof product information.

Click here to see information on cool roofs and emissivity from Energy Star.

Philip S. (Skip) Wenz is a freelance writer specializing in ecological design issues. He was a general contractor, residential designer, teacher and writer and founded and for 10 years directed the

Ecological Design Program at

the San Francisco Institute of Architecture.