How cool roofs can lower your AC costs

(Video: Illustration by Emily Sabens/The Washington Post; iStock)

Years ago, I walked onto the roof of the Palo Alto Research Center. The nondescript beige building in Silicon Valley, known for groundbreaking technology, is the birthplace of the mouse, laser printer and Ethernet.

That afternoon, researchers were sending heat from the roof into space.

Although the building’s rooftop was scorching hot under the burning sun, when I reached out to touch the prototype I was there to observe, a box covered with a white material, it felt inexplicably cool, as if I was touching a steel refrigerator door.

This was my first encounter with passive cooling technology — materials designed to reflect or radiate energy back into the world.

The easy way to do this is by painting something white, a method homeowners in places from the Mediterranean to the Middle East had pretty well figured out for millennia. But this mostly reflects the visible spectrum of sunlight. Getting rid of infrared radiation — the part you can’t see — cools a building as well.

That’s what the research center’s white material did. By reflecting and re-emitting much of the energy it encountered, it was even cooler than the temperature of nearby objects.

I’ve often thought about it on hot summer days, after getting into my car or while baking at home: Why isn’t everything covered in it? In theory, passive cooling could lower energy bills, save lives and even cool the planet itself.

It’s finally catching on. Let’s pick some cooler colors for your life.

For millennia, builders and architects have painted roofs in light colors to reflect sunlight. Over the past few centuries, however, asphalt and tar have blackened the urban landscape, says NASA. These materials, while durable, waterproof and cheap, inadvertently turn cities into urban heat islands.

Cities, on average, are now 2 to 5 degrees warmer than rural or natural areas. On hot summer days, that difference can soar to as much as 20 degrees. A black roof in summer in New York City, for example, might exceed 190 degrees Fahrenheit, more than hot enough to fry an egg.

This is deadly. There were more than 10,000 heat-related deaths in the United States between 2004 and 2018, according to the Centers for Disease Control and Prevention, a toll expected to worsen as temperatures rise.

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So researchers have long looked to passive cooling. Artificial cooling consumes 15 percent of buildings’ total energy use in the United States and about 70 percent in hot countries such as Saudi Arabia.

By installing cool roofs, air conditioning use could be cut by about one-third in warm regions, estimates Lawrence Berkeley National Laboratory. If all large North American cities — those with populations above 1 million — adopted passive cooling on roofs and pavement, we’d prevent the equivalent of the world’s total annual greenhouse gas emissions, or 57 billion tons of carbon dioxide.

“The real question is not whether we should move toward cool roof technology,” said Art Rosenfeld, a Berkeley Lab physicist affiliated with this research. “It’s why we haven’t done it sooner.”

Governments around the world are starting to incorporate passive cooling into their regulations and incentives. In 2014, Los Angeles became the first major city to mandate cool roofs for new residential construction. Today, Houston, Austin, Toronto, Miami Beach, Atlanta, Denver, Chicago and New York all have cool roof codes or ordinances on the books.

Previous generations of cool roofs were generally low-tech: the equivalent of a fancy can of white paint or gray shingles. These work. On a summer afternoon, estimates Berkeley Lab, a white roof will stay at least 31 degrees Celsius (55F) cooler than a darker one.

But for practical and aesthetic reasons, not everyone wants a white roof. New products approach the performance of white material in dozens of colors, including black.

For example, 3M’s cool roofing granules, the knobby texture in asphalt shingles, can reflect up to 25 percent of solar energy, even in dark colors, says Tim Hebrink, a senior staff scientist at the company. That’s about twice as much as conventional shingles. More advanced materials, such as polymer films, reflect more than 90 percent of incoming radiation at little to no extra cost across many colors. Hebrink says 3M plans to release them sometime in the next year or so.

More variety and falling prices are transforming the market, says Kurt Shickman, director of extreme heat initiatives with the Adrienne Arsht-Rockefeller Foundation Resilience Center. For years, advanced passive cooling roofing materials were priced about 30 percent higher than conventional ones. Today, they sell at a modest premium or even price parity.

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Shickman says most people should view redoing their roofs as they do remodeling their bathrooms: as a home improvement that pays off. “We’re not aspirational about our roofs,” he says. “We just want to it work. … But it’s a tremendous amount of money that’s being lost.”

The more advanced materials can pay for themselves in less than a year, at least in hot climates, estimates the Environmental Protection Agency. For example, the price premium to install a cool roof runs between zero and 20 cents per square foot compared to a conventional roof. Yet cost savings in California were estimated to be 50 cents per square foot — $500 in annual savings for a typical roof. In one trial in San Antonio, says Shickman, annual savings topped $1,000.

Reflecting radiation into space

All material absorbs and reflects heat. When sunlight hits your skin, or you touch a dark surface on a sunny day, you feel this as heat. But only about half of the solar radiation reaching Earth is visible. The rest is infrared radiation. That infrared radiation is why you feel a hot oven from several feet away.

Passive cooling materials reflect solar radiation and shed excess energy as infrared radiation. The most advanced passive materials — like the white material on the research center’s roof — go a step further. They radiate heat directly into space by taking advantage of something called the “atmospheric window” — a narrow gap that allows electromagnetic radiation to escape into the cold void above our atmosphere.

Imagine our atmosphere as a curtain. Most of it is drawn tightly, absorbing the sun’s radiation. But two narrow sections of the curtain are open slightly. These allow bands of electromagnetic radiation to pass through. The first gap, on the left side of the graphic below, allows visible sunlight and infrared radiation to reach Earth’s surface.

The second — the black range on the right side of the graphic — allows infrared radiation to go back into space. By shedding this excess energy, while also reflecting as much of the sun’s energy as possible, the temperature of passively cooled surfaces can fall below the objects around it. Under ideal conditions, the difference can be more than 30C (54F) cooler than a comparable black roof.

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Should you get a cool roof?

It depends. First, you can use online calculators to estimate your energy savings by analyzing your climate, insulation, building type and energy prices. Dow offers a Cool Roof calculator; a second, if more technical one, is available from Oak Ridge National Laboratory.

Next, talk to a roofer. Most are now familiar with these materials, but it pays to come prepared. You can browse a comprehensive database assembled by the Cool Roof Rating Council with an array of materials. For maximum summer savings, choose those with solar reflectance and thermal emittance values as close to 1 as possible — that is equivalent to reflecting 100 percent of incoming sunlight, or emitting 100 percent of infrared radiation.

New technology may also change the game: Researchers at the University of California at Berkeley have developed a cheap, passive cooling film that maximizes solar reflectivity and minimizes heat loss at lower temperatures.

If your roofer isn’t familiar with these materials, most manufacturers have a vetted list of installers in your area.

Shickman says almost anyone in the continental United States can benefit from a cool roof. While there is a small “winter penalty” for cool roofs in northern latitudes since they reflect heat, he says the modest increase in winter heating costs is typically outweighed by lower cooling costs during the summer.

But that’s not universally agreed upon. Not everyone needs a cool roof, says Ravi Kishore, a research engineer at the National Renewable Energy Laboratory. The farther north you live and the more insulation you have, the smaller the benefit, because less heat makes its way inside.

Instead, he uses a rule of thumb: “The hotter and drier the climate, the bigger the benefit,” he says. In the northernmost latitudes of the United States, the benefits may be minimal — at least for the building owner.

But as climate change advances, dark roofs will raise everyone’s thermostat because of the urban heat island effect, resulting in preventable heat-related deaths. Our roofs, in other words, are becoming a public health issue.

Expect more cities to follow the example of Los Angeles, especially in the southern half of the country, by mandating passive cooling technology. In the end, a cool roof is for everyone.

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