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 hadpretty well figured out for millennia. But this mostlyreflects 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.
A commercial building in Compton, Calif., with its roof painted white. Some roofs in Los Angeles’ South Bay are helping reflect over 60% of the energy from sunlight, according to research by the Berkeley Lab Heat Island Group. (Bing Guan/Bloomberg)
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.
What is a cool roof?
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.
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.
Nandini Chohan, 16, applies reflective paint to the roof of the home she shares with her family in Jodhpur, India. Communities are being trained to assess and implement measures to mitigate the impact and effects of climate change and rising temperatures including painting roofs white. (Rebecca Conway/Getty Images)
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.
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 (55 Fahrenheit) 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 sellat a modest premium or even price parity.
Paint is applied to a roof using a sprayer. (Lakeview_Images/Getty Images/iStockphoto)
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 it to 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.
Here’s how it works.
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 iswhy 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.
Incoming energy from the Sun and outgoing energy from the Earth relative to the electromagnetic spectrum. (NOAA)
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 30 degrees Celsius (54 Fahrenhei) cooler than a comparable black roof.
Should you get a cool roof?
It depends. First, you can use onlinecalculators 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 theCool 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.
This entry was posted on Saturday, July 15th, 2023 at 1:24 am and is filed under Uncategorized. You can follow any responses to this entry through the RSS 2.0 feed.
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As potential uses for building and parking lot roofspace continue to grow, unique opportunities to understand and profit from this trend will emerge. Roof Options is committed to tracking the evolving uses of roof estate – spanning solar power, rainwater harvesting, wind power, gardens & farms, “cooling” sites, advertising, apiculture, and telecom transmission platforms – to help unlock the nascent, complex, and expanding roofspace asset class.
Educated at Yale University (Bachelor of Arts - History) and Harvard (Master in Public Policy - International Development), Monty Simus has held a lifelong interest in environmental and conservation issues, primarily as they relate to freshwater scarcity, renewable energy, and national park policy. Working from a water-scarce base in Las Vegas with his wife and son, he is the founder of Water Politics, an organization dedicated to the identification and analysis of geopolitical water issues arising from the world’s growing and vast water deficits, and is also a co-founder of SmartMarkets, an eco-preneurial venture that applies web 2.0 technology and online social networking innovations to motivate energy & water conservation. He previously worked for an independent power producer in Central Asia; co-authored an article appearing in the Summer 2010 issue of the Tulane Environmental Law Journal, titled: “The Water Ethic: The Inexorable Birth Of A Certain Alienable Right”; and authored an article appearing in the inaugural issue of Johns Hopkins University's Global Water Magazine in July 2010 titled: “H2Own: The Water Ethic and an Equitable Market for the Exchange of Individual Water Efficiency Credits.”
