The Benefits of Cool Roofs
 Photo: Duro-Last Roofing
Cool roofs or light colored, reflective surfaces reflect more sunlight than dark ones, turning less of the sun's energy into heat. Increasing the reflectance of our roofs can can reduce the temperature of buildings, cities and even the entire planet. White roofs, and other cool surfaces, are an efficient and affordable tool to help alleviate some of the stress caused by rising local, regional and global temperatures. A cool roof is a roofing system, consisting of materials of white or light-color, which effectively reflects the sun's energy from the roof surface. Reflective surfaces do not need to be white. The term "cool roofs" is used to exemplify how materials of high solar reflectance come in a range of colors, including many shades of grey and even light red. Learn more.
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According to
Global Cool Cities Alliance:
Cool Roofs
Easy. Cheap. Efficient. 
Image: White Tops NYC | Click to enlarge
If all urban, flat roofs worldwide were whitened, the reduction in carbon emissions would be 24 Gigatonnes, or equivalent to taking 300 million cars off the road for 20 years. Learn more. |
A study by Lawrence Berkeley National Laboratory showed that if cool roofs were widely adopted, the Greater Toronto metropolitan area could save more than $11 million annually on energy costs. Learn more.
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Quick Facts About Cool Roofs
The latest research shows that cool roofs and pavements can help cool the entire planet. Permanently retrofitting urban roofs and pavements in the tropical and temperate regions of the world with solar-reflective materials would have an equivalent effect on global temperatures as stopping the emissions of 44 billion tonnes of CO2, or over one year’s worth of humanity’s CO2 emissions. 
Pavements and roofs comprise approximately 60% of city surfaces. Cool roofs and cool pavements can help reflect away the sweltering temperatures of summer urban heat islands, improving air quality and comfort. Widespread installations of cool roofs and pavements can reduce summer air temperatures in cities by 2 to 3 degrees Celsius (4 to 5 degrees Fahrenheit). |
After cool pavements were installed in a city park in Greece, surface temperatures were reduced by 12°C and ambient temperatures were reduced by 1.9°C. 'Cool' paving materials helps lower city temperatures
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Even School Buses Are Catching On!
 Photo: LemonReese/Flickr | Click to enlarge
"In a typical city, pavements account for 35 to 50 percent of surface area, of which about half is comprised of streets and about 40 percent of exposed parking lots. Most of these streets and parking lots are constructed with dark materials. Because dark pavements absorb almost all of the sun’s energy, the pavement surface heats up, which in turn also warms the local air and aggravates urban heat islands.” Like cool roofs, which are lighter-colored roofs that keep the air both inside and outside the building cooler by reflecting more of the sun’s energy, cool pavements reflect as much as 30 to 50 percent of the sun’s energy, compared to only 5 percent for new asphalt (and 10 to 20 percent for aged asphalt)." ~ Heat Island Group, 'Berkeley Lab researchers showcase cool pavement technologies'
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International Conference “Passive and Low Energy Cooling for the Built Environment”, May 2005, Santorini, Greece: Cool colored roofs to save energy and improve air quality, by Heat Island Group, Lawrence Berkeley National Laboratory |
The True Cost of Dark Roofs
"A new study of how different white roofing materials performed “in the field” in New York City over multiple years found that even the least expensive white roof coating reduced peak rooftop temperatures in summer by an average of 43 degrees Fahrenheit. If white roofs were implemented on a wide scale, as the city plans to do, this reduction could cut into the “urban heat island” effect that pumps up nighttime temperatures in the city by as much as 5 to 7 degrees Fahrenheit in the summer, said the study’s lead scientist, Stuart Gaffin of Columbia University." ~NASA |
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Most of the roofs in the world, including 90% in the United States, are dark-colored. In the heat of a full sun, a black roof surface can increase temperatures as much as 122 degrees Fahrenheit, reaching temperatures of 150-190 degrees Fahrenheit. This massive heat increase contributes to:
On a sunny summer afternoon the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C)., according to the EPA. The elevated air temperatures associated with this summer “urban heat island” (UHI) make air conditioners work harder to keep buildings cool, which can strain the power grid. They also accelerate the formation of smog, degrading air quality. Image courtesy of Heat Island Group, Lawrence Berkeley National Laboratory
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The "Heat Island" Effect
 On May 11-12, 1997, NASA used a specially outfitted Lear Jet to collect thermal data on metropolitan Atlanta, Georgia. Nicknamed “Hot-Lanta” by some of its residents, the city saw daytime air temperatures of only about 26.7 degrees Celsius (80 degrees Fahrenheit) on those days, but some of its surface temperatures soared to 47.8 degrees Celsius (118 degrees Fahrenheit). In this image, blue shows cool temperatures and red shows warm temperatures. Pockets of especially hot temperatures appear in white. Image courtesy of NASA/Goddard Space Flight Center Scientific Visualization Studio For hundreds of millions of people living in or near cities, urban heat islands are a growing environmental and health concern. If you turn on the local weather report, you'll most likely notice a strange trend. Temperatures are often higher in cities in comparison to their surrounding areas. According to the EPA, the annual mean air temperature of a city with 1 million people or more can be 1.8–5.4°F (1–3°C) warmer than its surroundings. In the evening, the difference can be as high as 22°F (12°C). This is a result of a phenomenon known as the urban heat island effect. "Heat Islands" are mainly caused by dark surfaces, which absorb and hold more heat from the sun, and from less vegetation, which would provide shade and cool the air. Elevated temperatures from urban heat islands, particularly in the summer, affect a communities environment and quality of life. Although the heat island impacts lengthen the plant-growing season, the EPA outlines the many detrimental effects of urban heat islands:
With the current rate of greenhouse gas emissions, studies by the National Academies of Sciences in the United States, show that that by
2100, a hot summer day would reach temperatures of roughly 137°F. Learn more.
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What About Winter?
Roof covered with snow/Wikipedia Commons
While it may seem that white roofs do more harm than good in colder climates, research shows that the heating benefits of a dark roof in the winter are very minor because the days are shorter, the angle of the sun is low, the skies are cloudier and sometimes the roofs are covered in snow. The amount of heat energy you may lose in winter may be at maximum 30% of the summertime savings. Also, hot air always rises. Thus, whatever heat is transmitted into the attic or into living areas from a dark roof will stay very near the ceiling trying to get back out again instead of circulating through the house. Aside from energy savings, a white roof also saves money on roof repairs. Since black or dark roofs become overheated in the sun, they are prone to warping and cracking. This means more roof repair. A white roof saves money on roof repair because it is more durable. A white roof can also extend the life of a buildings cooling equipment by decreasing use. If you live in a four-season climate where you need cooling in the summer and heating in the winter, a white roof will most likely save you money. For more information, see the Global Cool Cities Alliance Q&A page and how the White Roof Project answers the question, "What about the Winter?" According to the Cool Roofs Rating Council and other sources, "The roof is an insignificant source for heat gain in winter. While cool roof owners may pay slightly more to heat their homes, this amount is usually insignificant compared to the cooling energy savings during the summer". However, In much colder climates where annually there are far more heating days than cooling days, such as in parts of Alaska, cool roofs are not typically a worthwhile investment, in terms of energy efficiency and savings. In these much cooler locations, the cooling benefits of a reflective roof surface do not outweigh the winter months heating benefits on a less reflective, or dark roof surface. Learn more.
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Calculate Your Cool Roof Savings
Cool roofs are not just flat and white. There are many options for cool roofs, including cool asphalt shingles. Learn more. |
In addition to cool roof systems, communities and individuals can take four other main steps to reduce the heat island effect and help cool cities and the planet:
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The idea of white roofs isn't a new one. There are many locations in the world which fully utilize the benefits of cool roofing systems, especially in the Mediterranean and Middle East:
 Houses with white roofs, like these in Greece, would be able to reflect light back through the atmosphere. Image: Alamy |
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Cool Your Building. Cool Your City. Cool Our Planet.
Last Revised: 11/20/13
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