For one thing, buildings in Chinese cities tend to be much taller, making cool walls an important factor for energy efficient buildings. He noted that the energy savings were similar to what cool roofs would yield in the U.S., but said there are other differences between the two countries. But for the other cities studied, “there should be either credits or requirements for cool roofs in building energy efficiency standards for all hot summer climates in China,” Levinson recommended. For the factory in Foshan, which did not have air conditioning, summer measurements showed that the cool roof reduced the indoor air temperature by 1 to 3 degrees Celsius, and also reduced roof heat flow into the building by two-thirds.Ĭlimate zones with hot summers would benefit from cool roofs, the Berkeley Lab-led study found.įor colder cities in northern China, such as Harbin, Changchun, and Beijing, the study does not recommend cool roofs because the winter heating penalty exceeds the summer cooling savings. In the office building experiment in Chongqing, the researchers found that a white roof reduced air conditioning use on a summer day by 9 percent. Annual savings in emissions of NO x and SO 2, two pollutants associated with major health concerns, ranged from 2.9 to 13 grams of NO x per square meter of roof, and from 5.4 to 33 grams of SO 2 per square meter of roof. A small office building in Guangzhou with a 1,500-square meter cool roof would save 5 metric tons of CO 2 each year, about the same as taking a car off the road for the year. Residential cost savings are smaller mostly because residential electricity prices are lower.Īnnual reductions in emissions of CO 2, the principal climate-warming gas released by burning fossil fuels, ranged from 1.1 to 3.4 kilograms per square meter of roof in the four warmer cities. For residential buildings, the annual energy cost savings per square meter of roof ranged from $0.07 to $0.33. $0.16 to $0.49 per square meter of roof in the annual energy cost of an office building. The researchers found that increasing the albedo, or solar reflectance, of roofs to 0.6 from 0.2-for example, upgrading to an aged white roof from an aged gray roof-in four cities with hot summers (Chongqing, Shanghai, Wuhan, and Guangzhou) saves U.S. Additionally, while nearly 80 percent of the sunlight reflected from a roof can escape to outer space, the “thermal infrared” energy radiated by a hot, dark roof is trapped by greenhouse gases, such as CO 2 and water vapor, warming the atmosphere. Hot, dark roofs on any building also warm the city air, aggravating what is known as the urban heat island effect. For buildings without air conditioning, the sunlight absorbed by a dark roof heats the space below, making it less comfortable. “While the concept is the same everywhere, we wanted to show that cool roofs would also be effective for Chinese construction, in Chinese climates, and with Chinese building operation practices.”īy reflecting more sunlight, cool roofs reduce heat flow into the building, which lowers energy consumption and power-plant emissions if the building is air-conditioned.

“Cool roofs have been well demonstrated in the United States, Europe, and elsewhere,” said lead author and Berkeley Lab scientist Ronnen Levinson. A follow-up study will compare white, black, and garden roofs in China.

Chinese roofs