Urban planners who want green roofs in their cities need to remember that the roofs may not work the same way in different climates. Green roofs, which are covered with a layer of a vegetation to keep the building cool, perform differently according to the amount of solar radiation and precipitation present, according to Elie Bou-Zeid, an assistant professor of civil and environmental engineering.
In a study published in the journal Building and Environment, Bou-Zeid and his team found that the green roofs on the campuses of Princeton and Tsinghua University in Beijing performed similarly when the researchers controlled for the radiation and precipitation levels in the two areas, indicating the levels’ importance in green roof function. With support from the U.S. Department of Energy through Pennsylvania State University’s Energy Efficiency Building Hub and the National Science Foundation of China, the researchers used surface temperature, heat convection from the Earth’s surface to the atmosphere, and the amount of incident energy conducted through the roof as performance measures.
Bou-Zeid said he hopes his work will help city planners account for the specific climatic conditions in their cities when integrating rooftop gardens into their building decisions, and assess the potential benefits of irrigation that improves green roof performance in dry periods.
Highly effective green roofs are important in cities, which suffer from the “urban heat island” phenomenon: a sustained period of excessive heat in metropolitan areas caused by buildings that absorb heat and release it into the atmosphere, a lack of vegetation, and high human activity. Increasing the number of green spaces will trap rainwater, Bou-Zeid explained, thereby providing a “heat sink” in which evaporation of that water encourages heat loss and cools things down.
The New York City Office of the Mayor is taking the heat waves of the city particularly seriously, Bou-Zeid said. New York’s asphalt and concrete roads and buildings actively absorb heat, making the area sometimes up to seven degrees warmer than its neighbors. Bou-Zeid is working with representatives from the NYC Cool Roofs program, a citywide initiative to promote the use of reflective, white rooftop coating, to examine which areas of the city will suffer most during a heat wave. He later hopes to relate physical maps of area-specific heat stress in the city to physical health indicators.
“Heat waves are the deadliest natural disasters,” Bou-Zeid said. He noted that the 2003 European heat wave, which produced the Northern Hemisphere’s hottest-ever August, caused up to 70,000 deaths in the region. “They are silent killers.”
–By Tara Thean