Green Roofs in the Tropics Conserve Energy

Background: Concrete buildings on Guam are exceptionally strong but also accumulate large amounts of heat. In the tropical environment of Guam, where 24 h average temperature ranges from 28 to 29oC year round, air conditioning is used every day and continuously. Concrete roofs are often painted light colors, which make them more reflective and accumulate less heat. They are also suitable for establishment of vegetation, which results in a large decrease in roof temperature and therefore decreases the need for cooling. Objective: The objective was to determine the magnitude of temperature reductions resulting from light color and from vegetation covering roof tops and to use this information to estimate energy savings. Method: Temperature was measured on the undersides of concrete model roofs in both sunny and rainy weather. Results. The temperatures on the undersides of light-colored concrete model roofs rose up to 3oC less in the course of the day than did those of dark-colored ones. The temperatures of "green" (vegetation-covered) model roofs rose up to 12oC less than did those of either of the bare concrete models. Conclusion: The differences were so large that use of green roofs on the tropical island of Guam, where most buildings are concrete and air-conditioning is needed year round, could cut a typical household's electric consumption in half.