Green roofs are becoming an increasingly popular alternative to cool roofs. According to a published assessment in 2008, green roofs can provide energy cost savings of up to 50% annually, depending on the climate . A cool roof is designed to reflect sunlight, reducing heat transfer to the inside of the building. Cool roofs are usually white or other light color. A green roof, however, has vegetation on its surface. The name "green roof" is fitting, considering the green plants on top; nevertheless, this name refers to the environmental impact.
Typically, green roofs are implemented in locations with high rainfall such as the Midwest and the East Coast. However, reducing heat-loading inside buildings in cities such as Phoenix and Las Vegas is crucial to sustainability and reduces energy costs. University of Las Vegas researchers Milburn, Fernández-González, Jones, Solano and Martínez-Wong conducted a Greening Barren Rooftops study demonstrating the benefits green roofs offer in desert environments. The study quantifies the temperature impacts of a green roof on residential architecture in a desert environment.
The results of this study help developers determine which roof types reduce energy consumption during the day and which types reduce energy consumption at night. To conduct this study, researchers recorded temperature data with Onset HOBO® data loggers. Researchers placed data loggers within eight different pods simulating three different roof types: insulated cool, insulated green, and un-insulated green. A weather station was also installed to measure precipitation, wind speed, and temperature in order to monitor the effects of evapotranspiration and thus plant survival.
Researchers installed data loggers with thermistor temperature sensors in two locations inside each pod on the west wall - directly underneath the roof deck and under the soil layer. Researchers graphed the data using Onset's HOBOware® Pro software and then exported the data to Microsoft Excel for more detailed analysis.
Based on previous green roof research, 27% of solar radiation is reflected off of the plant material, 60% is absorbed by the plant material, and 13% penetrates the soil. Since the vegetation on top of the roofs in the current Greening Barren Rooftops study created a barrier from the sun, less solar radiation penetrated the soil on the green roofs than on the cool roof. And during the day, pods with green roofs allowed less heat to make its way inside the pods with cool roofs.
On average, the un-insulated green roof had the coolest daytime temperatures and the warmest nighttime temperatures, while the insulated cool roof had the hottest daytime temperatures and the coolest nighttime temperatures.
Figure 1 Indoor air temperature of green roof compared Z against the white (cool) roof (Pod #1)
Figure 2 Soil temperature of green roof pods compared against the white (cool) roof (Pod #1) exterior surface temperature
Based on the results of this study, for reducing energy consumption during the day, un-insulated green roofs are better than both cool roofs and insulated green roofs. For reducing energy consumption at night, insulated cool roofs are best and insulated green roofs are second best. Un-insulated green roofs did not fair well in this category since they act as heat sinks and heat interior spaces at night.
Other benefits of green roofs include reduced urban heat island effect, extended roof life, more available green space, cleaner air and water, and less noise pollution.
i Muga, H., Mukherjee, A. and Mihelcic, J., 2008. An Integrated Assessment of the Sustainability of Green and Built-Up Roofs. Journal of Green Building, 3, 2: 106-127.
ii Wasted Space: Altering building temperatures by greening barren rooftops in the desert Southwest. Lee-Anne S. Milburn, Alfredo Fernández-González, Travis Jones, Fidel Solano and Elvira Martínez-Wong. 2009.
iii Wong, N. H., Cheong, D. K. W., Yan, H., Soh, J., Ong, C. L. and Sia, A., 2003. The Effects of Rooftop Garden on Energy Consumption of a Commercial Building in Singapore. Energy and Buildings, 35, 353- 364.