Using Data Loggers to Monitor Green Roof Temperatures
We hear a lot these days about how we can build our environments more responsibly and efficiently, but how can we be sure that the money, effort, and expert advice put into a product or project is actually paying off?
That’s what the American Society of Landscape Architects (ASLA) wondered when it installed a demonstration green roof on its headquarters in Washington, D.C.
Green roofs are rooftops covered in soil and plants, underlain by a waterproof membrane and support and drainage structures. Such roofs are typically one of two types. An extensive green roof is topped with six inches or less of soil, and planted with shallow-rooted, ground-hugging plants. An intensive roof, on the other hand, has six inches to two feet of soil, and supports larger plants, shrubs, and even trees.
The ASLA’s green roof designer, Michael Van Valkenburgh Associates, Inc., combined the two types, creating a multilevel rooftop garden. The structure is complete with slopes, shrubs, and an innovative metal-grated walking area suspended over low plant cover.
The green roof is more than eye-catching, however. Green roofs are said to lessen the urban heat island effect, help in stormwater management, and decrease building energy use by insulating it from the elements, among other benefits. Since a major goal was to use the green roof for demonstration purposes, the ASLA wanted hard data on its performance.
That’s where battery-powered data loggers come in. Keith Swann works for the ASLA, and it’s his job to monitor rooftop temperatures. The heat island effect occurs when typical blacktopped roofs and pavements absorb solar radiation, increasing average temperature above them, and by extension, above a whole city. In contrast, green roofs don’t absorb as much solar radiation, and temperatures are cooler because of evapotranspiration, shading by the plants, and moisture held in the soil.
The ASLA’s green roof was installed in April of 2006, and in July, battery-powered temperature data loggers from Onset® were deployed. The loggers, called HOBO® Pendant temperature loggers, are capable of monitoring and logging temperature day and night at user-defined intervals for up to a year at a time. The size of a C-cell battery, the loggers are enclosed within a tough plastic housing that keeps them dry and clean in rough outdoor or indoor conditions.
The ASLA’s loggers monitor temperatures at three points on the green roof: on a planted slope, under the metal grating, and in the staircase. Additionally, loggers are situated on nearby conventional black tar and black non-tar rooftops.
“We have found that the HOBO data loggers are much easier to use than the large temperature monitors we used before,” Swann said. “Their portability is great for placement. We are accessing other buildings with the help of their personnel, and the loggers are barely noticeable on their roof, which is a benefit to us.”
Swann used the loggers’ software, HOBOware, to set the loggers to store readings every two hours. When it’s time to download the data, he simply grabs a laptop and climbs up to the roof. An optic base station connected to the laptop’s USB port downloads the data from each logger in less than half a minute, and Swann uses HOBOware to compare data from all sites on one graph. He applauds the simplicity of the data-collection process. “It is their ease of use that makes them so popular with the organization; we’re pleased to be able to pull the data so easily.”
The data show that from July 2006 to May 2007, at daily peak temperature, the green roof was as much as 32°F cooler than the surrounding rooftops. “The differences in the temperature of the green roof vs. the neighboring roofs is supportive enough to show its great insulating properties,” Swann explained. “But that’s not all. We also reviewed earlier electricity usage to determine if any savings were had after installing the green roof. Using ‘before and after’ data, it was determined that the first quarter of 2007 showed a 10% savings on the electrical use.”
The cooling and insulating effect is only expected to increase as the plants grow, and, thanks to the HOBO Pendant temperature data loggers, Swann and the ASLA will be able to show the almost daily stream of visitors that the green roof is, indeed, working.