Redefining the Roof
It’s highly likely that CEE PhD student Amy Nagengast has thought about your roof more than you have. Nagengast, who submitted her thesis in December 2012, studies the benefits of combining alternative roof technologies that are traditionally used alone, such as roof gardens and solar panels. She is a key figure in the movement to educate the public about the energy performance impacts of roof options and, more broadly, the value of roof space.
A Minnesota native, Nagengast inherited her love of buildings from her father, a carpenter. After earning a bachelor’s degree in civil engineering, she worked for several years at an environmental engineering consulting firm. “I enjoyed the outdoor and design aspects of civil and environmental engineering, but I wanted to engage in more frequent interdisciplinary work with multiple building stakeholders,” she said. “I realized I wanted to move into a field in which engineers are a part of the conversation from the beginning of a project.” In 2008, she decided to pursue a PhD in civil engineering with a focus on sustainable building technologies. The CEE program’s green design research program, within the Environmental Engineering, Sustainability, and Science group, proved to be a perfect fit.
An Evolving Roofscape
Nagengast’s thesis project centered on data collected at a local site: the headquarters of Scalo Solar Solutions, LLC, a Pittsburgh-based designer and installer of solar technologies. Scalo Solar had constructed a massive “living laboratory” on their roof to showcase alternative commercial roof options and to enable research on roof technologies. The state-funded project, called SunScape, occupied more than two National Hockey League rinks’ worth of low-slope, commercial roof. Currently, the project contains three types of roof surfaces, seventeen skylights, and three types of photovoltaic panels that produce enough energy to offset the electricity consumption of roughly ten average American households each year. After learning of the project at a green building conference, Nagengast approached Scalo Solar and asked to be involved. “They had begun acquiring a lot of data, but didn’t yet have someone to make sense of it,” she explained. “So I fit into the project well.”
Nagengast was particularly interested in the intersection of two broad categories of roof technologies that are typically used separately: renewable and surface technologies. Renewable technologies include photovoltaic (solar) panels, skylights, wind turbines, and other technologies that harvest renewable energy. Surface technologies cover a roof’s surface, and include green roofs (vegetative plants), black roofs (black membranes), and white roofs (white membranes). Each of these three surface technologies regulates heat flow through the roof in different ways. While many people study the benefits and cost--both financial and energy-based--of either surface or renewable technologies, Nagengast wanted to know what happened when they were combined. “People typically think about these two technologies separately as they often fight for the same space and money,” she explained. “But they can exist together and harness potential co-benefits.”
Advised by CEE’s Chris Hendrickson and H. Scott Matthews, Nagengast designed a study at the SunScape site to quantify the energy performance effects of pairing solar panels with two different surfaces: green roofs and black roofs. When solar panels reach high temperatures, their performance declines; however, solar radiation (and therefore energy production) tends to reach its highest levels in hot weather. Because green roofs have cooling properties due to evapotranspiration, Nagengast reasoned that combining a green roof with solar panels could reduce the temperature of the panels, thus increasing efficiency compared with solar panels above black roofs.
Nagengast found that while green roofs did increase panel performance, the energy efficiency benefits were minor and only occurred when the air temperature exceeded 77F. However, she notes that while her research focused on solar panel performance and heat flux components, there are additional environmental benefits of using multiple technologies on one roof. For example, on a green roof that includes solar panels, the solar panels generate electricity, while the green roof enables stormwater retention, reduces heat flux, improves air quality, and promotes biodiversity. In addition, the panels offer valuable shade to the green roof that can slow evapotranspiration rates and reduce the amount of heat transferred through the internal spaces.
The applications for Nagengast’s research are numerous. CMU Director of Facilities Management Services Don Coffelt hopes to use her data to make campus roof space more sustainable and energy efficient. Coffelt is also an adjunct professor in CEE and was a member of Nagengast's thesis committee.While several campus buildings currently feature green roofs, installing a combination of roof technologies as studied by Nagengast could lead to greater environmental benefits. Private sector organizations are intrigued because of the scale of the experiment, and have shown interest in extending her research to include additional combinations of roof technologies. In the near future, she will be presenting her work at conferences and giving talks to professional and trade organizations.
Ultimately, Nagengast hopes her work will raise public awareness of alternative roof technologies and the value of roof space. “A lot of interesting technologies can be deployed, but there isn’t one silver bullet across building types or across regions,” she said. “I’m not pushing people to use a particular roof technology; I’m trying to give them empirical data so they can make informed decisions about their roofs in a more robust and structured way.” She hopes to continue studying the interactions between buildings and the outside environment, perhaps by bringing together data on green technologies into a holistic decision-making tool for commercial building owners. “I love buildings, people, and the interactions of complex systems,” she said. “In the future, I see myself in the arena of helping quantify trade-offs and informing design decisions between infrastructure and scarce resources such as energy and water.”
More information about Amy Nagengast’s research can be found here.
While Amy Nagengast was able to conduct the majority of her research from Carnegie Mellon, she also had the opportunity to work on-site installing and maintaining sensors used to monitor conditions across roof types. Here, she is shown among the photovoltaic panels and green roof at the Scalo Solar headquarters.
The SunScape project includes three types of roof surfaces, seventeen skylights, and three types of photovoltaic panels that sit upon the roof of a commercial warehouse in Pittsburgh.
A closer view of the SunScape project. “One reason sustainable roof technologies are gaining popularity is that their benefits span many stakeholders,” Nagengast explained. “For example, alternative roofing choices not only help the building owner by reducing the electricity bills; they also help the local utility by curbing demand, and benefit the community by lessening the urban heat island effect and improving air quality.”