White roofing isn’t always green, Virginia Tech study confirms
A study out of Virginia Tech’s College of Architecture and Urban Studies’ Center for High Performance Environments presents a new scientific challenge to widely held industry assumptions that white roofing is the best option for commercial builders.
The study, led by Elizabeth Grant, associate professor in the School of Architecture + Design, with a team of student researchers, is an impartial, data-driven academic analysis of how black and white roofs differently affect temperatures of surrounding air and adjacent building materials and surfaces.
The team’s findings add to a growing body of evidence that roof selection isn’t just black and white – which carries broad implications for building industry and green building certification programs.
Both the U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) program and the international ASHRAE 90.1 (Energy Standard for Buildings Except Low-Rise Residential Buildings) standard require or reward use of white “cool” roofing to mitigate urban heat island effects. As more building clients adopt these guidelines, the use of white roofing in new buildings has become widespread.
“Roof selection is an architectural issue,” Grant said. “It’s not as simple as slapping a white roof on everything to gain a LEED point. Architects and roof consultants need to be aware of potentially adverse thermal effects when choosing roof membranes, particularly where roof surfaces are adjacent to walls that can be affected by bounced sunlight. We need to be intentional in which color roofing we use and consider where the heat goes.”
Grant co-authored an article on the study for the journal Architectural Science Review with research assistant and Ph.D. student Kenneth Black and Stephen Werre, a statistician with the Virginia-Maryland College of Veterinary Medicine. The study has also been published in Roofing and Home Energy and presented at the PLEA (Passive Low Energy Architecture) global conference in Scotland and the RCI International Convention and Trade Show in the U.S.
Roofing industry contacts approached Grant, associate director of Virginia Tech’s Center for High Performance Environments, in 2015 to conduct an impartial, scientific study. Supported by a grant from the RCI Foundation, Grant teamed up with Werre, Black, and nine architecture students to collect data on the effects of broadly implementing reflective white roofing on projects, regardless of climate, microclimate, or building-specific geometry.
The team found the perfect experimental site right in their own backyard. The vet school’s small animal hospital offered a low-slope roof surface next to to a west/southwest-facing opaque wall and a south/southwest-facing glazed wall that provided ideal features and safe access.
Using 126 sensors installed specifically to measure surface and air temperatures, they tested multiple roof locations covered in black ethylene propylene diene monomer (EPDM) and white thermoplastic polyolefin (TPO) membranes on bright, sunny days with little or no wind. The researchers controlled for as many variables as possible, taking temperature readings on the same days, times, and under the same atmospheric conditions.
Temperatures were recorded and compared on the membranes’ surfaces, in the air above at various distances, at electrical metallic tubing above, and at adjacent opaque and glazed wall surfaces. The data was then statistically averaged and analyzed.
The key findings? Not surprisingly, the surface of the black roof was significantly hotter than that of the white roof. But, while the black roof heated the air within 5 ½ inches, there was no difference in air temperature impact for black or white roofing beyond that zone. In addition, electrical metallic tubing and glazed and precast concrete wall panels near white TPO roofing were 3 to 9 degrees Fahrenheit warmer than those near black EPDM roofing.
Grant emphasizes that the study is one new piece in the ongoing black/white roofing debate, which has experts weighing in on issues ranging from energy efficiency and durability to climate change impact.
While her study doesn’t draw any conclusions about whether white or black roofing is good or bad, she does say it should serve as a suggestion that the industry should be more methodical about roof selection. As an educator, she also feels strongly that architects and designers need to be aware of basic building science concepts when making these decisions, a theme explored in her new book, “Integrating Building Performance with Design: An Architecture Student’s Guidebook” (Routledge 2017).
“We hope Virginia Tech’s research will enhance the ongoing industry discussion and awareness of the impact of reflective roofing on the adjacent building environment,” she said. “Our next logical steps will be to explore the interaction of roof reflectivity and insulation and examine thermal effects over time.”
Other researchers on the project included undergraduate architecture students Alyssa and Dana Brink, of Chesapeake, Virginia; Jeffrey Herring, of Virginia Beach; Sean O’Mara, of Skillman, New Jersey; Hannah Utter, of Concord, North Carolina; Jeffrey Rynes, of Norton, Ohio; Yonghoon Sheen of Taejeon, of South Korea; and master’s students Meera Murali of Chennai, India, and Christoph Opitz of Vienna, Austria.