White roofing isn't always green.
The industry has long held the assumption that white roofing, because of how it reacts with surrounding air temperatures--think about the idea that the color white reflects color rays, while black absorbs them--is more energy efficient than black roofing.
A new study by Virginia Tech’s College of Architecture and Urban Studies’ Center for High Performance Environments debunked this belief through an analysis of how black and white roofs differently affect temperatures of surrounding air and adjacent building materials and surfaces.
Led by Elizabeth Grant, associate professor in the School of Architecture + Design, a team of student researchers set up equipment on the roof of a small animal hospital at the Virginia-Maryland College of Veterinary Medicine to collect data on the effects of broadly implementing reflective white roofing on projects, regardless of climate, microclimate, or building-specific geometry.
The team created different test sites on the roof that were either covered in black ethylene propylene diene monomer (EPDM) and white thermoplastic polyolefin (TPO) membranes, and used 126 sensors to measure surface and air temperatures on each on bright, sunny days with little or no wind. 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 surface of the black roof was not surprisingly hotter than that of the white roof. However, the black roof heated the air within 5 ½ inches, but there was no difference in air temperature impact for black or white roofing beyond that distance. Also, electrical metallic tubing and glazed and precast concrete wall panels near the white roofing where 3-9 degrees warmer than those near the black roofing surfaces.
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. But Grant says there's much more to it than that.
“Roof selection is an architectural issue,” Grant said in a news release. “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.”
