The research team, led by André Desjarlais of ORNL, is conducting side-by-side experiments comparing ballasted roofs of differing ballast quantities/weights/methods to exposed black membranes and exposed white membranes. The purpose of the study is to investigate whether ballasted roofs could offer similar energy efficiency benefits as cool roofs and could be candidates for cool roof status from the U.S. Environmental Protection Agency (EPA) and other federal and code organizations.
Previous studies at Oak Ridge investigated the performance of reflective membranes. These studies demonstrated that both heavy ballasted and exposed reflective membrane systems can reduce the energy flow into building and therefore provide energy savings in cooling dominated climates.
The results of the study so far show that the maximum surface temperatures of the ballasted systems - 90-103 degrees - are significantly cooler than thos of the exposed black roof membrane - 146 degrees - and about equal to that of the paver system. The exposed white membrane stayed coolest, with peak temperature topping out at 86 degrees. The ballast systems show a delay in peak temperature of from 30 minutes to two hours, keeping these roofs warmer in the early evening hours.
By reducing peak roof temperatures and delaying heat flow into a building, the heavy mass of ballast upon any roof provides measurable energy saving benefits, according to the interim study. But these roofs do not meet the requirements of high solar reflectance and thermal emittance. So the EPA and other organizations do not recognize ballasted roofs as cool roofing systems.
For updated information about this study, visit SPRI or contact the association at info@spri.org. The completed interim report is available for free.
