Cool roofs have become a common component of building-envelope design. Most design professionals would define a cool roof as being white with a surface that reflects solar radiation and emits, rather than absorbs, thermal energy. This definition may soon change or at least expand. ”Evaluating the Energy Performance of Ballasted Roof Systems,” a recently completed study by Oak Ridge National Laboratory, Oak Ridge, Tenn., prepared for the Waltham, Mass.-based Single Ply Roofing Industry, or SPRI, has proven there are other options beyond white roofs to shield a building from solar heat gain.
The study was initiated in March 2004 at ORNL’s Roof Thermal Research Apparatus. Its goal was to address, through a series of experiments, whether ballasted roof systems offer similar energy-efficiency benefits as conventional cool roofs. Overseeing the project were André Desjarlais, group leader; Thomas Petrie, research scientist; and Jerald Atchley, research scientist with the Building Envelopes Program at ORNL; Richard Gillenwater, manager of advanced projects for Carlisle SynTec Inc., Carlisle, Pa.; and David Roodvoets, technical director for SPRI.
By the current definition, cool-roof materials have two important surface properties: a high solar reflectance, or albedo, and high thermal emittance. Solar reflectance is the percentage of solar energy that is reflected by a surface. Emittance is defined as the percentage of thermal energy a material can radiate after it is absorbed. The Atlanta-based American Society of Heating, Refrigerating and Air-Conditioning Engineers Inc. defines a cool roof in its standard 90.1, “Energy Standard for Buildings Except Low-Rise Residential Buildings,” as having a minimum solar reflectance of 0.7, or 70 percent, and minimum thermal emittance of 0.75, or 75 percent. The Sacramento-based California Energy Commission also follows this definition in its Title 24 standard. The Washington, D.C.-based U.S. Environmental Protection Agency requires a minimum solar reflectance of 0.65, or 65 percent, in its Energy Star program; EPA does not have a standard for thermal emittance. By comparison, a roof with ballast over a roof membrane also can reduce peak rooftop temperatures and delay the heat flow into a building. However, because ballasted roofs do not meet the traditional requirements of reflectivity, ballasted roofs had not been officially recognized as a cool roof.
To change that perception, the ORNL study used side-by-side experiments to determine whether ballasted roof systems mirror the energy-efficiency benefits of cool roofs. Six test sections were constructed and installed on the roof thermal research apparatus. One control roof featured a black single-ply roof membrane; the other control roof featured a white single-ply roof membrane. Three roof sections had black EPDM membranes installed and covered with No. 4 stone per ASTM D 448, “Standard Classification for Sizes of Aggregate for Road and Bridge Construction.” Stone sizes were a nominal 1 1/2-inch (38-mm) diameter. One roof section had 10 pounds per square foot (49 kg/m2) of stone, the minimum allowed for ballast. Another roof section had 17 pounds per square foot (83 kg/m2) of ballast, and the third roof section featured 24 pounds per square foot (117 kg/m2). A fourth section had 24-pound-per-square-foot (117-kg/m2) concrete pavers installed over an EPDM membrane.
The test sections continuously were monitored for temperature, heat flow and weather conditions for 36 months. Their surface properties also were periodically verified for solar reflectance and thermal emittance. Data gathered demonstrated comparisons between the ballasted and non-ballasted membranes with the ballasted systems performing as well as the white control roof surface.
Data were collected through April 2006, and an interim report was published by SPRI in September 2007; the report can be viewed at www.spri.org. Based on the ORNL research, ballasted roof systems performed in a manner that is equivalent to or better than traditional cool-roof membranes. After 19 months of testing, the data showed membrane temperatures below the ballast and paver assemblies peaked at temperatures similar to traditional cool-roof surfaces. The membrane temperatures below the 17-pound-per-square-foot (83-kg/m2) and 24-pound-per-square-foot ballast (117-kg/m2) assemblies peaked at 110 F (43 C). The paver assembly peaked at 105 F (41 C). The traditional cool-roof membrane surface had a peak surface temperature of approximately 102.5 F (39 C). When surface temperatures were taken, the traditional cool-roof membrane reflectivity still was above the minimum reflectance value of 0.65 for new roofs, indicating that the ballast systems are performing as cool roofs.
The study confirms that a ballasted roof system’s mass actually acts as a shield protecting a building from solar radiation, which reduces peak temperatures. A ballasted system also delays potential heat flow into a building so more of the cooling load can be moved to off-peak hours of the day. Washed river-rock ballast has an emittance of approximately 0.88 and a continuous reflectance of approximately 0.22, but it is the mass of the ballast that acts as a heat sink and simulates the performance of a traditional cool-roof surface with high solar reflectance and high thermal emittance. The interim report already has influenced ASHRAE and the California Energy Commission to revise their standards to allow an option for ballasted roof systems as cool roofs. A ballasted system option was included in the 2008 version of Title 24 that was just approved and will take affect in 2009. ASHRAE has included it in the early revisions being conducted on the next version for standard 90.1. There are efforts underway to encourage EPA and other regulatory organizations to re-evaluate their definitions of what it means to be a cool roof. In fact, these organizations may reconsider the use of solar reflectance as the metric to determine compliance.
By providing designers and building owners with more roof-system design options, such as cool ballasted roof systems, the market would achieve the desired results of cool roofing and provide designers the freedom to create roof systems that work most effectively in their geographic region for the application and lifespan of buildings. Standards and regulations, after all, only are meaningful when they allow desired goals for sustainability to be achieved by enhancing the overall building envelope system. Tom Hutchinson is a principal of Hutchinson Design Group Ltd., Barrington, Ill. He is former president and a fellow of the Raleigh, N.C.-based RCI Inc.—the Institute of Roofing, Waterproofing and Building Envelope Professionals and now serves as Building Envelope Committee chairman for the group. Hutchinson also is the technical consultant for the EPDM Roofing Association, Bethesda, Md. He can be reached at email@example.com or (847) 756-4450.