As the green building movement continues to grow within the commercial construction market, energy-efficient roofing technologies are often at the forefront. For many building and roofing professionals, however, much of the attention has been directly solely at white or light-colored reflective roofing materials.
Many roof designers are taking their cues from studies that show reflective roof surfaces reflect solar radiation back into the atmosphere, keeping the roof system cooler and ultimately reducing building energy usage through lower air conditioning demand. In some cases, a white thermoplastic polyolefin (TPO) membrane fully adhered over polyiso insulation may in fact be the most appropriate roof design for a given building. This is especially true in climates where cooling degree days outnumber heating degree days.
Yet, frequently missing from the discussion about creating eco-friendly roofs are several important aspects:
* EPDM roofing systems - both black and white - can provide a strong, energy-efficient option.
* Reducing a building's heating costs is often more important in many regions of the country.
* The roofing material's durability and life-cycle assessment should be considered.
EPDM's Proven History
According to the EPDM Roofing Association, ethylene propylene diene terpolymer (EPDM) rubber roofing membrane accounts for more than 1 billion square feet of new roof coverings in the United States each year. Although EPDM has been in use for over four decades, it has been the last 25 years where its growth has made it the single-ply market leader. Today, there are well over 500,000 warranted roof installations totaling more than 20 billion square feet of EPDM membrane in place nationwide.
Behind more than four decades of successful field performance, EPDM has become the trusted system of choice for many roofing contractors, specifiers and building owners worldwide. In addition to outstanding weathering characteristics, flexibility, durability, and life cycle costs, EPDM also offers superior wind, hail and fire resistance. Specifically, ballasted EPDM roofing systems are among the most common and economical installations available today.
However, when seeking a "cool roofing" option, many building professionals do not realize that black EPDM provides similar energy savings as its white, non-EPDM, counterparts. A cool roof, as defined by the California Energy Code, is a roof covering or surface that has been tested and labeled by the Cool Roof Rating Council as having an initial solar reflectance of a least 0.70 and an initial thermal emittance of at least 0.75. So where exactly does ballasted roofing fit among cool roofing options?
A three-year study initiated by the Single Ply Roofing Industry (SPRI) and conducted by the U.S. Department of Energy's Oak Ridge National Laboratory (ORNL) was completed in 2008 and provided extensive analysis of the energy performance of ballasted systems. As part of the study, titled Evaluating the Energy Performance of Ballasted Roof Systems, six roof panels - four ballasted EPDM featuring different stone/paver ballast weights and two control panels (a white TPO and a black EPDM membrane) - were subjected to daily weather cycles side-by-side from March 2004 through April 2006. Among the key findings
* Thee cooling loads for the heavy and medium stone-ballasted and uncoated paver-ballasted systems were approximately the same as for the white system.
* Cooling loads for the lightweight stone systems were slightly larger than for the white system but significantly less than for the black system.
* By the start of the second year of the project, temperature and cooling loads increased for the white system due to the effects of weathering.
* Heating loads for the ballasted systems showed random variation as loading increased and type changed. Except for the heavyweight stone system, they were about the same as for the white system.
* The heavyweight stone system showed slightly less heating load than the black system but this is considered an anomaly due to rain effects.
* All evidence on clear days of diurnal behavior showed the heavyweight stone and uncoated paver systems performing equally due to the same thermal mass despite different solar reflectance.
The study further revealed that the ballasted EPDM profiles offered better thermal emittance properties. While solar reflectance measures how well a roofing material reflects sunlight, emittance measures the roofing material's ability to release absorbed heat back into the atmosphere, rather than into the building. Both are important properties that help a roofing system remain cool.
In the study, the ballasted EPDM profiles delayed the temperature rise for up to three hours, effectively moving about 20 percent of the cooling load into off-peak hours of the day when energy costs are lowest. Summarizing the study's results, AndrŽ Desjarlais, program manager of the Building Envelopes Program at ORNL, stated that certain ballasted roof systems "are as effective as white-membrane roofs in mitigating peak energy demand."
As a result, several regulatory bodies across the country have adopted new standards in regard to cool roof materials. For instance, the California Energy Commission has approved the use of ballasted roof systems as a cool roofing option as part of its 2009 Title 24 energy standards. And, the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) has tentatively decided to update its cool roofing standards and will recognize ballasted roof systems as an acceptable alternative to light-colored materials. Likewise, the city of Chicago, which has earned international accolades for its commitment to sustainable roofing practices, has added ballasted EPDM as an accepted cool roofing alternative in its municipal code for low-sloped roofs.
In use since the 1980s, white EPDM roofing membranes feature the same characteristics and benefits of black EPDM, yet provide a highly reflective solution to coated membranes and thermoplastics. With its high solar reflectance index value, the bi-laminate, white-on-black cured membrane can help achieve points in the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) Green Building Rating System, specifically section SS 7.2 regarding the heat island effect, which requires the membrane to have a SRI value of greater than 78 for low-slope roof systems.
As a rubber-based material, white EPDM roofing systems are more flexible than other membranes, allowing for year-round application. In cooler temperatures, fully adhered EPDM membranes remain pliable and easy to install, while other membranes tend to stiffen and can be often more difficult to install, particularly on irregular substrates and vertical walls, such as parapets.
White EPDM roofing membranes are ideal for UL- and FM-rated systems, while exceeding ASTM D-4637 standards. They are well suited for new construction and re-roofing applications, and they can be installed over steel, concrete, wood and other common deck types.
Although reflective roofing materials have inherent value in the fight to gain energy efficiencies, the issue is not simply black vs. white. For any new or re-roof application, a multitude of factors must be weighed to determine the most appropriate roof design for each building. Choosing the right roof for the right situation is most important. That means moving beyond roof surface color and focusing on the building's entire roofing assembly, including decking, insulation and roof substrate.
Light-colored roofs can reflect sunlight and help reduce cooling costs, particularly in warm, southern climates. However, the energy required to heat a building is often a more significant factor in overall energy usage.
According the U.S. Department of Energy (DOE) Office of Energy Efficiency and Renewable Energy, commercial buildings are a large and growing consumer of energy. They account for 18 percent of total U.S. energy consumption. In a typical office building, energy use accounts for 30 percent of operating costs, the largest single category of controllable costs.
The DOE's ORNL also developed a Cool Roof Calculator that estimates cooling and heating savings for low-slope roof applications with non-black surfaces. A review of data available from the calculator indicates that among all 243 sites in its database, which includes cities within all 50 U.S. states, the Pacific Islands, Puerto Rico and eight Canadian provinces, only 35 (14 percent) have more cooling degree days (CDDs) than heating degree days (HDDs).
From cities like San Diego to Boston and Lubbock to Fargo, there are more HDDs than CDDs. As such, the need to reduce heating-related energy demands is much greater than air conditioning demands in many climate zones. Dark-colored roofing materials, such as black EPDM membranes, are often the most beneficial in such environments because they absorb solar radiation and transfer it into the building. By heating the building's interior, less demand is placed on the heating system.
Through its Building Technologies Program, the DOE also publishes the Buildings Energy Data Book. Table 7.4 of the 2007 book outlines energy use intensity in various commercial building types, comparing heating and cooling as a percentage of total energy consumed. The average results show that heating accounts for 29 percent of the energy consumed within a building, while cooling totals a mere 6 percent. The statistics are even more compelling when broken into specific building segments, such as health care and educational facilities, which feature 55 percent to 10 percent and 33 percent to 5 percent heating-to-cooling ratios respectively.
This information reaffirms that reflective roofing should not be the only consideration when seeking to improve energy efficiencies for commercial buildings. It also points toward the need for more focus on reducing heating costs, and not merely lowering cooling costs.
Other important aspects of designing an energy-efficient roofing system are product durability and the role of life cycle assessment (LCA), which evaluates the environmental impact of a product throughout its life cycle. Although the USGBC has incorporated LCA of building materials in its LEED rating system, some industry experts believe there is an overemphasis on environmental benefits without equal concern for durability.
In a white paper titled "Life Cycle Assessment and the LEED Green Building Rating System," Dr. James L. Hoff, DBA, research director for the Center for Environmental Innovation in Roofing, explained, "É no building product should be considered truly sustainable unless it also meets or exceeds the desired durability of the building itself."
In terms of roofing, the primary concern with LEED is the insufficient emphasis on durability. For example, a project can achieve one LEED point for painting the building's roof with a reflective coating, some of which may last less than five years. However, no credit is afforded for selecting a high-performance, non-reflective roofing system that may be designed and warranted to last 30 years or more.
Life cycle costs of EPDM membranes are lower than those of other popular low-slope roofing systems. EPDM membrane durability has also led to long life expectancy ratings, including more than 23 years in covered applications, more than 26 years in exposed applications and an estimate of more than 50 years for ultimate service life.
In the face of challenging economic times, choosing roofing materials that can help reduce overall energy costs involves looking beyond surface color. For more than 40 years, architects, roof consultants and contractors have relied on EPDM's weatherability, durability and other long-term performance attributes. Today, the environmental benefits of EPDM play an equally significant role in determining what roofing system is best suited for an individual building.