Although roofing still can be expected to fulfill its traditional responsibilities, it now also is expected to provide "cool" properties to respond to energy-efficiency issues. During the past few years, the term "cool" has become more popular, especially as it found its way into various energy codes and green building guidelines.
Even so, the definition of cool roofing does not seem to be widely understood, and cool roofing materials traditionally have been considered to be coatings and/or light-colored surface materials. But metal offers many cool roofing options, as well. For these reasons, whether you are a roofing contractor, building owner, architect, engineer or designer, you can benefit from knowing more about cool roofing—specifically, cool metal roofing.
There are two physical roof properties that define and typically are used to measure a roof's relative "cool" performance: solar reflectance and infrared emittance. Roofs that reflect and emit relatively high specified ratios of solar energy are considered cool roofs.
As total solar radiation strikes a roof surface, the roof's solar reflectance immediately redirects part of this energy back into the sky as reflected radiation. Solar reflectance can have the largest effect on reducing roof surface temperature. The energy that is not reflected is absorbed by the material, which can cause heat buildup. A smaller heat gain can lower building energy consumption for cooling and also potentially reduce the heat island effect.
The higher the solar reflectance of a roof, the more solar energy is reflected away from the roof as reflected radiation. With this measured ratio of reflected radiation leaving the roof surface, the radiation avoids conversion into heat energy on the roof surface. This allows a roof's surface to remain cooler, which means less heat ultimately can be transferred to the interior of the building. Therefore, one method to reduce the incessant heat gain potential from the sun is to reflect a high percentage of it back into the sky.
In some respects, cool metal roofing may appear to be little more than a casual marketing claim. However, the "cool" claim for metal roofing is well-earned and appropriate. Metal can be an excellent material for providing energy-saving and environmental benefits.
Cool metal roofs are available in three forms: unpainted, oven-baked paint finishes and granular-coated surfaces. This family of metal roofing materials can achieve an excellent solar reflectance measurement of near or more than 0.70—which means 70 percent is reflected—to help reduce heat gain.
As shown in Figure 1, the amount of solar radiation that is not reflected away has three paths for its disposition: re-emitted energy, convection or absorbed energy. As noted, infrared emittance is one of two physical properties that define and measure cool performance. The infrared emittance property of a roof is a measurement ratio expressed as a decimal of the absorbed solar radiation re-emitted to the sky. An important point is that this solar heat energy is being re-emitted as near-infrared energy, which is invisible to the human eye. The infrared emittance ratio of metal roofing varies with the surface finish. The infrared emittance measurement of painted or granular-coated metal roofing, reported as a decimal value, can be as high as 0.90.
In addition, some heat absorbed by a roof surface may contribute to the heat island effect through convection as local breezes blow over a roof surface, picking up the heat gradient and raising the ambient temperature 6 degrees Fahrenheit to 8 degrees Fahrenheit in some cities as reported by the Heat Island Group of Lawrence Berkeley National Laboratory, Berkeley, Calif. It should be noted cool roofing with high solar reflectance results in lower roof surface temperatures, which, in turn, help reduce ambient air temperatures that contribute to heat islands. This also improves air quality because less smog is formed at lower ambient air temperatures. The amount of air pollution associated with burning fossil fuels at utility plants also is reduced because there is less peak load demand on the electrical grid.
The final path for solar energy remaining on a roof surface is through absorbed energy that becomes the net heat flux into the roof and, depending on insulation and other factors, into the building envelope. This increases a building's cooling load and contributes collectively to the peak energy demand for a given utility service area. In colder regions, absorbed energy may be an energy-saving benefit because it can reduce the heating requirement for a building. In all cases, some level of insulation usually is prescribed to act with the cool roofing attributes and other roof system design components.
Federal, state and local energy code requirements; various green building guidelines; and tax credits are driving the continued development of cool roofing standards, as shown in Figure 2, for solar reflectance and infrared emittance. Although most code requirements in the figure stand alone in a given jurisdiction, as one might expect, others overlap with or are supplemented by voluntary green building guidelines adopted by states for government-owned buildings.
For example, California has its highly stringent 2005 Title 24 Energy Code for low-slope nonresidential buildings, prescriptively requiring minimum 0.70 reflectance and 0.75 emittance or alternative options using whole building and trade-off analyses.
California and select cities within the state also have adopted the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) Green Building Rating System® for government-owned buildings. The new LEED-NC version 2.2 cool roofing requirements call for specified values of the solar reflective index (SRI), an ASTM International calculation using solar reflectance and infrared emittance values. Therefore, two types of documentation must be submitted for a given government building project to approve these related but disparate code and green building requirements for cool roofing. (Another green building rating system, GreenGlobes,™ uses the same SRI values and is being used in some locations though it is not yet adopted in California.)
In addition, though the Title 24 Energy Code went into effect in October 2005 and still is not uniformly adopted throughout the state, the California Energy Commission is in the process of developing the 2008 code, which is expected to add requirements for steep-slope residential roofs.
It further should be noted there is an organization that registers cool roofing products through independent laboratories. The Cool Roof Rating Council (CRRC) now is listing more metal roofing products on its Web site, www.coolroofs.org, especially because it recently adopted a procedure to allow paints that share similar performance properties to be registered as a group to reduce the administration and cost associated with the vast array of metal roofing colors available.
The U.S. Environmental Protection Agency's ENERGY STAR® program is the oldest cool roofing credit program and has helped advance the energy-saving benefits of cool roofing considerably. The program not only sets values for reflectivity and allows self-testing but also uniquely includes an important feature—three-year-aged values. ENERGY STAR offers numerous cool metal roofing products on its Web site, www.energystar.gov. The program currently is being revised and may add a minimum emissivity value for low- and steep-slope roofing, as well as other updates and changes.
In 1999, the American Society of Heating, Refrigerating and Air-conditioning Engineers (ASHRAE) Inc. adopted its Standard 90.1, "Energy Standard for Buildings Except Low-Rise Residential Buildings," that established a trade-off or credit to the amount of insulation required in a roof assembly for buildings located in the southern regions of the U.S. ASHRAE 90.1 and its successors have been adopted in a number of states. Also, California adopted cool roof provisions in its California Energy Code; however, rather than permitting the use of cool roofing as an option, it was made a prescriptive requirement for specific regions within the state.
Generally, other cool roof policies are similar to the ones previously cited with some variations regarding minimum reflectance and emittance. An exception is the City of Chicago Energy Conservation Code, which, as originally proposed, was stringent. With strong industry interaction for practical steps, this code came to its present form. Because Chicago clearly is in the area of the U.S. where total annual heating days exceed total annual cooling days, the adoption of high reflectance and emittance values was not based on the effects of a cool roof for the entire year but rather was generated to address short-term heat island effects and peak energy (air-conditioning) demands experienced during the summer. (See December 2005 issue, "Cooling down Chicago," page 28.)
Cool metal roofing actually is a large family of sustainable, energy-efficient roofing products that includes unpainted metal, prepainted metal and granular-coated metal. It is available in a wide variety of finishes, colors, textures and profiles for steep- and low-slope applications. Not performing alone, cool metal roofing products all are part of an interdependent system of exterior roofing surfaces, substrates, underlayments, configurations, ventilation and insulation.
Cool metal roofing clearly should be considered to optimize energy savings and contribute to environmental protection. As determined by Oak Ridge National Laboratory (ORNL), metal roofing materials lose only 5 percent or less of their initial reflectance and emittance during a three-year time period and demonstrate this stability beyond 30 years per several long-standing test samples. The prescriptive requirements described among the previous codes and green building rating systems readily are achievable for some unpainted, granular-coated and painted metal roofing products. For some metal roof systems, extra steps may be required because certain governing code standards or green building guidelines specify highly challenging levels of solar reflectance and infrared emittance.
For example, unpainted acrylic-coated Galvalume® metal roofing is an all-around choice for many applications and has excellent reflectance. However, at 0.68 solar reflectance (the official value per ORNL tests done for ENERGY STAR) Galvalume falls just short of California's 0.70 requirement. But the code provides for an aged reflectance default value of 0.55 that easily would be achieved by Galvalume. New Galvalume doesn't qualify prescriptively, but when aged, it does. Therefore, if aged reflectance values are allowed to take precedence over initial values with the California 2008 Title 24 Energy Code, Galvalume would be considered quite cool with its solar reflectance.
Meanwhile, in contrast to most painted metal roofing products, unpainted Galvalume metal roofing has low infrared emittance—about 0.05—so it cannot meet the prescriptive requirement of 0.75. Galvalume may be the right choice for many low-slope nonresidential applications in California, but the code must be met on a performance basis using the whole building design or trade-off analyses. EnergySoft LLC, Novato, Calif., has developed a new cool metal roofing and metal building Wizard for its EnergyPro Title 24 compliance software so these analysis steps easily can be taken by an energy professional, including automatic preparation of submittals for approval.
Painted metal roofing with the modern super pigments, which are specially formulated to reflect more of the infrared and near-infrared portions of the spectrum, provide solar reflectance and infrared emittance for low-slope and steep-slope applications that easily conform to California's prescriptive requirements. These colors tend to be lighter in hue but are not relegated only to white. In fact, more midrange and dark colors are beginning to qualify as cool roofing, especially as granular-coated products help boost emittance even higher.
A simple concept
Cool roofing is intuitively simple. Dark roof surfaces usually are hotter than white ones. Cool metal roofing with good solar reflectance can reduce building heat gain and allow less potential for urban heat island effect. Good infrared emittance also can help though the difference in roof temperature is relatively small compared with the benefits of solar reflectance.
Painted metal roofing can be cool even with dark colors using modern super pigments, especially with granular-coated metal. Building code and green building guidelines for best design and application still are in development as more research is conducted. So when looking to specify a cool roof, remember that, above all, metal roofing can be cool.
Gregory L. Crawford is executive director of the Cool Metal Roofing Coalition.
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