Beyond Cool
Jul 1, 2010 12:00 PM, By Tyler Westerling
Advanced materials and technology have given roofing the opportunity to reduce operating costs and environmental impact.
Cool roofing is not without controversy. Some studies have shown that reflective roofs are not always best in cool climates, where low emissivity could benefit heating systems in cold weather. Potentially damaging condensation within a roofing assembly also can be aggravated in such climates because of the enhanced temperature differential between the warm interior and the cooler roof. This can be of particular interest for designers and facility managers of education institutions, which often are closed during the summer months and thus experience minimal air-conditioning demand for cool roofing to leverage against. The DOE’s Buildings Energy Data Book, for example, has noted that on average, education facilities use 33 percent of their total annual energy consumption on heating, but only 5 percent on cooling. To help estimate cooling and heating savings through use of cool roofing, the DOE’s Oak Ridge National Laboratory (ORNL) has developed a cool roof calculator. A similar calculator offered by the EPA for ENERGY STAR evaluations enables a designer to input specific details about a building, including heating and cooling systems, as well as location and the cost of energy. These calculators provide a simple payback analysis to determine the savings in electricity or natural gas through the installation of a cool roof. According to a paper issued by a single-ply roofing manufacturer, studies using this DOE calculator were instrumental in persuading the School Building Authority (SBA) of West Virginia to reverse an earlier blanket decision to require highly reflective roofing on all buildings in the state. Other investigations using the EPA calculator have shown that high-reflectivity, medium-emissivity roof coatings such as aluminum can yield greater savings in colder regions. Many designers believe that only white roofs meet the criteria for cool roofs, that flat or low-sloped roofs are the only ones that benefit from the specification of a cool roofing material, and that a cool roof is beneficial only in hot climates. However, roofing materials now are available with spectrally selective pigments that reflect infrared energy, even though their color still may absorb some of the visible light spectrum. In this way, roofing products can be both "cool" and dark-colored. This provides more opportunities for designers to incorporate aesthetic considerations. Although green properties and energy savings often receive the lion’s share of attention, design considerations should go beyond energy efficiency and green attributes to encompass long-term performance of the building envelope. Instances are beginning to surface in which LEED-certified and other green-rated buildings have developed performance problems, such as water leakage, which is not addressed by LEED criteria. Other examples include testing for the ability of the roofing system to resist uplift forces because of high winds and damage from hail impact. As always, full consideration of all performance challenges posed by a building’s location and use should be evaluated and addressed with optimized design solutions. Concentration on just one aspect or feature, especially as a means to earn rating credits, can result in over-building or under-building for the need at hand—both of which result in higher costs without improved performance.Too cool?
Total performance
Westerling, PE, is project engineer-solar department, with Architectural Testing Inc., york, Pa., a product testing service with ISO/IEC 17025-accredited capability to perform more than 2,000 defined test protocols at one or more of its nine laboratory locations. talkback@archtest.com.
