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.
A cool roof stays cooler than traditional construction, minimizing heat transfer to the building below.
Roofs always have been a major focus for building energy-conservation measures. The emphasis traditionally has been on beefing up insulation. More recently, advances in roofing materials technology and analytical techniques have found new ways to fine-tune the energy performance of roofing to reduce not only operating costs, but also environmental impact. The so-called "cool roof" is one of these.
Consequently, cool roof products are in vogue among architects, facility managers and building owners. Popular green building programs, such as the U.S. Green Building Council’s LEED and the Green Building Initiative’s GreenGlobes, offer credits for use of cool roof materials. A growing number of national building codes (e.g., ASHRAE Standard 90.1, California’s Title 24 Building Energy Efficiency Standards), as well as a number of local codes, including those in Chicago, Houston and Dallas, include cool roof requirements.
Keep it cool
A cool roof minimizes the solar heat gain of a building by reflecting the bulk of incoming radiation and then quickly re-emitting the remaining absorbed portion. The cool roof stays cooler than traditional construction, minimizing the transfer of heat to the building below.
Although cool roofs typically are white or other light colors, darker colors can be engineered for cool roof performance. They can be found in a variety of materials (shingles, slate or tile, single-ply or built-up, metal) and can be applied to virtually any building or roof slope in any location. They promise reduced energy costs and longer AC unit life because of reduced air-conditioning load, and increased roof longevity because of reduced thermal flux. In addition, they can improve air quality by reducing ambient air temperatures, thus mitigating the urban heat-island effect.
Cool ratings
To accurately determine a roofing product’s "coolness," the two radiative properties of solar reflectance and thermal emittance must be measured.
Technically, solar reflectance isthe fraction of solar energy reflected by a roofing material’s surface, expressed on a scale from 0 to 1.0. A solar reflectance of 1.0 (100 percent reflected) would mean no effect on surface temperature, and a reflectance of 0 (none reflected, all absorbed) would result in the maximum heating effect. Solar reflectance is determined in the laboratory (and sometimes in the field) in accordance with ASTM test methods C 1549, E 1918 and/or E 903, depending on the roofing material’s color and texture.
Thermal emittance also is expressed on a scale from 0 to 1.0, so a roofing material with a higher thermal emittance (closer to 1.0) will re-emit absorbed thermal energy more quickly than a material with a low emittance, resulting in a "cooler" roof. Thermal emittance is measured per the laboratory test procedure ASTM C 1371.
A force in the promulgation of cool roof technology is the Cool Roof Rating Council (CRRC), which has created an accreditation and reporting system for testing and listing the solar reflectance and thermal emittance of any roofing product. A manufacturer chooses a CRRC-Accredited Independent Testing Lab (AITL), which conducts testing according to designated ASTM and CRRC test methods.
The CRRC program does not specify minimum requirements for solar reflectance or thermal emittance and does not set a threshold definition for "cool," leaving that to code bodies and programs that reference CRRC ratings (such as the Department of Energy’s (DOE) ENERGY STAR Roofing Products program).
ENERGY STAR, however, specifies only a minimum solar reflectance and says nothing about emittance, requiring an initial solar reflectance of at least 0.65, and weathered reflectance of at least 0.50 for low-slope roof applications. For steep-sloped installations, initial reflectance must be at least 0.25 with an aged value of 0.15. (A low-slope roof is defined as that with a pitch less than or equal to 2:12; a steep-sloped roof has a pitch greater than 2:12. For aged ratings, product samples are exposed for three years at a CRRC-approved outdoor "test farm."
The Green Building Initiative’s (GBI) GreenGlobes system does take thermal emittance into account and awards points for roofing materials with a solar reflectance of at least 0.65 and an emittance of at least 0.90.
However, note that a high reflectance value or a high emittance value alone will not automatically result in a "cool" roof.
The U.S. Green Building Council’s Leadership in Energy and Environmental Design (LEED) program, arguably the nation’s most influential green building program, attempts to take this into account by not specifying separate values for solar reflectance and thermal emissivity. Instead, LEED—as well as a number of local building codes—defers to another quantity, the Solar Reflective Index (SRI), which combines these values into a single index. SRI is derived by a calculation method set forth in ASTM E 1980, which yields a number from 0 to 100. The standard black roof is given an SRI value of 0; a standard white roof is assigned an SRI value of 100.
Although not required for LEED certification, reflective cool roofs exhibiting an SRI of 78 or greater for low-sloped roofs, or at least 29 for steep-sloped roofs, and covering at least 75 percent of the roof surface, earn an additional Sustainable Sites Credit 7.2 point toward a project’s overall LEED score.
