A Cool Choice
Mar 1, 2006 12:00 PM, By Drew Ballensky
“Cool roofing” has been popular among education facility managers for several years, but rising energy costs and new regulations have made it an increasingly critical energy-management tool. Reflective roofing surfaces have been the fastest-growing sector of commercial roofing for many years. Their effectiveness at reducing energy consumption and air pollution will continue to make them an attractive choice.
In fact, if your school or university is in California or Chicago, your choices are limited. California Title 24 Building Code, which became effective in October 2005, mandates that, in most cases, new and replacement commercial roofs have a minimum initial thermal emittance of 75 percent and a minimum initial solar reflectance of 70 percent. Chicago Energy Code Section 303, adopted on April 1, 2003, mandates that all new low-slope roofs installed prior to Dec. 31, 2008, have a minimum initial and weathered reflectance of 25 percent. Roofs installed after that date must meet or exceed the criteria to qualify for an Energy Star label (currently set at 65 percent initial and 50 percent weathered reflectivity).
Clearly, cool roofing is considered by many to be an effective means of addressing energy efficiency and environmental challenges. This explains why all but a handful of the 50 states have established regulations or incentives to encourage cool roofing.
Cool research
The trend may be new, but people have known for centuries that white or light-colored surfaces are cooler than dark surfaces. Recent studies by Oak Ridge National Laboratory (ORNL) have demonstrated different levels of solar reflectance and thermal emittance among various materials:
Reflectance, also known as albedo or reflectivity, is the percentage of solar energy reflected by a surface. The higher the percentage of reflectance, the more heat energy will be reflected from the surface. Generally, white or light-colored surfaces have the greatest reflectance.
Emittance, or emissivity, is the amount of heat energy a material can absorb and then shed in the form of infrared radiation. Materials with low emittance tend to heat up more easily because they collect and trap heat.
Lawrence Berkeley National Laboratories (LBNL) has led related research into the dynamics of urban heat islands, or UHIs — even small cities are typically 3 to 10 degrees warmer than nearby suburbs or countryside. Using infrared photographs from outer space, LBNL has confirmed that most large American cities have a heat-island effect. The UHI chain of cause and effect is clear: As temperatures increase, more electric power is needed for air conditioning, and more fossil fuel is consumed, which causes higher levels of air pollution. The probability of smog creation rises 5 percent for each one-half degree increase above 70°F. Reduced vegetation accounts for the largest percentage of the UHI at 56 percent, but dark roofing surfaces run a strong second at 38 percent.
Cool roofing defined
Although there is no industry-wide definition of a cool roof, the EPA's Energy Star Roof Products Program has established a minimum standard, requiring that low-slope roof products have an initial reflectance of at least 65 percent and a reflectance of at least 50 percent after three years of weathering.
The Cool Roof Rating Council (CRRC) is a non-profit association that has established performance ratings for solar reflectance and emittance from roof surfaces. All tests are performed by accredited, independent laboratories and follow established American Society of Testing Materials (ASTM) protocols.
There are two primary types of cool roofing products on the market: protective paints and coatings, and single-ply roofing systems. Reflective paints and coatings, based on either acrylic or elastomeric chemistry, can be an effective short-term solution for reducing energy costs, but most facility owners looking for long-term, low-maintenance solutions opt for a complete single-ply roofing system.
Many single-ply roofing systems have emerged as good long-term cool roofing options:
Polyvinyl chloride (PVC) single-ply roofing systems have the best record of long-term performance among white single-ply cool roofing systems. The first white PVC systems were installed in Germany during the 1960s, and some of these are still in service.
Chlorosulfonated polyethylene (CSPE) single-plies were the earliest widespread white roofing systems in the United States, introduced in the 1970s. Although effective as a cool roofing system, CSPE has not achieved the same commercial success as PVC, EPDM and other single-plies.
More recent cool roofing single-ply developments include the introduction of thermoplastic polyolefins (TPOs) and certain new co-polymer alloys (CPAs) during the late 1980s and 1990s.
Today, many single-ply roofing systems are available in white, including EPDM and modified bitumen, but PVC and TPO systems are the most popular and best performing in terms of long-term reflectance. The coolest among these systems typically range from 70 percent to 85 percent solar reflectance.
Important factors to consider when selecting a cool roofing system:
Reflectance/emittance performance, both initially and after three years.
Long-term track record of durability and performance.
A good warranty backed by a well-established manufacturer.
Climate and weather extremes in a given location.
Maintenance requirements and ease of repair.
Energy management
High-performance cool roofing systems can reduce annual air-conditioning consumption by 10 percent to 40 percent, depending on location, building design, climate and other factors. This may allow schools to downsize their air-conditioning equipment. Air-conditioning units also will run less and have reduced wear.
Benefits from cool roofing are greatest in southern climates, but significant advantages can be realized in most North American cities. Indoor comfort can be improved dramatically in buildings that are not climate-controlled. Cool roofs have been shown to reduce interior temperatures by 15 to 20 degrees on hot days. Cooler indoor temperatures can reduce or eliminate the number of shortened school days and improve the productivity of students and faculty. In addition, reflective membranes typically last longer because they slow down the rate of chemical reactions, or degradation.
Ballensky is the general manager of Duro-Last Roofing Inc.'s Iowa plant and spokesman for the Duro-Last Cool Zone roofing system.
