Ben Franklin conducted the first reflectivity test in the 1760s. He took different colored swatches of fabric - from dark colored to light colored - and placed them in the Philadelphia snow. As the sun beat down, Franklin noted that the snow melted around the dark colored fabric prior to the snow melting around lighter colors. Since this time several studies have been conducted proving Franklin’s theory that dark colors absorb heat while lighter colors reflect heat. This theory is the basis of the urban heat island effect, which has led to the Energy Star program and the U.S. Green Building Council’s LEED program. For roofs, reflection of the sun’s rays decreases heat absorption in the building, leading to savings from lower cooling costs.
Since the mid 1990s there has been an intense effort to provide reflectivity rates for roof systems. This effort started with the development of the Energy Star program and has worked its way into the LEED program and adoption into some state codes. This trend will continue as sustainable construction becomes more prevalent. We have already heard that the Obama administration will push for a stricter environmental policy and the American Institute of Architects (AIA) recently announced that all registered architects must receive a minimum of four hours of sustainable continuing education points per year. Furthermore, Texas recently joined California in mandating roof reflectivity (70 percent) as part of the building code.
Roof reflectivity lowers energy costs by reducing the amount of cooling required on hot days. The greatest benefit of reflectivity will be seen in warm climates, where cooling is required throughout most of the year. In northern climates, proper reflectivity coupled with proper emissivity will provide year-round energy savings.
Reflectivity and EmissivityReflectivity ratings are based on the roof surface’s ability to reflect ultraviolet rays from the sun. Studies have indicated that reflective surfaces will keep the building cooler in the summer, decreasing the use of air conditioning, which reduces costs and uses less energy. The Energy Star program, which was developed by the U.S. Department of Energy, establishes the reflectivity rate of a roof surface for low-slope (less than 2:12) roof systems at 65 percent for the first three years and 50 percent after three years. LEED provides points based on this program. While there is currently some debate in the industry regarding this program, some municipalities have taken steps to enact it as code and all indications are that the Energy Star program - in some form - will become part of the national code.
Emissivity is the ratio of the radiation emitted by a surface to that emitted by a black body at the same temperature. Surface emissivity is generally measured indirectly by assuming that e = 1 - reflectivity. The LEED emissivity rating for roofing is a minimum of 0.90 based on ASTM E 408.
In layman’s terms, reflectivity keeps heat from entering the building, while emissivity keeps heat from leaving the building. Both of these functions serve their purpose throughout the year. A proper energy-saving roof design will be one that provides for proper reflection and proper emissivity. The notion that one element is more important than the other in certain temperate regional zones is incorrect.
Changes to the roof system can provide specific advantages in the reduction of a building owner’s energy cost and, if completed in a large enough scale, decrease the urban heat island effect in a given area. It is possible to reduce solar heat gain of a roof by applying reflective and emissive materials that reflect the sun’s radiant energy back toward the sky. Proper reflective surfacing can cool roofs even in cooler climates, and studies have indicated that white roofs are better at cooling the building than black roofs, even in colder climates. Reflective-surfaced roofs can reduce surface temperature by as much as 30 percent and can extend the service life of some membrane systems by reducing expansion and contraction. The reduction of the interior temperature decreases the need for air conditioning, which results in lower air conditioning costs and less intensive HVAC maintenance over the life of the equipment. Reflective roofs in conjunction with skylights can also increase interior ambient light, reducing yearly electricity costs.
Another way to minimize the urban heat island effect and reduce interior costs is by applying a green or garden surfacing. The application of garden roofs can provide an urban area with required greenery to reduce the urban heat island effect. Garden roofs can also serve as protective surfacing to improve the energy efficiency of the building, improve the quality of air that we breathe and improve storm water runoff - a major advantage in older urban areas with limited or worn infrastructure. It should be pointed out that cities that place percentage requirements on new buildings that have garden roofs will only reach the minimum required to reduce the urban heat island effect. Garden roofs must also be installed on a percentage of existing buildings for significant change to occur.
The increased use of insulation can also prove to be a source of energy efficiency. Some states - such as California - and municipalities throughout the country have initiated tax rebates and paybacks to building owners that increase the thermal value of the insulation that they provide on their roof systems. Proper research should be conducted prior to the installation of insulation on the roof system. Insulation can reduce emissivity loss, but it can also contribute to condensation. It is recommended that a dew-point calculation be conducted based on the buildings interior use to determine the correct thermal R-value prior to application.
The reduction of energy use will be a paramount concern as we move through the 21st century. Environmental regulations will further determine the materials and application procedures that we use in roofing in the future. Contractors must be aware of these changes and become acquainted with these technologies in order to succeed in the industry. Energy savings can be generated through proper reflectivity and emissivity in all parts of the country.