by Craig Tyler, CSI, AIA, CDT, LEED AP
Highly reflective roofs, or ‘cool roofs,’ have been used throughout the country as an urban heat island reduction strategy, and as an energy saving measure for many years. The implementation began in warm climates such as California, Texas, and Florida. However, the overall market shifted with the introduction of the Leadership in Energy and Environmental Design (LEED) rating system and other ‘green construction’ initiatives making cool roofing an urban heat island reduction strategy to be used on all building types, in every climate.
Major metropolitan areas like Chicago and New York began to require cool roofing in their local building codes or city ordinances. These large Northern municipalities will not see the same energy savings as their southern counterparts, but are still relying on the reduction of rooftop temperatures—through the use of reflective cool roofs—to lower the city’s overall temperature and potential for global warming contribution. Lower city temperatures mean less frequent use of air-conditioning, lower levels of pollution, and an increase of quality of life for residents.
Many architects, designers, and specifiers use cool roofing exclusively on all their projects for these reasons. However, like most parts of building design, cool roofing is more complicated than it seems.
Urban heat island reduction
Urban heat islands exist when areas of natural vegetation are removed and replaced by a built environment. Regardless of the size of the town or city, a temperature difference will exist when the buildings and pavement come in and green space is eliminated. This occurs because the removed vegetation no longer reflects a portion of the sun and cannot assist with localized temperature reduction by evapotranspiration or shading. Utilizing a cool roof, reflective coating, or reflective pavement relies on the principle strategy of reflection. The goal is to reflect as much light and heat, while retaining as little heat as possible.
Early heat island studies used calculations involving most rooftops in North America, with the premise being nationwide rather than local. (See H. Akbari and S. Konopacki’s article, “Calculating Energy-saving Potentials of Heat-island Reduction strategies,” in Energy Policy number 33, 2005.) Those studies showed reflectivity numbers higher than natural vegetation, large bodies of water, and mountains. This increase of reflection would reduce the amount of sunlight and heat absorbed by the built environment (namely, roofs) and could result in a lowering of local temperatures if a concentration of buildings could be utilized.
Over the past few years, further research has taken these climate models further and postulated two issues. The first is an issue of increased air temperatures using cool roofs. A 2011 Stanford University study suggested reflected heat from cool roofs could re-heat brown and black soot particles in the air—adding to local temperatures, rather than lowering them.(See Mark Jacobsen and John E. Ten Hoeve’s article, “Effects of Urban Surfaces and White Roofs on Global and Regional Climate,” in the September edition of the Journal of Climate.)
The second issue is one of decreased cloud cover and rainfall. A 2014 Arizona State University study suggested cool roof success was geographically dependent; in some areas, it suggested, the strategy may have a significant affect on precipitation reduction due to the deflected heat and sunlight back into the atmosphere. (For more, see Matei Georgescu et al’s article, “Urban Adaption Can Roll Back Warming of Emerging Megapolitan Regions,” in the Proceedings of the National Academy of Sciences of the United States of America, January 2014 edition.)
An alternative to cool roofing is the practice of increased vegetation throughout a city by tree planting and added rooftop flora. Such vegetative assemblies can replace some of the natural vegetation lost during construction and can provide other benefits, including stormwater retention and rooftop protection.
Cool roofs’ energy savings is contingent on many factors, including building use, geographic location, size of roof, and building orientation. A greater savings is achieved when used in southern climates, on buildings with lower levels of insulation, and when the building has a larger roof surface area than wall surface area.
Initial research studies in the late ’90s were conducted ‘in situ’ on retail and residential buildings to identify the levels of energy savings a homeowner or building owner could save by using a cool roofing material. The buildings had low levels of insulation—a common construction practice in the south—and many employed insulation below the deck and between the roof joists or supports. Both of these practices are no longer common, as model building codes and energy standards now require higher levels of insulation. A certain percentage of this insulation must also be in a continuous layer. Therefore, the savings associated with these earlier studies may not be achieved today.
These changes in design and construction practices go beyond the roof and are carried over into the walls and foundations. Sustainable building practices have fostered more ideas and better implementation of building products. This means energy savings in all aspects of building design and construction—from harvesting, manufacturing, shipping, installing, to recycling.
These early studies have shifted from in situ to computer models that can aid the architect, specifier, and building owner in making decisions regarding roof color and insulation levels appropriate to the building. Oak Ridge National Laboratory (ORNL) has pioneered a simplified computer model called the Roof Savings Calculator (RSC). (Visit rsc.ornl.gov to access the calculator.) This tool has been modified over the years to model much of the real-world data ORNL has collected in its test roof assemblies. This model is not the same as those aiding mechanical engineers in building envelope design alongside HVAC systems, which require the size, shape, and orientation of the building. The ORNL version has a simple interface for understanding the possible savings or costs of various insulation and roof reflectivity levels.