Reflective Roofs and Urban Heat Islands

Protecting people, the environment and the economy
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Sponsored by Duro-Last®, Inc.
Dr. James Hoff
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What Can We Do About Urban Heat Islands?

Although the urban heat island effect could be addressed simply by removing a city’s buildings and pavements and replacing them with open land and vegetation, such a solution would effectively eliminate the urban area itself along with the heat island. As a consequence, the only practical way to address this challenge is to transform these urban surfaces rather than eliminate them. And this transformation must somehow eliminate or reduce the tendency of these surfaces to absorb and retain heat. Without a doubt, pavements and roofs offer the best opportunity to reduce urban heat absorption. When combined, roofs and pavements account for over 60 percent of the entire surface area of modern urban areas, with roofs contributing 20 to 25 percent and pavements contributing 30 to 35 percent.6

Because transformation of roofs and pavements appears to be the most effective approach to heat island mitigation, almost all current global strategies are directed toward making the roofs and pavements of cities less heat absorptive. In lieu of this solution, we can plant more trees and create more parks. But given the scarcity and economic value of urban real estate, such a strategy may have only a limited impact on overall urban temperatures. As a result, this article will focus primarily on the various ways to reduce the heat absorption of roofs and pavements.

It’s easy to understand why there is so much public reference to the word “cool,” as in cool pavements and cool roofs. If the fundamental problem is related to excessive heat in urban pavements and rooftops, then the obvious solution would be to reduce their temperatures and make them much cooler. Without getting into a complex discussion of thermodynamics, there are a number of ways to make hot surfaces cool, and a brief review of these different approaches should be helpful.


Hot urban surfaces may be cooled through the use of shading, or blocking the sun’s radiation before it is absorbed by the surface. Common examples of this strategy include the planting of trees to provide shade as well as the installation of awnings or solar shades to block sunlight from hitting highly absorptive surfaces. Although shading may be a useful strategy for small heat reductions in highly targeted areas such as sidewalks and patios, it is difficult to envision large-scale applications that would address more than a small percentage of current urban area.


The temperature of urban surfaces also may be reduced by evaporation, or the heat transfer achieved when water moves from a liquid to a gaseous state. As an example, water fountains can be very effective in cooling nearby surfaces as well as the surrounding air. Unfortunately, just like shading strategies, the overall reduction in urban heating achieved through evaporation may be relatively small. In addition to the direct evaporation of a water fountain, it should be noted that a portion of heat reduction attributed to trees and other plants involves a form of evaporation called transpiration. During transpiration, plants give off water vapor in order to cool their leaves and facilitate the movement of nutrients. The most obvious applications of transpiration to reduce surface temperatures in urban areas would include any garden or park area where pavements have been replaced by extensive plant cultivation. But just like shading strategies, the amount of urban area that may be dedicated to, or suitable for, gardens and parks likely is limited. One final application of transpiration in urban areas involves the installation of “green” roofs consisting of a variety of plants that may be installed in a depth of planting media directly over new or existing roof surfaces. Many cities across the globe have adopted programs to encourage the installation of green roofs, but the cost of a vegetated roof may be difficult to justify for the sole benefit of surface cooling. As a result, green roofs tend to comprise a very small proportion of urban roof installations. For instance, a survey of green roof installations estimated that slightly over 20 million square-feet of green roofs were installed in North America in 2013.7 Although 20 million square-feet may appear to be a sizeable number, it accounts for less than 1 percent of the 3 billion square-feet of low-slope non-residential roofs installed annually in North America.8 Therefore, the potential for green roofs to make a significant impact on heat islands appears to be almost as limited as previously discussed shading strategies.


The temperature of urban surfaces also can be reduced through advection, or the effects of wind blowing across a heated surface and cooling it. As an example, a 2005 modeling study of rooftops in downtown Chicago suggested that roof surface temperature may be reduced by the effects of wind.9 However, other studies using larger “urban canyon” models suggest that the effect of wind on urban surfaces is very complicated and in many cases can actually lead to increased heat absorption.10 Regardless, it should be obvious that whatever benefits attributed to wind would be highly unpredictable and probably not useful as a heat island strategy.


Source: Global Cities Cool Roofs and Cool Pavements Tool Kit11

Figure 2. Reflective versus Non-reflective Roof: Where Does the Heat Go?

Finally, the temperature of urban surfaces can be reduced through reflection, or the redirection of solar energy away from urban surfaces and the heat island itself. Although both roof and pavement surfaces can be made to be reflective, cost-effective technology suitable for immediate and scalable application is only available at this time for reflective roofs. Given the urgency of reducing heat island effects, it is likely that new reflective paving products will become commercialized in time; but for today’s cities, reflective roofing offers an immediate answer to reducing heat island effects.

Because reflective roofs offer the primary technology available today to address heat islands, it is important not to underestimate their potential contribution. As previously stated, roofs account for approximately one-quarter of urban surface area; and if reflective roof technology can be used to significantly reduce heat absorption over such a sizeable surface area, its potential contribution would be substantial.

In order to better understand the potential contribution of reflective roof technology, it may be useful to evaluate how a typical reflective roof compares with a traditional non-reflective roof in removing solar heat from urban areas. Figure 2 provides such a comparison based on data developed by the Heat Island Group of Lawrence Berkeley National Laboratory and published by the Global Cool Cities Alliance.11

As shown in Figure 2, when sunlight strikes a traditional non-reflective roof surface, only 5 percent of the total heat energy is effectively removed from the urban heat island by reflection back toward space. Instead, 52 percent of the energy heats the air directly around the urban area, 38 percent heats the larger atmosphere above the heat island, and 5 percent heats the building or area directly beneath the roof. However, when sunlight strikes a reflective roof, up to 80 percent of the total heat energy is reflected back toward space, leaving only 8 percent of the energy to heat the city air, 10 percent to heat the atmosphere above the city, and 2 percent to heat the building. As a result, the reflective roof may effectively remove up to 80 percent of the heat energy that otherwise would impact the heat island. Assuming that roofs account for 25 percent of the urban area, the overall potential decrease in solar heat absorption would be 80 percent times 25 percent, or a 20 percent total reduction.*

*Because cool roofs tend to lose some level of reflectivity as they age, the overall energy savings would likely be less than the percentage calculation shown.

Benefits of Reflective Roofs

For cities and their inhabitants. Just as elevated temperatures in urban heat islands pose numerous negative consequences, reflective roofs offer an equally long list of potential benefits to improve the environment and quality of life in urban areas. Important benefits of reflective roofs for cities and their inhabitants include:

  • Lower heat island temperatures. Simulations run for several cities in the United States suggest that citywide installations of highly reflective roofs and pavements, along with planting shade trees will, on average, reduce ambient air temperatures by 4 to 9 degrees Fahrenheit in summer months.12
  • Reduced peak energy demand. By reducing air temperatures and the associated demand for air conditioning at critical peak periods during the day, the installation of reflective roofs and surfaces may reduce overall peak electricity demand in urban areas by as much as 5 to 10 percent.2
  • Lower air pollution. The combination of lower overall temperature and reduced peak demand offered by reflective roofs and surfaces may lead to reduced air pollution by lowering the amount of power plant emissions and by reducing the temperature- related formation of smog, or low-level ozone.
  • Reduced risks from blackouts. By lowering peak electricity demand, reflective roofs may reduce the risk of power blackouts. And in the event a blackout does occur, reflective roofs continue to divert solar heat away from buildings and help occupants stay cool without the use of air conditioning.
  • Improved quality of life. Reductions in overall air temperature and urban air pollution combine to provide a healthier and a safer environment for a city’s inhabitants, leading to improvements in work productivity and leisure activity.

For Building Owners

Perhaps the most noteworthy aspect of reflective roofs is that so many of their benefits accrue directly to the building owners who invest in them. Important benefits of reflective roofs for building owners include:

  • Lower electric bills. Because many electric utilities, especially in urban areas, add peak demand charges to their electric bills, the dollar savings available from installing a reflective roof may be many times more than the actual reduction in peak usage. For example, a recent study of peak electric charges suggests that the costs associated with peak electricity demand charges may account for over 50 percent of some electric bills during the summer. In addition, this study suggests that the annual savings available from a reflective roof installed on an existing 20,000-square-foot commercial building may vary between $880 and $3,040, depending on utility rates and climate zone.5
  • Reduced equipment sizing/improved service life. Because air conditioning equipment must be sized to accommodate peak cooling loads, reflective roofs may help lower the overall size of the compressors and air handlers needed to cool a building. And because reflective roofs may lower roof surface temperatures by as much as 50 to 60 degrees Fahrenheit, rooftop air conditioning units will operate at reduced temperature differentials, which may extend the service life of equipment.
  • Competitive cost. In many cases, a reflective roof may cost no more than non-reflective roofing options. Table A provides a comparison of reflective surface cost premiums for the most popular commercial roofing materials, as estimated by the U.S. Department of Energy (DOE).13
  • In addition to the DOE estimates shown in Table A, the cost effectiveness of reflective roofs may be demonstrated by the significant increase in market share they have achieved in the past 10 to 15 years. From less than 20 percent of the low-slope commercial roofing market in 2000, reflective roofs have grown to around 50 percent of the market ten years later in 2010 and probably well over 50 percent today.8
  • High return on investment. Because there is little or no cost premium associated with reflective roofs, the return on investment may be very high, especially for new buildings or existing buildings requiring a new roof simply due to age or other reasons. So when a building owner selects any of the roofing options shown in Table A that require no cost premium, all of the dollar savings from reduced electricity bills drop directly to the bottom line.


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Originally published in Architectural Roofing and Waterproofing
Originally published in September 2015