Study: Reflective Roofs Can Save Lives By Keeping Washington, D.C., Cool

Reflective roofs and vegetation have long been known to improve energy efficiency, manage storm-water runoff and make buildings more comfortable. A new study, “Assessing the Health Impacts of Urban Heat Island Reduction Strategies in the District of Columbia”, by the Global Cool Cities Alliance, funded by a grant from the District Department of the Environment and the Department of Consumer and Regulatory Affairs, has identified a new benefit: saving lives during heat waves by keeping the District cooler.

Annually, heat kills more than 1,500 people in the United States, more than any other natural disaster in an average year. The District is particularly susceptible to extreme heat events because its downtown is sometimes 10 to 15 degrees hotter on summer afternoons and stays warmer at night than surrounding rural areas. This so called “urban heat island” effect is caused by the fact that cities tend to have more hot dark roofs, less vegetation and more heat-generating human activity than rural areas.

In the 20 years between 1990 and 2009, the District experienced about 14 dangerously hot summer days each year. The District has experienced a marked increase in such days during the last three years, with 33 days in 2010, 28 days in 2011 and 27 days in 2012.

The District experienced its hottest month on record in July 2011, including a record 25 days above 90 F. Extreme heat takes a disproportionate toll on people of color and low-income urban populations that often live in neighborhoods that have older, lower-quality building stock, less tree cover and fewer buildings with air conditioning.

“The health of our city’s residents is a cornerstone of the Sustainable DC plan,” says Bill Updike of the District Department of Environment. “This study shows that we can actually save the lives of our city’s most vulnerable populations by also doing what is right for the planet.”

The study found that installing more reflective and green roofs and planting more vegetation would have reduced the number of deaths by 6 to 7 percent during four local heat waves. During the next decade, these cooling measures could save the lives of at least 20 District residents.

“We found that even seemingly small changes in temperature and humidity could shift weather conditions into less dangerous, more manageable types. For vulnerable populations, like the elderly and sick, a little cooling can make a big difference,” adds Dr. Laurence Kalkstein, president of Applied Climatologist Inc., professor at the Miller School of Medicine at the University of Miami and lead author of the report.

“The District continues to be a leader in addressing its urban heat island including a cool roof requirement in its energy code. Our study shows, though, that urban heat island reduction is truly a regional issue. We found that the District’s cooling strategy actually helped Prince George’s County stay cooler during extreme heat waves. Likewise, Arlington County’s policies will affect District conditions. Regional collaboration is key,” says Kurt Shickman, executive director of the Global Cool Cities Alliance.

4 Comments on "Study: Reflective Roofs Can Save Lives By Keeping Washington, D.C., Cool"

  1. Michael McGuire, Ph.D. | December 20, 2013 at 10:31 am |

    Reflecting solar energy is absolutely the correct thing to do. Reflected solar radiation consists of wavelengths that penetrate the atmosphere and go back into space un absorbed by earth’s atmosphere, just as high cloud cover and snow cover do. Dark roofs and other objects absorb and eradicate heat at wavelengths that are absorbed by our atmosphere and cause warming.

    Counter intuitively perhaps, in winter some reflective roofs, such as stainless steel (Reagan Airport) lose much less heat than dark or EPDM roofs because of low emissivity, a characteristic of many metal high reflectivity materials, so there is a net summer and winter benefit. In a Northern climates this effect is dominant, while in warmer climates the summer reflectance is more important.

    The key is to reduce earth’s heat gain in summer by reflectance while avoiding heat losses in the winter by low emissivity. Only reflective metal roofs do that.

  2. Kurt Shickman | November 26, 2013 at 11:46 am |

    The same properties of reflective roofs that allow them to keep buildings cooler in the summer may also cause them to make buildings colder during the winter. This “winter heating penalty” is a consideration when installing reflective roofs in some northern locations, but research and real world examples from the marketplace indicate that the potential for extra heating does not outweigh the potential for cooling energy reductions in most of the country. The marketplace for cool roofs in “northern” climates has been growing rapidly for 25 plus years. There are billions of sq. ft. of cool roofing saving money and providing great extra benefits for people across the country – even in areas that experience cold winters.

    A 2010 study from Lawrence Berkeley National Laboratory (LBNL) determined that in nearly all U.S. locations, the extra winter heating energy needed as a result of switching to a reflective roof is just a fraction of the avoided summer cooling energy. In their analysis, the only locations that saw a net increase in energy use over the course of the year were in Alaska. That 2010 study applied then-current local energy prices to the extra heating energy used and cooling energy saved (shown in the contour map above) and determined that energy bills would not increase as a result of switching to reflective roofs anywhere in the country—even in Alaska, where the switch would lead to greater annual energy use. Updating this analysis with current (2013) energy prices shows that building owners in all states can still save on average energy costs over the course of a year. When the energy cost differences of installing a reflective roof are broken down by state, the average money saved on cooling energy outweighs the heating energy cost penalty by a wide margin in all states, with the heating penalty hovering around 27% of cooling energy saved on average.

    A 2005 LBNL report that performed comparable simulations to the 2010 did assume the insulation levels now appearing in codes (R30) to buildings in cities throughout the U.S. That study found only one city (St. Paul, MN) in which an office building with insulation that meets today’s IECC standard would pay more for energy after switching to a reflective roof.

    A Columbia University study that measured both heating and cooling energy use before and after the installation of reflective roofs in New York City found no detectable winter heating penalty, but observed average summer cooling energy savings of roughly $200 per building each year.

    A common assumption is that reflective roofs are unsuitable wherever heating degree days exceed cooling degree days and Ms. Thorpe is correct that Washington DC has more heating degree days than cooling degree days. And yet, studies indicate that the winter heating energy penalty is about 33% of the cooling energy saved. How can this be? Because of the way the earth tilts at different times throughout the year, the U.S. receives lots of direct sunlight in summer and less in winter, particularly in northern locations. As a result, dark roofs will absorb less sunlight in winter (when it’s beneficial) than in summer (when it’s harmful), and reflective roofs will reflect less sunlight in winter (when it’s in short supply) than in summer (when there is too much).

    The vast majority of research currently available on the global warming impacts of cool roofs, undertaken by scientists from around the world, indicates that reflected sunlight from cool roofs has a net global cooling effect. Even the studies that produce the most negative results for cool roofs global impact acknowledge their ability to reduce urban heat islands and note that global findings are highly uncertain.

  3. This article is a prime example of wishful thinking regarding both climate control and energy savings. While the overall goal of the study – saving lives during a Washington DC heat wave – is beyond reproach, the science and solutions it offers may, in fact, be counterproductive. Washington is very hot in the summer. It is just as important to remember, however, that it can be extremely cold in the winter.

    As members of the EPDM Roofing Association (ERA), we represent the dominant manufacturers of roofing in the United States. We make both black and reflective membrane. We have spent thousands of hours looking at roofs, inspecting roofs, standing on roofs and fixing roofs. We are concerned about energy costs and sustainability, but we are also concerned that in the rush to simple solutions, our products will not be used correctly. We want our customers to find out what they need to know to get the most out of their investment in our products. This means understanding the complex interaction between building design, climate, insulation and all of the other factors that impact the efficiency of our products.

    It is simply common sense to look at both cooling costs AND heating costs in a climate like the one in Washington, DC. White roofs may save energy in the summer, but they will also drive up heating costs in the winter and require more energy to keep a building and its occupants warm. And while that could keep the residents of DC more comfortable during a hot summer, it will only add to their discomfort during a long, cold winter.

    We invite the decision-makers in Washington to go to our website, http://www.epdmroofs.org, where we have assembled resources that explain the factors involved in the appropriate choice of roofing color. We also would welcome the opportunity to meet with these leaders, share our expertise, and help them – and our customers – avoid costly mistakes in their rush to solve a complex problem.

  4. As more and more people are beginning to question the logic of saving energy by using cool roofs in Northern climates, where energy bills are dominated by heating costs, the “white only” crowd is shifting emphasis to other arguments like global warming and reducing heat-related illness or deaths.

    Studies have also begun to surface questioning the claim that white roofs help address global warming. Reflective membranes do just that…they reflect heat back into the atmosphere or onto surrounding surfaces. There have been numerous news stories about the unintended consequences of reflected heat due to highly reflective surfaces. Is reflecting heat back up into the atmosphere really what we should be doing? Doesn’t it make more sense to capture the solar energy absorbed by dark-colored membranes to help reduce heating costs, which in turn reduces natural resource consumption and, consequently, CO2 emissions?

    It has also been documented in a study by Indur Goklany titled “Death and Death Rates Due to Extreme Weather Events” (November 2007) that nearly twice as many people die from exposure to extreme cold than extreme heat. According to data from the Center for Disease Control, extreme cold, on average, claims more lives than tornadoes, floods, lightning, hurricanes, and extreme heat combined.

    For a more in-depth look on the issues of roof color and roof choice, visit http://www.epdmroofs.org

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