The Impact of Cool Roofs on the Urban Heat-Island Effect

In Losing Our Cool (The New Press, 2010), Stan Cox examines how indoor climate control is helping send our outdoor climate reeling out of control and explores how combining traditional cooling methods with newer technologies can keep both us and the planet comfortable. In this excerpt from Chapter 9, “Coming Out of the Cold,” Cox discusses the urban heat-island effect common in cities as well as methods such as installing reflective roofs and green roofs that can have a natural cooling effect. 

Urban heat-island effects have burdened air-conditioning systems with increasingly heavier loads as the years go by, and much research has gone into reversing that trend. Hashem Akbari of the Lawrence Berkeley National Laboratory and his colleagues have long advocated increasing the reflectivity of whole cities, so that the sun’s energy is sent back to the sky and not absorbed. They point out that energy can be saved both directly, by keeping heat out of individual buildings, and indirectly, by keeping the entire city cooler and thus putting less stress on cooling systems. Lighter-colored, more reflective roofs and exterior walls have demonstrated summertime energy savings of 15 to 40 percent in individual buildings. But those savings are realized mostly in warmer climates, because replacing a dark, heat-absorbing roof with a more reflective roof means giving up a source of solar warming in winter. In a 2001 review of cool roofs, Akbari skirted a direct comparison between summer energy savings and winter losses by using different units of measurement. After citing an impressive projected savings of 10 billion kilowatt-hours per year for cooling nationwide if all residences and commercial buildings had cool roofs, he estimated the resulting increase in natural gas consumption for heating at 26 trillion Btu annually. Translated into common energy units, the net savings are only a little over 2 billion kilowatt-hours, equivalent to less than one-half of 1 percent of air-conditioning energy consumption that year. Clearly, cool roofs will have their greatest positive impact in warmer climates. 

Before World War II, U.S. urban areas were often cooler than the surrounding countryside because of extensive tree planting in cities and the spread of agriculture in formerly forested lands in the East and Midwest. When air-conditioning replaced shade as a primary means of cooling, cities became universally hotter than rural areas. Tree planting is now widely advocated as an anti-heat-island measure. Recorded reductions in cooling energy through direct shading of buildings have reached as high as 50 percent in very hot climates. Trees can reduce heating energy as well, by shielding buildings from winter winds. Furthermore, planting and maintaining trees in densely populated urban areas contributes in a way to social justice: in addition to improving a city’s aesthetic appeal, trees also provide shade and evaporative cooling of the immediately surrounding air, giving some relief to people nearby who cannot afford air-conditioning. Through those same mechanisms, trees have a general indirect cooling effect on the city as a whole. Overall temperature reductions by trees have been estimated at 1.8° to 5.4° for ten U.S. cities. Trees also reduce air pollution both directly and through cooling. 

For maximum conservation, it is recommended that trees be positioned to shade west, southwest, and south windows, so that at maturity, the edge of the tree’s canopy comes close to the house. Deciduous trees, which do not shade the house in winter, are also beneficial when situated on the south side. Along the south side of our own house in Kansas, there is not enough space for a shade tree, so we grow a strip of the giant reed Arundo donax. This perennial, thick-stemmed, fast-growing grass with cornlike leaves emerges in spring and shades most of the house’s south face during the hot period of July, August, and September. It covers even the second-story windows and dies back to ground level with the first frost, allowing winter sunlight to warm the side of the house. 

Covering a roof with vegetation shields it from the more intense solar radiation to which it is subject, dampens temperature fluctuations through the thermal mass of the soil, and cools further by transpiring water, which absorbs heat and escapes as vapor. Energy savings by “green roofs” vary widely, depending on local climatic conditions and the method and type of vegetation used. One study found that ivy had a stronger cooling effect than grass planting, because ivy had a larger leaf area for transpiring water. Topping a house or office with a layer of soil and growing plants, however, is no simple matter. Green roofs must be designed and maintained with great care to avoid the host of easily imagined problems that can plague them. They are expensive and can consume a lot of water, yet can dramatically reduce energy consumption under the right conditions. And unlike reflective roofs, they don’t reject solar warmth in winter.