The dog days of summer are upon us, and many of us are sweltering in the heat. If you live in a big city, you may feel worse than your country cousin due to the “urban heat island effect”. In an urban heat island, temperatures may be 6-8ºF warmer than surrounding rural areas, fields, or woodlands. Urban heat islands are caused by a combination of factors:
- Loss of vegetation that shades buildings and cools the air through evapotranspiration
- Hard surfaces (buildings, streets, and parking lots) that absorb solar radiation and radiate it back to the air/atmosphere
- Waste heat from electrical use, water heating, car & truck exhaust, and other thermal and mechanical sources
The temperature differentials are worse at night, when hard surfaces like asphalt pavement, brick, and concrete radiate the heat they absorbed during the day back into the air. Calm air also makes it worse.
The urban heat island effect causes a number of problems. It exacerbates normal summer discomfort for everyone—causing people to crank up the air conditioner (and yes, generate more waste heat), and it makes elderly or sick people more susceptible to problems brought on by heat waves. It’s tough, and sometimes dangerous, for athletes and for those who work outdoors–such as construction workers, roofers, police officers and highway workers. Higher temperatures also increase smog formation, which boosts health risks to the elderly, children, asthmatics, allergy sufferers, and those with respiratory problems. Finally, stormwater runoff from paved urban areas causes excess warming in creeks and lakes, which can damage ecology in those environments.
There are a number of green building materials and practices that urban individuals and organizations can use to mitigate or reduce the heat island effect:
Plant trees. In summer, they create shade which reduces air conditioning loads on a building, and in winter, they shield buildings from wind and unwanted heat loss. They also produce oxygen, and create bird & animal habitat.
Install green roofs and walls. These are vegetated, carefully engineered surfaces that support plant life, and keep the building cool. They provide other benefits, too, including improved drainage (protects underlying roof and reduces stormwater runoff), recreation for employees or occupants, and even herb or vegetable growth. Green roofs also improve air quality because plants convert carbon dioxide to oxygen, and filter out other contaminants. Although green roofs may cost $10-$15/sf more than traditional roofs, these costs are offset by energy savings, longer roof life and reduced maintenance costs, and by various grants and tax incentives.
Green Roofs have enjoyed increasing popularity in Europe over the last decade, with government incentives stimulating a multi-million dollar industry. North American planners, builders and consumers are also beginning to consider green roofs for malls, schools, hospitals, and homes. In New York State, green roofs have been installed at the Bronx Zoo, Cornell University, Pace University, the Bronx County Courthouse, and at Rockefeller Center Roof Gardens.
Install a white roof to increase reflectivity (albedo), so that solar radiation is not absorbed by your roof to be re-radiated later. For flat roofs, Green Depot recommends Bulldog Durex Elastomeric White Roof Coating. It’s a flexible, mildew- and UV-resistant waterborne roofing topcoat made with 100% acrylic resins. It reflects heat from the roof surface: lowering cooling costs, and extending the life of roof membranes and surfaces. It can be used over a variety of existing roof surfaces, including metal, asphalt, masonry, and EPDM.
To learn more, visit Lawrence Berkeley Laboratory’s Urban Heat Island site, or the independent site Urban Heat Islands, or the US EPA’s site on the Heat Island Effect.