Many cities around the world experience temperatures up to 10°F (5.6°C) warmer than the surrounding countryside. These areas are called “urban heat islands.” Their air surface temperatures are elevated more than usual. Drought conditions are also more pronounced than in nearby rural areas.
The effect occurs because urban surfaces, such as roads and buildings, tend to absorb heat. There are also far fewer trees and other plants to provide cooling through the plant process known as transpiration.
Among the biggest heat absorbers in cities are rooftops. Rooftops typically make up 5 to 35% of the urban landscape. The U.S. Environmental Protection Agency (EPA) reports that over 90% of the rooftops in the United States are dark in color. They also reflect very little sunlight – meaning they absorb all the incoming heat. As a result, city roof surfaces can reach temperatures of 150°-190°F (66°-88°C) during the summer. Yes, that’s hot enough to cook an egg!
When the roofs are this hot it’s harder to cool the interior spaces of buildings. That results in higher peak electricity demands, which can overburden city power grids. The level of air pollution in a city can also be intensified by the urban heat island effect. For example, higher temperatures in cities can increase levels of ground-level ozone. Ozone is a pollutant that causes respiratory problems.
counteracting the heat island effect
Green roof technology can help reduce urban heat islands and conserve energy. Green roofs have been shown to keep rooftops cooler than conventional roofs. They also have a cooling effect on a city’s surface temperatures. A study in the Baltimore-Washington metropolitan area showed that green roofs could cool the surface by nearly 2oF (1oC)! The project studied green roofs that covered at least 30% of the roof surface.
The biggest factor that influences how well green roofs work is the volume of moisture in the soil. It is important to keep the soil from becoming very dry or near the plants' wilting point. While it’s important to monitor how much water the plants are using and irrigate appropriately, adding too much water doesn’t help. It doesn’t help the cooling – and it’s wasteful of the water.
Green roofs can also reduce energy use across different climates. The amount of cooling from the green roof depends on the material used for the roof. Green roofs are typically covered with a growing media that is different from the soil on the ground. It is specially-made mixtures of soil and inorganic materials like crushed clay and perlite. This keeps the material lightweight and provides good water drainage.
Intensive green roofs offer more cooling and energy savings benefits than extensive green roofs. The increased soil depth and plant density of intensive green roofs prevents heat from moving down to ceiling height. Energy use can be reduced 10 to 30% when a building has a green roof. This effect is more significant in hot, dry climates than in humid or temperate climates. For example, intensive green roofs reduced indoor heat by 2.5oF (1.4oC) in the hot, dry urban area around Cairo, Egypt. The same type of roof only reduced indoor heat by 1.4oF (0.8oC) in the humid climate of Hong Kong and 1oF (0.6oC) the temperate climate of Paris.
Plants are also vital parts of a strategy to reduce the urban heat island effect. Plants produce a cooling effect on their environment through the process of evapotranspiration: Heat energy is lost as water evaporates and transpires from vegetation. Cities can maximize this cooling effect by using green roof plantings as well as expanding vegetation at the ground level as much as possible.