White house, white roofs: Obama's low-cost plan to cut electric bills
White roofs are especially useful in cities, where large stretches of black asphalt roads and black tar roofs massively drive up temperatures. This phenomenon, called the "urban heat island effect," results in urban temperatures that are far higher than those of surrounding rural areas. As asphalt and tar absorb heat, they drive up temperatures, pushing residents to use air conditioners. This, in turn, drives up electricity expenditure and can release further heat into the atmosphere.
While one might expect cities to cool down at night, evenings are when the biggest differentiations between urban and rural temperatures emerge. Asphalt and tar roofs, which have spent the day absorbing heat, continue to radiate it long into the night. Further, as tall buildings block access to the night sky, they keep cooler air from reaching the ground.
White roofs and roads, working in tandem, help combat this effect. By reflecting more light out into space, they keep urban temperatures down. In the process, they reduce the need for air conditioning, which reduces energy expenditures.
The shortcoming, however, is that many cities, particularly those in colder areas, benefit from asphalt and tar roofs in the winter. As temperatures drop, old fashioned black roofs and roads help retain heat, reducing winter heating bills and generally mitigating the misery of cold weather. Further, the heat retained by asphalt streets could ease snow removal.
Rooftop planting, also known as green roofs, offer a solution that is useful in both the winter and the summer. In the winter, empty plant beds and dead organic material provide insulation from the cold. In the summer, green roofs reflect light into space, convert carbon dioxide into oxygen, and evaporate water, producing a cooling effect. More important, they also slow the rush of water after storms, reducing the burden on sewer systems. In New York, where almost every substantial storm results in the release of raw sewage into surrounding rivers, green roofs could take the place of billions of dollars worth of infrastructure expenditures.
The planting solution also translates well to street areas. Trees in urban areas offer the cooling effects of evaporation and shade while blocking the sun from hitting asphalt streets. Further, in the winter, when sunlight is so vital, they lose their leaves, allowing more heat to reach the ground.
Unfortunately, green roofs are fairly expensive. Although some experts claim that they can be installed for as little as $10 per square foot, critics cite prices as high as $30. Moreover, the permitting process is extremely difficult in some areas, including New York. However, for all their potential difficulty, they represent an ideal solution for many areas. Here's hoping that, even in his search for an economic solution, Professor Chu finds the time to push a truly green one!