A little on the top, for environment’s sake

If you want a greener urban environment, start at the top. A cover of vegetation on the roof can help reduce energy bills and improve air quality. Green roofs have become popular in Europe, and they are beginning to catch on in North America. Some use ground-cover plants like sedum and other succulents while others are more like gardens, with shrubs and even trees. Green roofs can provide immediate benefits, but what about the long term? Scientists at the University of Toronto investigated the environmental impact of a green roof over the expected life of a building. They modelled the impact over 50 years on an eight-story apartment building, designed by Susana Saiz, an architect who is one of the researchers.

The building, in Madrid, currently has a conventional flat roof, although it could support a green one. Christopher Kennedy, a professor of civil engineering at the university and an author with Saiz and others of a paper describing the work, said that such environmental ‘‘life cycle assessments’’ evaluate elements like the cost of construction materials and maintenance as well as energy use. The main effect, however, comes from energy savings. Vegetation absorbs less sunlight than a conventional dark roof, and some of the energy that is absorbed is used in evapotranspiration from the plants. So a green roof stays cooler, and less energy is required to cool the living space beneath it. The paper, published this month in Environmental Science and Technology, showed a 6 percent energy savings in summer and 1 percent overall. —NYT