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