FIGURE 3.5 White roof installed on the headquarters building of the Department of Energy; green roof installed on Chicago’s city hall.
CO2 (carbon dioxide) per unit area of roof, because of their greater solar reflectance. A notable advantage of green roofs, however, is that they retain up to 75 percent of incident rainwater, which subsequently evaporates, cooling the roof itself and the building below.
Although well-insulated white roofs can be ensured through performance-based procurement processes, systems-based thinking, combined with integrated work processes, can be used to develop cool, white roofs that also capture 100 percent of their rainwater to store and use on site, so they provide many of the advantages of green roofs. Rainwater capture for both types of roof typically involves installing a cistern at ground level and using the rainwater for irrigation or for equipment such as cooling towers. The commitment to on-site rainwater capture reduces the use of potable water for nonpotable uses, which can result in measurable energy savings and reductions in the use of hazardous chemicals for water treatment and transport.
These examples are intended to illustrate the importance of identifying the lever or set of levers most critical to enabling each technology or integrated design solution to rapidly advance the energy and environmental performance of federal buildings.
Examples of best practices that showcase the effective use of these technologies and others, as well as the other levers of change, are the focus of Chapter 4.