Worldwide, buildings account for 32 percent of energy use and 19 percent of energy-related greenhouse emissions. They pull from the electric grid or natural gas lines to heat, cool, and light the spaces within them and to power appliances and machinery. As much as 80 percent of the energy consumed is wasted—lights and electronics are left on unnecessarily and gaps in the building’s envelope allow air to seep in and out, for example.

Much of the attention paid to green buildings is in new construction, but retrofitting brings energy efficiency to the existing built environment. The world has 1.6 trillion square feet of building stock, 99 percent of which is not green. Retrofitting addresses how heat and cold are escaping or entering the building, the systems that cool or warm inhabitants, and how spaces are illuminated. I