Zero-energy homes start with well-sealed and well-insulated attics, walls and basements or slabs. They often use triple-pane windows, especially in places with cold winters. Inside, energy-efficient appliances, highly efficient LED lighting and smart thermostats help avoid energy waste. Their designs often take natural lighting into account, too, and position windows and overhangs for additional solar heating in the winter and shade in summer. Since the homes are sealed to avoid letting cold or hot air in—and cool or warm air out—they also have ventilation systems customized to maintain comfortable circulation.