An energy-efficient building is one that is designed, constructed and operated to minimize the total energy consumption and environmental impact, while providing a comfortable and productive environment for its occupants. It is designed to reduce energy usage and waste through the use of efficient heating, cooling, lighting and other systems that require minimal electricity and other energy sources. The building's envelope, or structure, is usually designed to reduce heat loss and gain, and take advantage of natural ventilation, daylight and renewable energy sources. Energy-efficient buildings can also incorporate features, such as insulation, that help to reduce the environmental impact of the building and its occupants.