The first step is to reduce the amount of energy our houses use. The design principles used to achieve this are deceptively simple: make a house well-insulated so that heat from passive sources – the Sun, inhabitants" body heat, even pets – stays indoors. The orientation of the house and the position of its windows can ensure it makes the most of the Sun"s rays, while shutters, overhangs, and even trees can keep houses cool in summer. In colder climates, only a small amount of additional heating should be needed in winter. Because an energy-efficient house needs to be relatively airtight in order to retain its heat, the final element is a ventilation system to circulate fresh air.