A passive house is built so that it barely needs heating. Very thick insulation, windows with three layers of glass and an airtight envelope keep in the warmth produced by people, appliances and sunlight, and a ventilation system with a heat exchanger supplies fresh air without losing that warmth. Heating bills fall by around ninety per cent. The extra construction cost is typically five to ten per cent, which the savings repay over a period measured in decades rather than years.