Passive House, or Passivhaus in German, is a building construction and design standard that results in buildings that require very little energy input in maintaining comfortable indoor temperatures, even during the coldest days of winter and warm days of summer. In order to be certified as a Passive House, a building's annual energy requirements for space heating must not exceed 15 kWh per square meter of floor space. Passive Houses lack traditional active heating and cooling systems; in a sense, they heat and cool themselves by dint of their energy efficient design. In order to achieve this, Passive Houses typically sport very thick insulation, good airtightness, southern exposures, heavily glazed windows that hold in solar energy, and ventilation systems that circulate fresh air while minimizing heat loss. Conceived by German and Swedish engineers in the 1980s, Passive House principles have been used in the construction of over 6,000 buildings to date, mostly in Northern and Central Europe.