of heat loss by long-wave radiation from a warm surface (roof) to another body at a lower temperature (sky). On a clear night, a typical sky-facing surface can cool at a rate of about 75 W/m2 (25 BTU/hr/ft2) This means that a metal roof facing the sky will be colder than the surrounding air temperature. Collectors can take advantage of this cooling phenomena. As warm night air touches the cooler surface of a transpired collector, heat is transferred to the metal, radiated to the sky and the cooled air is then drawn in through the perforated surface. Cool air may then be drawn into HVAC units. See also Ventilation applications By drawing air through a properly designed air collector or air heater, solar heated fresh air can reduce the heating load during sunny operation. Applications include transpired collectors preheating fresh air entering a heat recovery ventilator, or suction created by venting heated air out of some other solar chimney. See also Solarwall Passive solar building design Passive house Low-energy house Zero-energy building List of low-energy building techniques Sustainable architecture References Heating Solar energy Solar architecture Sustainable building Low-energy building