 A radiative air well is designed to cool a substrate by radiating heat to the night sky . The substrate has a low mass so that it cannot hold onto heat , and it is thermally isolated from any mass , including the ground . A typical radiative collector presents a condensing surface at an angle of 30 ° from the horizontal . The condensing surface is backed by a thick layer of insulating material such as polystyrene foam and supported 2 – 3 metres ( 7 – 10 ft ) above ground level . Such condensers may be conveniently installed on the ridge roofs of low buildings or supported by a simple frame . Although other heights do not typically work quite so well , it may be less expensive or more convenient to mount a collector near to ground level or on a two @-@ storey building . 
