A roof pond system can provide both radiant cooling and radiant heating with no alteration of its components, simply by changing the operating cycle. It consists of a large mass of water contained in a plastic bladder and liner, lying on a metal ceiling deck, which should be corrugated to increase the thermal coupling with the house below. Above the water lies a movable insulation panel. The water mass above the metal ceiling collects, stores and dissipates heat energy to maintain constant comfort conditions inside the building. Typical roof pond systems use a water mass from 100 mm to 250 mm (4 to 10 inches) in depth.

The primary cooling mode for a roof pond system is radiation, which can cool the water significantly below wet bulb temperature. The night sky (on days with clear skies and low humidity) gets very cold, well below freezing if you aim an infrared thermometer at it. So, the "sky cooling" effect is achieved by exposing a warm mass to the night sky and preventing the daytime sunshine from reheating it (via the movable insulating panel).

Because of its reliance on radiative cooling, the Skytherm concept is best suited for areas of low humidity and clear nights.

Source: Roofpond Simulation Software Manual at http://suntzu.larc.calpoly.edu/ehhf/

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