Radiation cooling of buildings at night

The cooling of small buildings at night by radiation loss to the sky has been investigated by monitoring the thermal performance of two huts: one roofed with galvanised steel decking painted white, which acts as a [`]black body' for wavelengths greater than 3 [mu]m; the other with aluminium decking to which aluminised [`]Tedlar' sheet had been glued, the [`]Tedlar' acting as a selective surface absorbing and radiating mainly in the 8-13 [mu]m band. The hut with the painted roof was cooled marginally better than that with the [`]Tedlar' covered roof. Useful cooling powers of 22 Wm-2 were achieved at a roof temperature of 5°C, ambient 10°C, and the gross cooling power probably exceeded 29 Wm-2. Calculations based on a simple simulation of the sky radiation yield an upper limit of 40 Wm-2 for the cooling power of the surfaces and suggest that an ideally selective surface operating under the best possible clear-sky conditions has little advantage over a black body radiator unless the temperature of the surfaces is significantly lower than the ambient air temperature.