Extra-atmospheric space is practically a pure sink of radiation, and can be used as a nonconventional energy source. Surfaces with an emissivity matched with the atmospheric (8 ÷ 13) µm transparency window interact with cold space when exposed to clear sky at night, and undergo a sizeable cooling effect. During the day the sky radiative temperature remains low, but solar radiation is so much larger that no cooling effect can be obtained : it is however possible to shade the radiating surface using selective covers which stop the solar radiation without preventing infrared emission. In this paper we describe the optical properties of some cheap plastic films for radiative cooling, and present a simple analysis of the net radiated power as a function of the optical properties of the selective radiator and cover.