The experiments were carried out in three semi-cylindrical "tunnel" plastic greenhouses, measuring 6 x 20 m and 3 m high, and covered with double-skinned PE which contained UV+IR stabilizers. The ground surfaces on the floor of all the plastic tunnels were insulated by black PE films and, therefore, the effect of the heat stored in the soil was neglected. The tunnels were not used to grow agricultural crops during the experiments. The thermal screens, made of PE with UV+IR additives and polyester materials, were placed at a height of 2 m in the tunnels, and supported by wires. The tunnels were heated by air heaters. The overall heat loss coefficients of the greenhouses were calculated when the thermal screens were drawn during the cold night periods. The relationships between the overall heat loss coefficient and the wind speed and outside temperature were obtained.