The actual rate of heat loss from the plastic tunnels and overall heat loss coefficients, which were used to compare the results from the different greenhouses were determined. In addition, the effects on internal air temperature and the air-tightness of the screens were also investigated. For the PE-screened plastic tunnel, the heat loss was calculated to be 15 % lower than that of the unscreened plastic tunnel. The rate of heat loss per square meter of floor area from the polyester-screened plastic tunnel was 20 % lower than that from the unscreened tunnel. It was calculated that average increases in temperature above the outside temperature were 5, 4.2 and 0.2 degrees C in the polyester-screened, PE-screened and unscreened plastic tunnels, respectively.