A study of the overall heat transfer coefficient of greenhouse cladding materials with thermal screens using the hot box method

A laboratory guarded hot box was designed and constructed to investigate the overall heat transfer coefficient, U (W m−2 K−1), of different greenhouse polyethylene plastic films with or without thermal screens. Tests were made on plastic films, in both dry and wet (in the presence of condensation) states, with or without thermal screens, as functions of wind velocity and temperature difference. It was found that the overall heat transfer coefficient U increases with increase in temperature difference. For a given temperature difference, the U value increases as the wind velocity increases. The condensation phenomena favored the plastic film, since its U value was always less than that of the corresponding dry film. It was found that using a thermal screen would reduce the U value by about 30%, an energy saving of about 30%.