ONSET OF CONDENSATION ON THE INNER AND OUTER SURFACE OF GREENHOUSE COVERS DURING NIGHT

A one-dimensional steady-state model is used to describe heat transfer to, through and from greenhouse covers by radiation, convection, conduction and phase change and the threshold value of relative humidity for occurrence of condensation on the greenhouse roof is determined. Variables investigated include wind speed, inside and outside air temperature, radiative sky temperature and emissivity of the inner and outer surfaces of the greenhouse cover. Results are presented for standard horticultural glass and for low emissivity glass under various climatic conditions and for various states of the internal environment. It is shown that the threshold value of inside air relative humidity for the occurrence of condensation on the inner surface of the cover is increased by a decrease of wind speed and inside air temperature and by an increase of outside air and radiative sky temperature. Furthermore, it is demonstrated that the threshold value of outside air relative humidity for the occurrence of condensation on the outer side of the cover is influenced in the opposite way by the same variables. The use of low emissivity glass, however, increases the threshold values of both inside and outside air relative humidity at the onset of inside and outside condensation respectively. (C) 1995 Silsoe Research Institute