EFFECT OF COVERING MATERIALS ON ENERGY-CONSUMPTION AND GREENHOUSE MICROCLIMATE

The objective of the study was to conduct an extensive energy and micr oclimatic assessment of different greenhouse covering materials. Singl e glass (GL) and three types of double polyethylene (PE) claddings wer e compared. The double polyethylene cladding consisting of an anti-fog thermal film for the inner layer and a standard PE film for the outer layer was the most energy efficient, It had an average measured heat transfer coefficient (U value) of 2.9 W m(-2) K-1. The average U value for the other PE cladding was 3.4 W m(-2) K-1. The use of thermal scr eens in the PE houses during the night reduced heat loss rates by 23-2 4%. The differences in climates under different claddings is presented in terms of PAR transmission and humidity levels. The measured averag e PAR transmission during the winter months (November-March) were 0.68 , 0.62, 0.65 and 0.60 for glass, anti-fog 1-year, anti-fog 3-year and anti-fog thermal claddings, respectively. In the summer months (April- October) the values were higher. The average vapour pressure deficit i n the double PE houses was found to be 0.2 kPa lower than under single glass during the winter season, but no significant difference was obs erved between various anti-fog films. The use of a thermal screen in a double PE houses caused only a slight increase in greenhouse humidity . The contribution of supplementary lighting to greenhouse heating dem and is also presented and discussed.