THE EFFECT OF GUTTERS AND ROOF VENTS ON LIGHT IN A MULTI-SPAN GREENHOUSE

To effectively ventilate large, multi-span gutter connected greenhouses, in the Mediterranean region, it is usually required to use both side and roof vents. Since the roof vents and gutters are constructed of opaque structural elements and since the vents are generally covered with insect-proof screens, they may partially block the penetration of solar radiation into the house. The present work characterizes the disturbance induced by the vents and gutters to the level and distribution of light reaching the plants. Experiments were carried out in a typical multi-span greenhouse with roof vents on which insect-proof screens were installed. To measure the PAR (photosynthetically active radiation) above the plants a sensor was placed on a cart that moved along a horizontal beam positioned 2.6 m above ground level inside the greenhouse; a motorized system moved the trolley continuously across more than one greenhouse span back and forth throughout the day. Two additional sensors were used as reference: one was located inside the greenhouse, just below the plastic cover at the greenhouse ridge, to compensate for the effect of the cover type and accumulated dust on the cover, and the other was located outside the greenhouse to measure ambient PAR levels. The mean daily PAR level immediately below the greenhouse cover was 64% of the level outside the greenhouse. Above the crop the mean daily PAR level over one greenhouse span (including regions that were shaded by the gutters and vents) further decreased and was only 47% of the outside level. Results show that the structural elements of the roof vents significantly affect light distribution. At midday the light level in the region below the roof vents may drop by nearly 27% in comparison to the level measured at the centerline of the greenhouse span. Thus, although roof vents improve natural ventilation they may deteriorate light level and its distribution.