Teitel, M., Agricultural Engineering Institute, Agricultural Research Organization, Volcani Center, P.O.B. 6, Bet Dagan, 50250, Israel
Liran, O., Agricultural Engineering Institute, Agricultural Research Organization, Volcani Center, P.O.B. 6, Bet Dagan, 50250, Israel
Barak, M., Agricultural Engineering Institute, Agricultural Research Organization, Volcani Center, P.O.B. 6, Bet Dagan, 50250, Israel
Tanny, J., Institute of Soil, Water and Environmental Sciences, Agricultural Research Organization, Volcani Center, P.O.B. 6, Bet Dagan, 50250, Israel

Natural ventilation is an important process in greenhouse cultivation. It is usually done by roof openings, side openings or a combination of both. Two modes of ventilation roof alone and side alone, were considered in this study to find their effect on ventilation rate and air velocity within a greenhouse in which roses were grown. Ventilation rate was estimated by the decay rate of a tracer gas (N2O) and by the mass balance of water vapor in the greenhouse air. A comparison between the results of the two methods gave a good agreement. Air velocity at four heights above the soil was measured at the center of the greenhouse with fine hot wires. It is shown that the mean air velocity obtained by dividing the ventilation flow rate by the cross sectional area of the greenhouse was a good estimate for the directly measured velocity when the greenhouse was ventilated by roof openings, but gave less good results when side ventilation was applied.