Using a single-span double-polyethylene greenhouse without plants, the effect of natural ventilation rate on humidity and water use for fog cooling was investigated. A simple and unique control algorithm for fog cooling was proposed and tested. The greenhouse was equipped with high-pressure fog nozzles, roll-up side vents with insect screens and a roof vent. Fogging was operated cyclically with an air temperature set point of 24.5°C. Under several configurations of vent openings, the greenhouse environmental conditions and the outside weather conditions were monitored. The natural ventilation rate was measured continuously by the tracer gas method. The fog generated was collected and measured at 15-min intervals. Results showed that the inside relative humidity decreased with an increase in ventilation rate as expected from simulations based on the steady-state energy balance equations using a software Visual VETH, while the water used for fog cooling increased. For example, the humidity decreased from approximately 80 to 65% on a clear day when the ventilation rate was increased from 1 to 3.5 m3 m-2 min-1, while the water use increased from 18 to 21 g m-2 min-1. There was good agreement between the measured 45-min averages of ventilation rate and the predicted ventilation rates by Visual VETH. The control algorithm which incorporated the adjustment of vent openings demonstrated the possibility of maintaining relative humidity and air temperature simultaneously within a desirable range (65-75% and 24-25°C, respectively) while reducing the water used for fog cooling.