How does increasing the humidity in the greenhouse reduce the need for ventilation? Is it due to latent heat transfer reducing the greenhouse temperature? Or something else?
So I think this is because you can exchange a lot more energy by venting out the same volume of air when the air contains higher latent heat (humidity). Latent heat is originating from the plant transpiration, which is originating from the net radiation absorbed by the plants. So by allowing more humidity in the greenhouse, you can eject potentially more energy efficiently with lower ventilation rate.
This is because humidity is a major driver for ventilation. In general ventilation is performed for one of two reasons: to get rid of excess heat or to get rid of excess humidity. Less commonly it could be done to let in CO2 from the outside air (if the indoor concentration is lower than the outdoor). In cold weather, ventilation is done almost exclusively to let out excess humidity. In that case, ventilation would be done whenever humidity reaches a certain setpoint X. Increasing this setpoint, which means allowing higher humidities, or in practice, increasing the indoor humidity, directly means ventilating less
Another case would be if a greenhouse is too hot, one can use fogging instead of ventilation, which could be desirable, e.g., because you don’t want to lose your indoor CO2 concentration. This would be another case where humidity is increased and the need for ventilation is decreased.