The positive pressure ventilation concept is to draw fresh air from outside to inside the greenhouse.
Positive pressure ventilation is achieving maximum insect exclusion and temperature uniformity inside the greenhouse. It maximises the performance of the crops and help to reduce the usage of chemicals. The air entrance has to be filtered (with insect-proof net for exemple) before pressurising the whole greenhouse. The pressure must be high enough to exceed the flying speed of insects.
Positive ventilation system is more economical for an individual greenhouse. Indeed, it requires a more developed system for gutter-connected greenhouses (larger surface, volume…), which is therefore much more expensive.
This active ventilation strategy can be used in the Northern Hemisphere during the beginning and end of the season to recycle the air in the greenhouse without causing thermal shock. In fact, it is not recommended to ventilate with the sides roll-up because cold temperature can have a negative effect on the crops.
Add a positive pressure fan linked to a perforated poly tube is widely used for mid-season ventilation in Canada for example. The diffused fresh air introduction will help to control humidity, CO2%… inside an individual greenhouse.
Moreover, a cooling system can be linked to positive pressure ventilation. For example, the air is pushed through an evaporative cooling pads, which forces the cool air into the greenhouse.
D. R. Mears and al., 2020. A positive pressure ventilation system with insect screening for tropical and subtropical greenhouse facilities. Acta horticulturae 578(578):125-132