The tasks of ventilation are: the limitation of the temperature level; the regulation of the air humidity; the gas-exchange with the free atmosphere.
The limitation of the temperature-level depends on the difference of the temperature between the air intake and the spent air, the radiated solar energy, the relative atmospheric humidity as well as on the degree of saturation with water of the soil and plants.
Only by ventilation without any auxiliary means the temperature of a greenhouse can hardly be decreased more than 1–3 centigrades below the outdoor temperature.
Ventilation can highly help to avoid stagnant air with high relative humidity.
By the movement of the air the plants can dry better and the danger of an infection by fungi diseases is reduced.
Finally, in times of high difference in the temperature the gas exchange depends especially on the airing to guarantee a sufficient supply of CO2.
The exchange of air between greenhouses and outdoors is performed by free uncontrolled exchange of air favoured by perviousness; by free controlled exchange of air through ventilation outlets and by air exchange by forced ventilation.
The influence of the free exchange of air is highly dependent on the thermics in the house, the difference of the relative atmospheric humidity between indoors and outdoors as well as influences from outside on the roof and side walls of the house.
To be able to measure the amount of exchanged air the ventilation figure (unit) was introduced; it indicates how often the air volume of the house is exchanged in one hour.
In 1959 Stein proposed the introduction of an exchange figure so as to have a better possibility of comparing the ventilation efficiency.
This figure says how many cubic metres of air per square metre ground are exchanged in a time unit.
In a closed hothouse the exchange of air determined by its imperviousness (dependent on how well panes, doors, vents close) ranges between 1–5/h.
According to several authors it should be up to 20/h by free controlled ventilation and 20–40/h by forced ventilation and 40–60/h by using cooling mats.
The best known ventilation types are the ventilation by side vents, roof vents, top vents, gable vents, and the forced ventilation.
Besides these we known other methods as flue ventilation, lifting gable ventilation, lifting roof ventilation.
The latter, however, are more suitable for specific purposes.
The effectiveness of the ventilation depends primarily on the width of