A greenhouse should have one top ventilator and one side vent for every 6 to 8 feet of length assuming a width of up to about 10 feet. For alpine plants, and many of the more hardy annuals and cut flowers, more ventilation is desirable.
It is an advantage not to have the vents all on one side of the greenhouse so that they can be opened according to wind direction. In rare cases ventilators are supplied as extras. Should this occur, do not be tempted to cut costs by doing without a ventilator or two. You don’t have to open ventilators if it is not necessary, but it is wise to have them so that they can be used freely when the plants require plenty of air.
The conventional hinged ventilator with stay bar is still fitted to many greenhouses, but there are now louvred vents in some aluminium framed houses, and sliding vents (patent) in Alton greenhouses that can be fitted as optional extras or alternatives in their timber range. Unfortunately some louvred vents do not close to give an airtight seal. This can lead to draught in winter and should be checked when purchasing. The sliding vents on the Alton range are reasonably draught free, and have the advantage that they are fitted as bottom ventilation. This is in the glass-to-ground type of house only. Some other types of greenhouse are also designed to permit the vents to be fitted at staging level or at ground level. There is some controversy about which is best, but to keep the greenhouse cool when the weather is very warm the ground-level vents are preferable.
The hot air tends to quickly rise and escape through the top ventilators, and this draws cool air in at ground level, thus giving a complete air change. However, it has been argued that cool air, being heavier than warm air, will fall to ground level on entering from a staging-level vent.