FEATURESTORY
by Jessica Batchelor
Choosing Between Passive or Active
Greenhouse Ventilation
M
aintaining stable greenhouse tempera-
tures during seasonal changes keeps
growers on their toes year-round. When
correctly designed and installed, passive
(natural) and active (mechanical) ventilation systems
help to ensure optimal plant health and crop produc-
tion. Less obvious to growers however, is whether a
passive or active ventilation system is ideal for their
individual greenhouse operation. When considering
which types of ventilation tools to integrate, growers
must first determine how aggressive their grow needs
to be, their proximity to an on-site power supply and
their budget.
Passive Ventilation
Passive ventilation relies on two factors: the wind
effect and thermal buoyancy. Thermal buoyancy oc-
24
curs when denser, cold air lifts warm air up and out
of a building. When wind passes over the backside
of the roof vents, a sort of chimney effect is created.
As hot air is drawn out of the roof, cooler outside air
is drawn in through the wall vents. When done prop-
erly, this can be a very efficient and cost-effective
way to ventilate a greenhouse.
The wind effect simply refers to the fact that
wind is almost always blowing around structures. It
is hardly ever completely still. These air movements
create small pressure differences on the windward
and leeward side of a structure. By placing ventila-
tion windows strategically, pressure differences will
move air through the greenhouse. The wind effect
is the dominating force as long as it is blowing at
around 2.3 mph. Therefore, only under very wind
still conditions will the wind effect have a major im-
pact on ventilation.
www.GardenandGreenhouse.net
June 2018