Passive ventilation typically uses a combination
of roof and wall vents that work with the forces of
wind and temperature to create a natural air ex-
change. Other ventilation passages include doors,
roll up sides, louvers and shutters. Rolling up a
portion of the greenhouse wall will suffice in some
cases. However, the best performing systems make
use of a combination of accurately-sized roof and
wall vents. It’s important to remember that insect
screening adds additional air resistance; the tighter
the screen, the less room there is for air to flow.
One of the main advantages of passive ventila-
tion is affordability. The initial purchase price is
inexpensive, especially when buying a manual sys-
tem. Operating costs are also very reasonable since
very little electricity used. Another fringe benefit is
having access to crops from the end walls and side
walls. For crops that can handle wider temperature
and humidity fluctuations or for growers who just
grow for themselves and have flexible timelines, a
passive system makes more sense. Passive ventila-
tion is an efficient option for growers who live on
the site of their greenhouse and don’t mind keeping
a constant eye on it.
Important to note is that what users of passive
ventilation gain in savings, they lose in control.
Passive systems are less precise than their active
counterparts, because they rely on wind and tem-
perature. They are also more labor-intensive, as they
require someone to open and close the vents regu-
larly. The vents have to be sealed well, or air is lost
26
through infiltration. Improperly installed vents can
cause heating costs to go up. Labor costs associated
with running such equipment add up also, so own-
ers should assess the option to automate upon initial
investment.
Active Ventilation
Active ventilation relies on mechanical devices,
like inlet vents, exhaust fans, horizontal airflow fans
and evaporative coolers, to produce air circulation
and temperature control. Similar to passive ventila-
tion, fans exhaust hot air and bring in cooler outside
air to lower the greenhouse temperature.
Fans have a set of guidelines developed by
AMCA (Air Movement and Control Association) to
confirm capacity, static pressure, ventilation effi-
ciency ratio and power requirement. Growers look-
ing to buy fans for greenhouse ventilation should
keep an eye out for a sticker indicating it has passed
AMCA inspection.
With fan ventilation alone, growers may not
be able to lower temperatures to the ideal level. If
greenhouse temperatures must be brought down
even further than those outdoors, evaporative cool-
ing is the most effective method. Evaporative cool-
ing works by taking warm outside air through a
damp pad to cool it. Fans at the other end of the
structure then pull the cool, moisture-laden air from
the pad and across the greenhouse to cool the envi-
ronment. This action not only cools the air, but also
provides some ventilation. This type of cooling can
www.GardenandGreenhouse.net
June 2018