Ventilation is
crucial for a productive green house and healthy vegetation.
The purpose of ventilation is to exchange carbon dioxide and oxygen, to
remove hot air or cool the nursery and to lower relative humidity. Hobby
greenhouses can be vented by natural flow-through ventilation or by forced-air
ventilation.
Flow-through
ventilation relies on side and top vents that pull cool outside air into the
greenhouse through the side vent as warm air rises and exits through the top
vent. The combined side and top vent area should equal about 20 percent of the
roof area. Vents can be manually controlled, but this means you’ll be
constantly adjusting them as outside temperature and conditions change. Another
option is using a thermostat paired with an electric motor or auto-opener. A
single set-point temperature determines when an automatic system opens and
closes your vents. More advanced systems open and close vents in stages based on
multiple set-point temperatures. Natural flow-through ventilation costs less
than forced-air ventilation to operate; however, it may not be adequate to cool
the greenhouse during the summer or in hot climates.
Forced-air ventilation systems use a
louvered intake vent and electric fan, or multiple fans, controlled by a
thermostat. The fans take cool air into the greenhouse from outside through an
intake vent and push warm inside air out. Fans should be mounted in a waterproof
housing with air-activated louvers to protect electrical components from
inclement weather and to keep cold air out during the winter. Be sure to install
a screen over the inside of the fans to prevent injury and to keep out airborne
pests. Be sure that the distance between the fans and adjacent structures equals
at least 1-1/2 times the fan diameter.
You’ll want to be sure that your fan
capacity is large enough to exchange the air in the greenhouse at least once per
minute. Recommendations for warm climates call for a fan capacity to remove 12
to 17 cubic feet per minute per square foot of floor area. If the greenhouse is
attached to the east, west, or south wall of another building, solar heat will
collect inside the greenhouse from this wall. If this is the case, add half the
area of the attachment wall to the floor area when calculating the ventilation
requirements. Work with a greenhouse supply or construction company to determine
the proper size fan or vent for your situation.
