Even on a cool day, a greenhouse in full sun that is closed up tight can get
quite hot. Roof vents are used to allow hot air to escape, to be replaced by
cooler air from the outside. But during the early part of the season, you want
to trap as much of the warm air inside to last into the night, to save on heating
costs.
This fine balance can best be achieved by the use of optional automatic openers
for the vents. These openers can be adjusted to begin operating at different
temperatures, depending on the plants' needs. As the temperature inside the
greenhouse increases, the vent automatically begins to open. If the temperature
continues to rise, it opens more. As it cools off later in the day, the vent
automatically closes in the same manner. These openers require no electricity
or batteries to operate. In the heat of summer, the vents will often remain
open both day and night.
Additional ventilation can be achieved by adding a louver window, usually installed
opposite the door, down low to allow for the entry of cool air. If needed, one
or more glazing panels can be removed for the summer. Remember to use the door
as a vent also when needed. (In the winter, the automatic openers should be
disconnected.) Shadecloth will be needed to prevent damage to tender plants
during warm weather. Usually 50% shade is adequate, though 70% may be preferred
for tropical plants.
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