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
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.