Gardening under glass requires provision for circulation of air and control of temperature during hot weather as well as when the heat is on. Methods of ventilation range from a simple built-in wall sash to an automatic, thermostatically controlled roof system. When excessive heat from the sun or an artificial source is trapped inside the glass structure, the heat rises and the air stratifies—it is warm near the roof, cooler down through the house toward the floor. Then air becomes stagnant, moisture is not evaporated properly, and it is difficult to maintain a constant temperature. For this reason, some means of ventilation and air circulation must be provided.
Since hot air is trapped under the ridge, the logical place for an opening to relieve the concentration is along the ridge, ideally along the full length of the structure. These openings, or roof sash, can be regulated by push rods, by manual gears, or automatically by thermostatic control.
In the sectionalized hobby greenhouses, most push-rod-operated roof sash cover two sections alternately—two operative, two stationary. Some models provide push-rod sash in side panels only, with only one panel operative. (The rods are the type used on basement sash in the home—a flat metal rod fastened to the window allows it to be pushed open and held in place when a hole in the rod is slipped over a fixed pin on the bottom rail of the sash opening.) Several holes are provided in the rod. The amount of ventilation depends upon the distance the rod is pushed out before a hole is fitted over the pin.
The next-best method of ventilation is by manually operated gears. Here again, sash is placed along the ridge line of the greenhouse, each sash fastened to a pipe shaft that is centered through a rotary gear with a continuous leather strap or chain. Pulling on the strap or chain activates the gear that operates the sash, opening or closing it to the degree desired. The number of turns regulates the amount of ventilation. Each gear operator controls one line of ridge sash; long runs of sash are divided into two or more shorter units.
The drawbacks to either push-rod or manual types of ventilation lie in the temperature variation inside the greenhouse, which in turn depends upon the weather outside, the amount of sun, and the intensity and direction of the wind. As these affect the inside temperature, frequent changes of sash opening are necessary. Constant attention to these fluctuations is a chore if an even temperature is to be maintained, and if the greenhouse must be left unattended for some time, plants may be damaged seriously.
Most hobby greenhouses now come with automatic controls. A moisture-proof thermostat, set 10 degrees higher than the setting of the night-temperature thermostat, regulates the sash electrically. There is now available a motor that provides automatic ventilation for pennies a day; it uses only 10 watts per motor when sash are open and it has a built-in safety device that closes the roof sash if electric power fails, thus retaining heat in the greenhouse until current is restored. Each motor will lift about 5 feet of sash; therefore multiple motors will be required, depending on the length of the greenhouse.
The thermostat that regulates the vent sash is placed above the bench line at about the center of the area in which a given temperature is to be maintained. It can be suspended on electric conduit from a roof bar, or it can be installed on a pipe stand permanently attached to a bench. In either place, the thermostat must be protected from direct sunlight or blasts from the heater. An aluminum shade around back, sides, and over the top of the thermostat is the most practical protection, but there must be an air-circulation space between the shade and the thermostat. The same type of thermostat can be used to control both heat and ventilation, but there must be a separate one for each operation. Automatic systems open and close the sash as often as is necessary to maintain the temperature setting.
Where summers are very hot and the greenhouse is used all year, additional ventilation is helpful. Built-in wall sash, push-rod operated, will give under-bench air circulation without direct drafts on plants; this type of sash can also be installed at the eave line. If you want to provide extra air circulation and dress up your greenhouse at the same time, replace one or more glass sections in the side or gable end with hand-operated jalousies (overlapping horizontal glass slats set in an aluminum frame).