Ironically, too much heat can be as much of a problem as too little.  A bright, unobstructed sun burning from sunrise to sunset can turn the greenhouse into a steam room.  As a result, some way to ventilate excess heat must be provided.  Most greenhouses are equipped with hinged ridge panels, powered by a motor or operated by hand.  Because temperatures will fluctuate, thermostatically controlled motors can prevent overheating automatically--whether you are there or not.

But too much heat is not the only reason for proper ventilation.  Good air circulation, even during the winter months, increases the supply of carbon dioxide to plant leaves and stems, prevents the spread of disease, and contributes to the even distribution of heat.  Small vents can be installed under the benches to allow fresh outside air to enter.  Remember, though, that strong drafts can affect plants adversely.  It's also a good idea to put screening material over all vents so insects won't invade the greenhouse.

Some form of shading, especially during spring and summer, can be an effective way to keep excessive heat under control.  Reflective paint, colored plastic film, or slatted curtains are easy to install and provide relief from intense sunlight.  If the greenhouse will be used in summer, some form of air conditioning may be necessary when temperatures are not sufficiently reduced by shading and ventilation.