Most greenhouses will have roof windows for ventilation. Warm air rises and escapes through these vents and cooler air is drawn in from the door or side windows. Several firms market a. simple expanding tube which can be fitted to the roof ventilators to open and shut automatically, depending on the temperature. A pole with a hook on the end can put manually operated ventilators within reach. The most effective type of ventilator is the thermostatically controlled extractor fan. These are easy to install and cheap to run, providing a continuous movement of air and reducing condensation. They are normally fitted to the gable end of the greenhouse. For polythene tunnels, ventilation is usually provided by opening the end doors.
Shading has the dual functions of at least limiting the sun's rays - thus avoiding scorching to leaves, fruit, etc - and of preventing too much radiant heat from building up, so keeping the greenhouse to a reasonable temperature. Some shading can also be used to insulate the house and thereby keep in the heat at night. As shading reduces the amount of light into the greenhouse it should not be used on dull days. 'Paint on' shading powder, that is mixed with water and painted on the outside of the glass, remains until wiped off (usually in autumn). If you prefer this method, be aware of the limitations posed by the lower light levels. All forms of shading should only be used on the south side of the greenhouse. Roller blinds can be fitted either to the inside or outside of the house and rolled down when required. However, these are expensive.
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