Greenhouse Ventilation


A good system of ventilation is required in any greenhouse or conservatory to reduce the temperature and to prevent a build up of damp, stale air. It is always worth asking for extra ventilators when ordering a greenhouse as they are very rarely supplied with a sufficient number as standard, and may be difficult to add once constructed. This is especially important if your greenhouse is heated with a gas or paraffin heater, as water vapour and plant-toxic fumes can soon build up to an unacceptable level.

Air Circulation

Air may be circulated around the greenhouse by one or more of the three following methods: wind effect, chimney effect or fan ventilation.

The wind effect relies on vents placed in the roof and sides of the greenhouse, through which fresh air blows and is circulated around the area before escaping through an open vent on the opposite side. Vents should be staggered to ensure that the wind does not simply blow in one vent and straight out of another.

The chimney effect is based on warm air rising from the base of the greenhouse and escaping through vents in the roof, which is then replaced by fresh air drawn through side vents. Finally, a fan system may be used that draws air out of the top of the greenhouse using a fan, which is then replace by cool, fresh air from an open vent placed low down on the opposite side.

To create a good ventilation system, make sure that the area covered by ventilators is equal to at least one fifth of the floor area.

Ventilation Systems

Hinged ventilators may be positioned in the roof or sides of a greenhouse or conservatory to provide a good source of airflow. Make sure that they can be securely fixed when open; an angle of approximately 45° is best to allow a good flow of air whilst preventing any gusts of air from damaging plants.

Louvre ventilators are a good source of air flow during the winter months when hinged roof ventilators would allow too much heat to escape. They are usually placed fairly low down in the greenhouse, below the height of the staging and may be set to a range of positions to allow just the right amount of air to flow through. However, to prevent draughts, make sure that the vents close firmly when not in use.

Automatic vent openers may be fitted to open and close hinged and louvre windows at pre-set temperatures. Some autovents work by means of a number of metal rods that change shape with temperature, which then activate the vent opener, whilst others rely on a plug of mineral wax that expands with heat to work a piston which causes the vent to open. As the temperature drops, the wax contracts and a strong stainless steel spring closes the opener. It is always best to set your autovent temperatures to just below the maximum and just above the minimum for your greenhouse conditions to ensure the vents will open or close before the greenhouse becomes too hot or too cold for your plants.

Extractor fans may be used within the greenhouse to draw out warm, stale air from the top of the greenhouse to be replaced by fresh, cool air through a louvre window or hinged vent set low down on the opposite side. Fans should be fitted with a thermostatic control to ensure they keep the greenhouse at the optimum temperature, and should also have louvre slats to prevent draughts when not in use.

Fan power is usually expressed in cubic metres or cubic feet per hour. You should always try to choose a model that is slightly more powerful than you need; this will prevent the motor from having to run at maximum capacity, thus extending the life of the fan. A fan with a capacity of 400 cubic metres (14,000 cubic feet) per hour will change the air in a 3.5 x 2.5 m (12 x 8 ft) conservatory every 3.5 minutes.