For a Healthy Greenhouse Supply Good Air Ventilation
For optimal functioning of your greenhouse, supply sufficient air ventilation at all times.
The air inside a greenhouse is heated by re-radiation of solar energy by objects inside the greenhouse, or, sometimes, by means of artificial heating.
It may reach a point where the temperature inside the greenhouse grows uncomfortably high for the plants. If this happens, you need to implement a good greenhouse ventilation system to exchange some of the warm air inside for cooler air from outside.
Because warm air rises, it is convenient to allow it to escape through vents at the highest point of the greenhouse. Cool air can gain access through natural means - for instance, leaks where glass overlaps - or through side vents set lower down in the sides of the greenhouse. It's especially important to provide lean-to or circular greenhouses, or greenhouses used to grow alpine plants, with an adequate fresh air supply.
A rule of thumb for small greenhouses is that the vents should be equal to 25 - 30 percent of the floor area.
It's best to have an automated system to control the opening and closing of vents. This can be achieved by thermostatically controlled electric motors, or, on a smaller scale, by expansion vent units. Roof vents are best positioned alternately on both sides of the roof, with side vents set to match. Don't forget - if side vents are placed at ground level, you need to cover them with wire netting to prevent unwanted guests like mice or cats from gaining access to the greenhouse.
To prevent cold or wet spots, circulating fans can be used to mix the air inside the greenhouse. Extractor fans will pull air out of the greenhouse, and you obviously need a means of allowing fresh air in to replace the lost air. It's possible to use fans to pull air into a greenhouse, but, in practice, extractor fans work more reliably. Air vents are the normally preferred means of allowing air in.
Extractor fans are controlled by an air thermostat - and they must be large enough to move sufficient quantities of air. A good greenhouse supply retailer should be able to recommend the best type of fan for your requirements. Normal domestic extractor fans are not suitable - those used in greenhouses have larger blades which turn more slowly.
Moisture jets or absorbent pads can be used alongside fans to add or remove moisture from the incoming air to help maintain the correct humidity level in the greenhouse atmosphere. With a little planning and forethought it's quite easy to maintain a good greenhouse supply of fresh air to keep your plants happy!
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