Manually this is done by damping down. The moisture in the greenhouse atmosphere will also be governed by the moisture in pots and on the staging, and by any method of automatic watering employed. With automatic watering the humidity may also look after itself. In some cases it may be necessary to improve humidity or to control humidity but retain manual watering. A high humidity in summer will, in any case, substantially lower the need for manual watering. The best way to get water into the atmosphere is by misting jets, and those automatic systems for watering described earlier which can be adapted to supply jets can be used to improve humidity.
Automatic ventilation is easy and effective. However, it is always advisable to combine automatic ventilation with automatic watering or humidity control. Where there is no way of getting moisture into the greenhouse atmosphere, automatic ventilation or unattended manual ventilation for that matter can cause the air to become very dry and increase the water demand of the plants.
New electric thermo-expansion devices
There are a few different designs of this device on the market, but all work on the same principle. A special petroleum compound is sealed inside a cylinder. This expands and contracts with temperature change, just like the mercury in a thermometer. Owing to precision engineering and a system of levers, this movement is transferred by operation of a piston to the ventilator. The mechanism can be adjusted to open a vent at any desired temperature, and it is sometimes possible to work both a side and atop ventilator together.
Normally one device per ventilator is necessary. Where there are many vents this can therefore prove expensive. Nevertheless, this device has now been in use for many years and has proved reliable and trouble free.
Electric fan ventilation
Fan ventilation is very efficient and ensures a rapid air change in the greenhouse. The fan is best set at one or both ends of the greenhouse in the side high up in the apex. Special greenhouse fans are obtainable. They blow air out of the greenhouse and are fitted with louvres so that wind cannot blow back into the greenhouse when the fan is not running. The arrangement is similar to that used for ventilating in most other domestic buildings or factories. Even so, the louvres are best fastened down during the winter when such automatic ventilation is rarely needed in the greenhouse.
Greenhouse fans are available according to the air change required and the size of the greenhouse. The makers will advise which of their models should be bought if given the greenhouse dimensions. Control is by thermostat, and this must be of a type that switches off with rise in temperature. Some thermostats can be used for heating or ventilation and have terminals for both, but most employed for heating do not have this facility. It is usually best to obtain a thermostat designed to go with the fan. Again, look for reasonable accuracy, but since fans use little electricity there is no need for too great a precision.