Greenhouse Ventilation and Humidity

One of the advantages of growing plants in a greenhouse is that the condition of the atmosphere can be controlled. Unfortunately many beginners seem to give this little consideration. Here we explain how best to regulate the circulation and humidity of the air in your greenhouse.

The vital constituents of the air — carbon dioxide, oxygen, water vapour and nitrogen — are visible. The old saying ‘out of sight, out of mind’ applies! Yet air should be considered a ‘fertilizer’ because it is essential as a plant food. In photosynthesis, carbon dioxide (CO2) in the atmosphere is used by plants, in the presence of water and light, to make starches, sugars and celluloses, and many other chemicals of which they are composed. Oxygen is also used and exchanged, and some plants — such as the pea family — make use of certain beneficial bacteria in their root nodules to take in nitrogen directly. Normally nitrogen is taken up only as nitrates from the soil.

The humidity of the air, the moisture it contains, is also unseen — but its presence becomes apparent when drops form as condensation on the greenhouse structure. A high humidity can be bad in winter and good in summer, so this also has to be controlled.

Adjustment of ventilators

Make sure that there are plenty of ventilators in the greenhouse structure. There is no need to be afraid of having too many, since you don’t have to use them all at the same time. Although there should be a generous number they must fit tightly: ventilation does not have to mean draughts. Many times in the winter the vents may have to be kept closed, and even a small icy draught will counteract any benefits a heater may give. Sliding doors, now commonly fitted to greenhouses, are useful for extra ventilation in summer. They can be adjusted easily and will not slam. Both side and roof ventilators should be fitted, preferably on both sides of the greenhouse. This enables them to be opened on the side away from the prevailing wind. Where to position side vents is a matter of some controversy. Often there may be little choice, especially if the greenhouse has a base wall. However, in summer, vents set as low as possible will give good air circulation, since hot air rises rapidly and draws cool in from below.

Adjustment of ventilation will affect both temperature and humidity. Apart from the obvious cooling effect of the air change, a flow of air increases evaporation of water from the greenhouse. When water evaporates it absorbs energy, in the form of heat, from the surroundings. If the greenhouse interior is well damped down and watered in summer this will have a further cooling action. On the other hand, a fast flow of air may hasten evaporation excessively and you will have to be constantly watering to keep the air moist and the plants from drying out. Ventilation must therefore be given in moderation, and the amount you give will depend on the type of plants you grow and the outside weather conditions.


In summer most plants prefer a moist atmosphere since this reduces the rate at which they lose moisture from their leaves — the process called transpiration. This, in turn, lessens the rate at which the roots dry out, and so the plants are able to absorb nutrients through their roots more easily. A moist atmosphere in summer also discourages red spider mite, a very common pest when the air is hot and dry.

In winter the aim is quite different —every attempt must be made to keep down humidity. In most home greenhouses the rule is, the drier the air, the better. When conditions are cool, perhaps cold, and daylight poor, many fungoid diseases flourish when the air is humid. These include moulds, mildews, and fungi causing stem and storage organ rots (of bulbs, corms, tubers, and the like). Ventilation again becomes very important, but many beginners are reluctant to give it. However, there are many winter days in Britain when the outside air is well above freezing, and you can then ventilate freely. Again, much depends on the plants you are growing, but in most home greenhouses a temperature higher than 5-8°C (40-45°F) is rarely necessary.

In winter the air is further kept dry by not damping down and by watering, sparingly, only those plants that are still growing. Dormant plants should require no water at all. Staging that is kept moist automatically in summer, or covered with moist peat or grit to maintain humidity, must be allowed to go dry.

Use of heaters

Paraffin wick heaters are very popular and are probably the most widely used method of warming a greenhouse. Unfortunately, they are also frequently associated with winter plant troubles attributed to fumes. Typical symptoms are browning of leaf edges, leaf shrivelling, and scorched spots. Often this is caused by oxygen starvation, due again to lack of ventilation. Oil must have oxygen to burn properly. If a greenhouse is tightly sealed, with an oil heater burning, the time will come when the air becomes short of oxygen. The oil will not burn properly so that oil vapour and other results of incomplete combustion — harmful to plants — are produced. Where oil heaters are used there must always be some ventilation, however cold the weather. Because oil forms its own volume of water, as water vapour, when it burns. This contributes to humidity and is another reason for carefully checking your ventilation.


When the weather is cold some condensation is normal and indeed inevitable. If there is so much that drips form everywhere and the glass (or plastic) is coated with droplets, ventilation is seriously at fault. You are also probably overwatering. Plastic greenhouses are particularly affected by condensation because the water forms droplets instead of a film as it does on glass. The droplets interfere with light entry so that plants may become pale and weak. Efficient ventilation is the answer, together with reduction in quantity of water applied.


In summer it is a good idea to water early in the morning. This keeps up humidity during the day and there is adequate moisture for the plants at their time of greatest need — during bright daylight when photosynthesis is most rapid and plants are growing quickly. In the night there is little growth and little water requirement. Moreover, a high humidity during the cool of the night can encourage diseases like grey mould even in summer, something you may often see in tomato houses. High daytime humidity is good for tomatoes, aiding pollination.

Fan ventilation is now popular in greenhouses in summer owing to the ease with which it can be operated by a thermostatic control. It is wise to have some form of automatic humidification in conjunction with fan ventilation, prevent rapid drying out of the house. Below you will learn more about humidity and the most desirable levels according to the plants grown, how to use a wet and dry bulb hygrometer to help measure the changing levels of humidity in your own greenhouse; automatic humidifiers described on Automatic Watering Equipment help cut down manual watering.

wet and dry bulb psychrometer - greenhouse typeThe temperatures recorded (see image right) on the dry bulb thermometer(referred to as T) and the wet bulb thermometer (t) are noted, and by reference to tables supplied with the instrument, the relative humidity of the air can be found. The relative humidity, called RH for short, is the measure usually used. An RH of, say, 60 per cent — a roughly ‘normal’ figure, means that the air contains 60 per cent of the water that it is possible for it to hold at the temperature when the reading is taken.

The direct-reading hygrometer is simply a dial calibrated in RH, and usually also has sections marked ‘dry’, ‘normal’ and ‘moist’. It works by the expansion and contraction of a special fibre that moves a needle on the scale, but the instruments are rarely accurate at the extreme ends. Fortunately for the greenhouse this does not matter. For most purposes, readings from about 35 to 50 can be considered ‘dry’, 50 to about 70 as ‘normal’, and 70 to 85 as ‘moist’. In summer the RH should generally be kept not below about 70 per cent and in winter not below about 50 per cent. This is because the humidity level must relate to the outdoor temperature.

Regulating humidity

damping down greenhouse pathsBy siting plants carefully in the greenhouse it is possible to provide a single plant (or a small group) with its own microclimate. Trays of moist sand placed near, or on, an automatic capillary watering bench, will give localized moist air. Plants needing drier conditions can be put on shelves, near vents or on slatted staging. In summer many of the subtropical species can be freely sprayed with a fine mist of water to moisten the foliage thoroughly. The water should be clean and lime-free; if the mains tap water is limey, use clean rainwater instead so that the leaves do not become marked with lime deposits. In summer nearly all plants, whatever their nature, will benefit if you damp down the floor and staging from time to time. How much damping down you do depends on the humidity requirements of your plants.

Most forms of automatic watering will also keep up humidity automatically. Special automatic humidifiers are obtainable and usually consist of a fan blowing air over a spray of water, this operation being controlled by a special switch that is sensitive to air moisture. The photoelectric method of automatic watering can also be adapted to control humidity by being connected to misting jets that damp down the floor or staging.

For hot-water pipes, and similar types of heat radiator, simple water reservoirs with wicks can be fitted (as is done with domestic radiators). But when the heating is in operation in winter it is often desirable to keep humidity down. For the same reason the humidity’ trays fitted to some older designs of paraffin oil heater should rarely be used.

10. July 2011 by admin
Categories: Greenhouse Cultivation, Greenhouse Equipment | Tags: , , | Comments Off on Greenhouse Ventilation and Humidity


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