Knowledge · greenhouses

Greenhouse dehumidification: ventilate or dehumidify?

A growing crop transpires water. That moisture has to leave the greenhouse, or you get wet leaves and disease. The classic answer is to open the vents. In a well-insulated greenhouse with screens that works less well, and it costs heat. This page explains how active dehumidification with heat recovery helps.

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Where does the moisture in a greenhouse come from?

Plants take up water through their roots and release it through their leaves. This is called transpiration. The plant needs it to grow: it cools itself and takes up nutrients that way.

It is a lot of water. According to Wageningen University & Research, a fully grown tomato crop transpires about 25 grams of water per m² per hour at night. During the day, with sunshine, it is more.

With every gram of water that evaporates, heat also goes into the air as latent heat. That heat comes from the greenhouse: from the heating or from the sun.

Why too much moisture is a problem

• Condensation on the crop: wet leaves give fungi such as botrytis a chance.
• Drops falling from the screen or the roof onto the crop.
• More fungal pressure, especially in the autumn.

Venting: simple, but it costs heat

When you vent, you open the windows. Humid greenhouse air goes out and drier outdoor air comes in. But the air that goes out takes its heat with it, including the latent heat in the moisture. The heating has to make up for that heat.

Research by Wageningen University & Research states that dehumidifying by ventilation in a tomato crop easily costs 8 to 10 m³ of natural gas per m² per year.

Growers insulate their greenhouses better and better, with more and better screens. The same research states that better screen insulation almost automatically leads to less moisture removal. So the moisture stays unless you do something extra. That is why active dehumidification is part of Next Generation Growing (Het Nieuwe Telen).

Ways to dehumidify actively

Under code 220304, the Dutch agency RVO lists these and two other versions of a system for dehumidifying horticultural greenhouses.

Dehumidification with heat recovery

In heat pump dehumidification, greenhouse air flows past a cold evaporator. The moisture condenses and drains away as water. The heat pump takes up the latent heat released in the process. It returns that heat, together with the sensible heat, to the greenhouse air or to a buffer.

So the moisture goes out without venting, and the heat stays in the greenhouse.

The X-Cross Green Line works this way. It heats, cools and dehumidifies in one unit. If the unit stands outside the growing area, with ducts to the greenhouse, you do not have to go through the crop for maintenance.

What to look out for

• How much moisture your crop releases, especially at night and in the autumn.
• Your screening regime: how often and for how long the screens are closed.
• Where the unit stands and how the air is distributed over the crop.
• What you do with the recovered heat: straight into the greenhouse or into a buffer.
• For businesses taxed in the Netherlands: whether the investment is on the RVO Energy List. Read more about Dutch tax incentives.