Quick answer: A semi-closed greenhouse keeps its vents closed for much of the time and uses air-handling units, usually with air tubes under the crop, to supply treated air. It removes moisture by mixing in drier outside air, by cooling coils that condense water, or by dehumidifiers. A fully closed (sealed) greenhouse has no vent exchange at all, so every litre the crop transpires must be condensed out by a coil or dehumidifier. In both, recirculating air distributes humidity but does not remove it: there must be a named device that takes the water out, and a drain where it leaves.
Quick answer
- Semi-closed: outside-air mixing plus cooling or dehumidification; vents open only at peaks.
- Closed: all moisture removal by condensation; condensate ≈ crop transpiration and can be reused.
- Benefit: CO₂ held in, more even climate, less heat lost; cost: equipment and energy.
Three operating modes
| Mode | How moisture leaves | When it is used |
|---|---|---|
| Recirculation only | It does not; humidity rises | Short periods; spreads dry air from elsewhere |
| Outside-air mixing | Drier outside air replaces humid air, then is heated or cooled | When outside air holds less water than inside |
| Condensation (cooling coil or dehumidifier) | Water condenses on a cold coil and is drained | Humid outside air; closed operation; CO₂ enrichment |
Write down which mode runs in each period (day, evening, night, morning) before calculating any equipment duty. The same house can need very different removal rates in each mode.
How much water must be removed
In a closed period, the removal duty equals the moisture entering the air:
removal (L/h) ≈ crop transpiration + evaporation from substrate and floors − any outside-air exchange
Crop transpiration in productive greenhouse crops is often several litres per m² per day in summer, much of it in daylight. Condensing it releases heat: about 0.68 kWh per kg of water. That is why fully closed greenhouses in summer need very large cooling capacity, and why the Dutch closed-greenhouse concept paired cooling with seasonal heat storage. Wageningen research evaluated how semi-closed greenhouses perform on climate and crop management (WUR, Climatic evaluation of semi-closed greenhouses; Crop management in semi-closed greenhouses).
Worked example (night, closed mode): a 1 ha semi-closed tomato house with assumed night transpiration of 15 g/m²·h has a load of about 150 kg/h (3,600 L/day). Condensing it adds roughly 100 kW of heat back to the house, which offsets night heating. The assumption sets the answer; measure or estimate transpiration for your crop and stage.
Delivering dry air to the crop
- Air tubes under the gutters or between rows distribute treated air evenly through the canopy; WUR research on dehumidification with air movement studied its effect on crop climate (WUR).
- Slight overpressure keeps outside air and insects out and pushes air out through small openings.
- Return air should be drawn from the humid zones, not from next to the supply.
- Measure at canopy level along the tubes: the far end of a long tube gets less air.
Where dehumidifiers fit
| Situation | Role of a dehumidifier |
|---|---|
| Semi-closed house with air-handling units already installed | Add capacity on humid nights; ducted units can feed the same air tubes |
| Closed or sealed growing room | Main moisture removal, sized for the full transpiration load |
| Existing greenhouse moving towards semi-closed operation | Run with vents closed more often; spread units through the house |
Specify:
- capacity at the house condition, for example 18 °C / 85 % RH, not only at 30 °C / 80 % RH;
- airflow and external static pressure for ducted connections;
- the condensate route and whether it will be reused;
- how the climate computer stages it with outside-air mixing and heating.
See our ceiling-mounted and ducted dehumidifiers, greenhouse dehumidifiers and ducted dehumidifier design.
Checking it works
Run each mode for a full period and record, at the same time:
- canopy temperature and RH along the air tubes;
- outside temperature and RH;
- which devices were on (fans, coils, dehumidifiers, vents, heating);
- condensate collected.
If the house is still humid in closed mode, compare the condensate collected with the estimated transpiration. A large gap means the equipment is too small, the air is not reaching the crop, or the coil is not cold enough.
FAQ
Frequently asked questions
What is a semi-closed greenhouse?
A greenhouse that keeps vents closed much of the time and uses air-handling units, usually with air tubes under the crop, to heat, cool, dehumidify and distribute air.
How does a closed greenhouse remove humidity?
By condensing water on cooling coils or in dehumidifiers. With no ventilation, all transpired water must be removed this way.
Does recirculating air dehumidify a greenhouse?
No. It spreads humidity more evenly. Water only leaves through outside-air exchange or condensation.
Why use a semi-closed greenhouse?
To keep CO₂ in, reduce heat loss, keep pests out and control climate more evenly. The cost is equipment and energy for cooling and dehumidification.
Sources
- Wageningen University & Research, Climatic evaluation of semi-closed greenhouses; Crop management in semi-closed greenhouses; Dehumidification of greenhouses.
- Sapounas et al. (2020), TNO greenhouse design study (TNO).
- ASHRAE Handbook—Fundamentals, Psychrometrics.
