Quick answer: Control greenhouse humidity in this order: limit the water you add (irrigation timing, wet floors), move air through the crop so leaves stay at air temperature, vent and heat when the outside air is drier, and dehumidify when venting costs too much heat or the outside air is too humid. The deciding number is the difference in absolute humidity between inside and outside air: the smaller it is, the more air you must vent and heat to remove the same water, and the stronger the case for a dehumidifier.
Quick answer
- Crop transpiration is the main water source, and it continues at night.
- Venting works only when the outside air holds less water than the inside air.
- A dehumidifier keeps the heat in: condensing water releases about 0.68 kWh per kg back into the house.
The moisture balance
Water enters the greenhouse air from:
- crop transpiration, by far the largest source, highest in the day but continuing at night;
- evaporation from wet substrate, floors, paths and irrigation water;
- outdoor air when it is more humid than inside (humid climates, rain, fog).
Water leaves through:
- ventilation and leakage, when outside air is drier;
- condensation on cold glazing, an uncontrolled loss that drips back on the crop;
- dehumidification, which condenses water inside a machine and drains it away.
The humidity stays where it is when what enters equals what leaves. To lower it, either reduce the input or increase a controlled exit.
The five methods
| Method | What it does | Works when | Limits |
|---|---|---|---|
| 1. Reduce inputs | Water early in the day, avoid wet floors, fix leaks, drain paths | Always | Cannot reduce transpiration much without hurting growth |
| 2. Air movement (HAF fans, air tubes) | Keeps leaves at air temperature, breaks up wet pockets, evens out zones | Always; essential in dense canopies | Moves water around but does not remove it |
| 3. Vent + heat | Exchanges humid inside air for drier outside air, then heats it | Outside air holds clearly less water than inside | Heat loss; poor in humid weather; limited with screens closed |
| 4. Screens and cover management | Reduce heat loss and cold surfaces | Cold nights | Trap humid air under the screen; need a gap or a moisture exit |
| 5. Dehumidification | Condenses water out of the air and returns heat to the house | Vents closed for heat or CO₂; outside air humid; strict night targets | Equipment and electricity cost; must be sized and distributed |
Most commercial houses combine all five. The question is which one does the last part of the job at night.
Worked comparison: venting vs dehumidifying at night
A 2,000 m² greenhouse at night, held at 18 °C / 85 % RH. Outside air is 10 °C / 90 % RH.
Assumed crop transpiration at night: 15 g per m² per hour, a moderate value used for this example; measure or estimate it for your crop and stage.
- Water to remove: 2,000 × 15 g = 30 kg/h.
- Inside air holds about 11.0 g of water per kg of dry air; outside air about 6.9 g/kg. Each kg of air exchanged removes about 4.1 g.
Option A: vent and heat
- Air to exchange: 30 / 0.0041 ≈ 7,300 kg/h, about 6,100 m³/h.
- Heat to warm that air from 10 to 18 °C: 7,300 × 1.006 × 8 / 3,600 ≈ 16 kW, continuously, and the latent heat in the vented vapour leaves with it.
- If outside air were 14 °C / 95 % RH (a humid night), each kg would remove only about 1.5 g, and the airflow and heat would need to be almost three times larger.
Option B: dehumidify
- Removal needed: 30 kg/h, about 720 L/day, at the house condition (18 °C / 85 % RH), not at the 30 °C / 80 % RH catalogue rating.
- Condensing 30 kg/h releases about 20 kW of heat into the air, and the machine's electrical input also ends up as heat.
- Vents can stay closed, so CO₂ and screen insulation are kept.
The comparison turns on your energy prices and on how often the outside air is humid. Venting is usually cheaper when the outside air is cold and dry; dehumidification gains on humid nights, in warm climates, and wherever heat and CO₂ are expensive. Wageningen research on greenhouse dehumidification makes the same point: the choice depends on climate and energy balance (WUR, Dehumidification of greenhouses). See also ventilation vs dehumidification in controlled agriculture.
Why RH alone misleads
RH falls when air warms, even if no water is removed. Heating a house from 16 °C to 20 °C drops RH by about 20 points without taking out a gram of water. For control, use:
- humidity deficit (g/m³) or VPD (kPa) for the crop;
- dew point compared with leaf and glazing temperature for condensation risk.
The night setpoints in greenhouse humidity and disease and the nighttime dew point guide apply this.
Fixing wet zones
Humidity is rarely even across a house. A Wageningen study with 100 wireless sensors in six commercial greenhouses found horizontal temperature and RH differences that persisted in five-day averages (WUR). Lower canopy air is usually wetter than air above the crop.
| Symptom | Likely cause | Fix |
|---|---|---|
| One end or side always wetter | Cold wall or gable, poor air movement, door | HAF fans along the house; heat at the perimeter; dehumidifier discharge aimed at that zone |
| Lower canopy wet, upper air fine | Dense canopy, no air through the leaves | Air tubes or fans under the canopy; leaf removal where possible |
| Leaves stay wet long after irrigation | Late irrigation, cool air, no air movement | Irrigate earlier; run fans after irrigation; raise temperature slightly |
| Drips from roof or gutters | Condensation on cold glazing | Screens, roof heating, lower dew point at night |
Multi-span houses: map conditions in several spans and heights at the same time, including perimeter bays and the leeward side. A sensor in one aisle does not represent the whole house; place dehumidifiers or air tubes so each span has its own dry-air supply.
Rooftop greenhouses: treat greenhouse air, the host building's air and roof waterproofing as separate questions. A wet ceiling below is an envelope or drainage problem first; lowering greenhouse RH alone will not fix it. Agree with the building team how condensate is drained and whether any air is shared.
Choosing and placing dehumidifiers
- Size for the night peak at the house condition, not the catalogue rating.
- Spread several units, or use ducts and air tubes, so every zone gets dry air.
- Link them to the climate computer so they run before the crop gets wet and stage with vents and heating.
- Route condensate to a drain or collection tank.
Our greenhouse dehumidifier range covers 168–960 L/day fixed units and 60–138 L/day portable units (rated at 30 °C/80 % RH). See how to choose a greenhouse dehumidifier and greenhouse humidity sensor placement.
FAQ
Frequently asked questions
How do I lower humidity in a greenhouse?
Water earlier in the day, run horizontal airflow fans, vent and heat when outside air is drier, and use a dehumidifier when vents are closed at night or outside air is humid.
What is the best humidity for a greenhouse?
It depends on the crop and the time of day. Day RH of about 60–80 % suits most crops; at night, keep a margin below saturation (for example about 89 % at 16 °C) so leaves do not get wet.
Does heating lower greenhouse humidity?
Heating lowers RH but does not remove water. Combine it with venting or dehumidification to take the water out.
When is a dehumidifier better than venting?
When outside air is humid, when venting would lose expensive heat or CO₂, and when screens must stay closed at night.
Why is one part of my greenhouse more humid?
Cold walls, poor air movement, dense canopy or a door. Map the zones and add air movement or dry air where the problem appears.
Sources
- Wageningen University & Research, Dehumidification of greenhouses; Using a wireless sensor network to determine climate heterogeneity in greenhouses.
- UMass Extension, Reducing humidity in the greenhouse.
- UF/IFAS Extension, Greenhouse ventilation (AE030).
- UC ANR, Condensation on leaf and flower surfaces.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
