Greenhouse night humidity control is mainly a dew point and leaf-wetness problem. When the greenhouse cools after sunset, the same amount of water vapor can push RH sharply upward and bring leaves, fruit, glazing or curtains close to condensation.
Why Night Humidity Rises So Fast
Relative humidity is temperature dependent. As greenhouse air cools, its moisture-holding capacity falls. If water vapor is not removed, RH rises even when the absolute moisture content stays nearly the same.
Michigan State University Extension explains that vapor pressure deficit connects humidity, temperature and plant water loss more directly than RH alone (MSU Extension). For night control, this matters because a room can move from acceptable daytime RH to condensation risk during the cooling transition.
The practical target is not only "hold RH below a number." It is to keep plant and surface temperatures safely above dew point while maintaining enough air movement through the canopy.
Disease Risk Depends on Wet Surfaces
Penn State Extension notes that good air circulation and lower relative humidity inhibit gray mold activity in greenhouses (Penn State Extension). UC IPM's disease-model guidance also emphasizes that leaf wetness is site-specific and sensor placement must represent the crop canopy (UC IPM).
For greenhouse operators, the night check should include:
- Are leaves, flowers, fruit or pruning wounds still wet after dusk?
- Does condensation form on glazing, screens, pipes or structural metal?
- Does the crop-zone sensor show a different condition from the room sensor?
- Does RH rise after curtains close or heating pipes reduce output?
- Does air movement reach row centers, gable ends and cold corners?
If the answer is yes, the issue is not only capacity. The control sequence, airflow path and sensor location may need to change.
A Better Night Control Sequence
A stable night strategy usually starts before the room has already reached saturation.
| Control step | Purpose | What to verify |
| Pre-dusk moisture removal | Lower absolute humidity before temperature drops. | Dehumidifier starts early enough and does not overcool the crop. |
| Curtain coordination | Avoid trapping moisture without a removal path. | Operation around thermal screens or blackout curtains. |
| Air circulation | Break humid boundary layers around leaves. | Air reaches canopy pockets and row ends. |
| Dew point or VPD monitoring | Track condensation risk more clearly than RH alone. | Sensor location and calibration. |
| Heating coordination | Keep critical surfaces above dew point when needed. | Heating, dehumidification and ventilation do not fight each other. |
The sequence should be tested with trend data. If RH spikes after curtain closure, the moisture removal or air mixing may need to start earlier. If leaves remain wet while the room RH looks acceptable, the sensor probably does not represent the crop zone.
What Data to Send Before Equipment Selection
Send these inputs before asking how many units are required:
| Input | Why it matters |
| Day and night temperature targets | Determines dew point margin during cooling. |
| Crop type and canopy density | Changes transpiration and airflow resistance. |
| Water use or irrigation schedule | Helps estimate moisture released into the room. |
| Curtain or screen schedule | Explains when air volume and heat transfer change. |
| Ventilation and CO2 limits | Defines when venting is allowed or undesirable. |
| Existing fan layout | Shows whether dry air reaches the crop zone. |
| Sensor locations | Determines whether control data reflects the canopy. |
For a worked greenhouse load example, use the 5-hectare greenhouse sizing walk-through. For crop-specific application notes, read tomato greenhouse dehumidification.
Where Mechanical Dehumidification Fits
Ventilation is still useful when outdoor air is drier in absolute terms and heat or CO2 loss is acceptable. Mechanical dehumidification becomes important when night venting would waste heat, break CO2 strategy, introduce cold drafts, or fail during humid outdoor conditions.
The equipment decision should compare:
| Option | Best fit | Limitation |
| Ventilation | Drier outdoor air and acceptable energy loss. | Weather dependent. |
| Refrigerant dehumidifier | Warm greenhouse with moderate dew point target. | Performance changes with inlet condition. |
| Desiccant dehumidifier | Low-temperature or low-dew-point operation. | Needs reactivation heat and exhaust planning. |
| Combined strategy | Seasonal greenhouse with changing conditions. | Requires clear control logic. |
Use the commercial dehumidifier sizing calculator inputs guide to structure the RFQ before selecting industrial dehumidifiers.
FAQ
Frequently Asked Questions
Why does RH rise at night even after irrigation stops?
Because the air cools. Cooler air reaches saturation with less water vapor, so RH can rise quickly even when the absolute moisture content has not increased much.
Is dew point better than RH for night greenhouse control?
Dew point is clearer for condensation risk because it can be compared with leaf and surface temperature. RH is still useful, but it should not be the only control signal.
Should the dehumidifier run before sunset?
Often yes, when trend data shows a predictable dusk humidity spike. The goal is to reduce absolute humidity before temperature falls, not to wait until leaves are already wet.
