Greenhouses naturally produce a lot of moisture. Plants release water vapor through transpiration, irrigation adds more moisture, and cooler nighttime temperatures can quickly raise relative humidity and cause condensation.
Greenhouse dehumidification can help when ventilation, heating, air circulation, and irrigation management are not enough to keep moisture under control. The goal is not to make the greenhouse as dry as possible. It is to maintain conditions that suit the crop while reducing prolonged leaf wetness, condensation, and large humidity swings.
This guide explains where greenhouse humidity comes from, when a dehumidifier is useful, and how to choose and operate one without disrupting temperature, airflow, or plant transpiration.
Where Does Greenhouse Humidity Come From?
Most greenhouse moisture comes from plant transpiration and evaporation from growing media, floors, irrigation systems, and standing water. Outdoor air entering through vents, doors, and gaps can either lower or raise indoor moisture, depending on its dew point.
Humidity often rises after irrigation and again at night. As the greenhouse cools, the air can hold less water vapor, so relative humidity increases even if no additional moisture enters the structure. When plant leaves, glazing, pipes, or framing fall below the air’s dew point, condensation can form.
This is why one daytime RH reading does not tell the whole story. Measure temperature and relative humidity near the plant canopy throughout the day and night. In larger greenhouses, use several sensors because dense plants, corners, exterior walls, and areas with weak airflow can develop different conditions.
Why Does Excess Greenhouse Humidity Matter?
Condensation and Leaf Wetness
Condensation forms when a surface falls below the air’s dew point. In a greenhouse, water may collect on glazing, framing, equipment, or plant surfaces as temperatures drop overnight. Wet leaves and stagnant air can create conditions that favor several fungal and bacterial diseases.
A dehumidifier can lower the amount of water vapor in the air, but it is only one part of the solution. Air circulation, watering time, plant spacing, ventilation, heating, and drainage also affect how long leaves remain wet.
Plant Transpiration
Plants rely on transpiration to move water and dissolved nutrients. When the air around the leaves remains very humid, transpiration may slow. If the air becomes too dry, plants may lose water faster than their roots can replace it.
For this reason, growers should not use one universal RH setting for every greenhouse. Crop type, growth stage, leaf temperature, air temperature, and vapor pressure deficit all influence the appropriate target.
Disease Pressure
High humidity alone does not cause every plant disease, but prolonged moisture and limited airflow can create favorable conditions for pathogens such as Botrytis. Removing infected material, improving sanitation, spacing plants properly, and keeping air moving remain important even when mechanical dehumidification is used.
UMass Extension recommends combining ventilation, heating, air circulation, appropriate watering times, and moisture management when reducing humidity in a greenhouse.
Greenhouse Materials and Equipment
Persistent condensation can also affect framing, fasteners, electrical components, benches, insulation, and other greenhouse materials. Before installing a dehumidifier, inspect the structure for leaks, standing water, blocked drains, damaged glazing, and areas where humid air becomes trapped.
When Does a Greenhouse Need a Dehumidifier?
Not every greenhouse needs a dehumidifier. Ventilation may be enough when outdoor air is drier and temperatures allow the greenhouse to exchange air without losing too much heat. Morning irrigation, horizontal airflow fans, proper plant spacing, and good drainage may also reduce local moisture problems.
A dehumidifier becomes more useful when the greenhouse is enclosed or heated, outdoor air is too humid for effective ventilation, valuable heat or supplemental CO₂ must be retained, or RH repeatedly rises above the crop’s target range at night.
Look at the pattern rather than one high reading. If RH remains elevated for hours, condensation repeatedly forms on leaves, or some parts of the greenhouse stay damp despite adequate airflow and ventilation, supplemental dehumidification may be appropriate.
Mechanical dehumidification should not replace repairs or basic moisture management. Fix leaks, standing water, overwatering, clogged drains, and poor airflow before adding more equipment.
Greenhouse Dehumidification Methods
Ventilation and Heating
Ventilation can remove moisture when the outdoor air contains less water vapor than the greenhouse air. During cool weather, growers may briefly exchange humid indoor air for cooler outdoor air and then heat the incoming air. This lowers its relative humidity and gives it more capacity to absorb moisture.
However, ventilation may waste heat or supplemental CO₂, and it becomes less effective when outdoor conditions are already humid. Compare indoor and outdoor dew point rather than relying only on outdoor RH.
Air Circulation
Horizontal airflow fans help reduce humid pockets around leaves and in dense plant canopies. They do not remove water from the greenhouse, but they distribute moisture and temperature more evenly and may reduce condensation on plant surfaces.
Fans should support dehumidification rather than replace it. A greenhouse can have strong air movement and still contain too much total moisture.
Refrigerant Dehumidification
A refrigerant dehumidifier draws humid air over a cold coil, condenses the water vapor, and returns drier, slightly warmer air to the greenhouse. It can work well in enclosed, heated growing spaces when the temperature remains within the unit’s rated range. If nighttime temperatures drop sharply, check the manufacturer’s specifications and understand how low temperatures affect dehumidifier operation before relying on the equipment for overnight moisture control.
Desiccant Dehumidification
A desiccant system uses a moisture-adsorbing material to remove water vapor. It may be useful at lower temperatures or when the grower needs a lower dew point than a refrigerant system can efficiently maintain.
Desiccant equipment requires regeneration energy and careful integration with greenhouse airflow and temperature controls. It is more common in specialized or commercial applications than in small hobby greenhouses.
How to Choose a Greenhouse Dehumidifier
Do not size a greenhouse dehumidifier by floor area alone. Two greenhouses with the same dimensions can have very different moisture loads depending on plant density, crop stage, irrigation volume, outdoor-air infiltration, temperature, and ventilation schedule.
Start by estimating how much water enters the greenhouse each day through irrigation and how much returns to the air through plant transpiration and evaporation. Then consider nighttime peaks, outdoor weather, air volume, desired RH or VPD range, and whether the greenhouse will continue ventilating while the dehumidifier operates.
Important equipment factors include:
- Moisture-removal capacity at the greenhouse’s actual temperature and RH
- Minimum operating temperature and defrost performance
- Airflow and discharge direction
- Continuous drainage or a condensate pump
- Adjustable humidistat and external control compatibility
- Automatic restart after a power interruption
- Remote status and control options
- Filter, coil, and drain access
- Heat added to the growing space
- Corrosion resistance in a humid environment
A small greenhouse may be able to use one standalone unit. Larger or densely planted greenhouses may need several units or a ducted system to prevent wet zones and provide redundancy. For commercial installations, have the moisture load and equipment layout reviewed by a qualified greenhouse or HVAC professional.
Where Should a Greenhouse Dehumidifier Be Placed?
Place the dehumidifier where it can draw humid air without its dry-air discharge returning directly to the intake. Keep the intake and outlet clear of leaves, plastic sheeting, storage materials, and walls.
The driest air should mix with the greenhouse air rather than blow continuously onto one group of plants. Use circulation fans to move conditioned air through and above the plant canopy, especially in corners and densely planted areas.
Do not rely only on the dehumidifier’s internal sensor. Place independent temperature and RH sensors near the crop canopy and in areas that have previously shown condensation. Large greenhouses may need several sensors and more than one dehumidifier.
Use continuous drainage where possible and inspect the hose, floor drain, or condensate pump regularly. Standing water from an unreliable drain would add moisture back into the same environment the equipment is trying to dry.
The Bottom Line
Greenhouse dehumidification works best as part of a coordinated moisture-control plan. Start with irrigation timing, drainage, plant spacing, air circulation, ventilation, and nighttime temperature management. Then use environmental data to determine whether mechanical dehumidification is still needed.
If a greenhouse dehumidifier is required, size it according to the actual moisture load rather than floor area alone. Check its performance at the greenhouse’s normal temperature and RH, account for the heat it adds, provide reliable drainage, and verify conditions with sensors near the plant canopy. Once these requirements are clear, use this greenhouse dehumidifier buying guide to compare capacity, low-temperature performance, drainage, controls, and installation options.
For smaller enclosed greenhouses that need remote equipment control, compare Wi-Fi-enabled dehumidifiers by capacity, operating temperature, drainage method, airflow, and control features. Confirm that the selected model is appropriate for the greenhouse environment before installation.
Frequently Asked Questions
Do I need a dehumidifier in my greenhouse?
Not always. Ventilation, morning irrigation, proper drainage, plant spacing, heating, and air circulation may be enough. A dehumidifier becomes more useful when the greenhouse is enclosed, outdoor air is humid, heat or CO₂ must be retained, or nighttime RH remains above the crop’s target range.
What humidity level should a greenhouse maintain?
There is no single level that is right for every greenhouse. The appropriate range depends on crop type, growth stage, temperature, leaf temperature, and vapor pressure deficit. Follow crop-specific recommendations and monitor conditions near the canopy.
Why does greenhouse humidity rise at night?
As the greenhouse cools, relative humidity rises because cooler air holds less water vapor. Plant transpiration, wet growing media, and water on floors can add more moisture, increasing the likelihood of condensation.
Can fans reduce greenhouse humidity?
Fans improve air circulation and reduce humid pockets, but they do not remove water from the greenhouse. They should be combined with ventilation, heating, drainage, or dehumidification when total moisture remains too high.
How do I size a greenhouse dehumidifier?
Estimate the daily moisture added by irrigation, plant transpiration, evaporation, infiltration, and ventilation. Then choose equipment based on its rated performance at the greenhouse’s actual temperature and RH, not just its advertised coverage area.
Does a greenhouse dehumidifier add heat?
A refrigerant dehumidifier normally returns both the heat removed during condensation and the heat produced by its components to the space. This may help during cold weather but can add to the cooling load during warm conditions.
