Quick answer: Choose a greenhouse dehumidifier by the moisture it must remove during the most humid hours, usually at night with vents closed and screens drawn, at the temperature it will actually run at. For warm houses above about 15 °C, a refrigerant dehumidifier is the efficient choice; for cold houses or strict dew-point targets, consider desiccant. Then make sure the dry air reaches the canopy, the condensate can drain, and the unit can talk to your climate computer.
Quick answer
- Size for the night peak, not the floor area.
- Refrigerant above about 15 °C; desiccant for colder or drier targets.
- Several smaller units spread through the house usually beat one big unit in a corner.
Compare models on our greenhouse dehumidifier page: fixed-installation units from 168 to 960 L/day and portable units from 60 to 138 L/day, rated at 30 °C/80 % RH.
When does a greenhouse need a dehumidifier?
When outdoor air cannot carry the moisture away. That happens when:
- vents are closed at night or in cold weather to save heat;
- thermal or blackout screens are drawn and trap humid air under them;
- the outdoor air is already as humid as the target (warm, humid climates and rainy periods);
- CO₂ enrichment is running and venting would waste it.
If the outdoor dew point is well below your target indoor dew point, venting with heating is often cheaper. Our guide to ventilation vs dehumidification shows how to compare the two.
The 7 selection criteria
| # | Criterion | What to check | Why it matters |
|---|---|---|---|
| 1 | Moisture load | Water released by the crop, wet floors and irrigation during the most humid hours | Sets the required L/day; floor-area rules undersize dense crops |
| 2 | Temperature range | Day, night and winter temperatures at the unit | Refrigerant coils lose capacity and may ice in cold air |
| 3 | Target | RH, VPD or dew point the crop needs, especially at night | Sets how dry the air must be and which technology fits |
| 4 | Air distribution | How dry air reaches the canopy across the whole house | Dry air near the unit and wet leaves at the far end cause disease |
| 5 | Drainage | Where the condensate goes from each unit | Hundreds of litres a day need a planned drain or reuse route |
| 6 | Corrosion | Fertilizer, sprays, chlorine or salt air | Coil coating, casing and fixings must survive the environment |
| 7 | Controls | Link to the climate computer, alarms, staging with vents and heating | Stops the dehumidifier fighting the vents and heating |
1. Size for the night peak
The hardest hours are usually between sunset and early morning. Plants keep transpiring, irrigation water is still evaporating, vents and screens are closed, and the air cools toward its dew point. Size the dehumidifier for those hours. Our 5-hectare tomato greenhouse walk-through shows the inputs needed for a large project, and our greenhouse moisture load calculation explains the method.
2–3. Match the technology to temperature and target
| Greenhouse condition | Start with |
|---|---|
| Night temperature above about 15 °C, RH target around 70–85 % | Refrigerant dehumidifier |
| Night temperature below about 15 °C, or frost risk | Low-temperature refrigerant model or desiccant |
| Strict dew-point target, or very dry air for propagation and storage | Desiccant or hybrid |
| Outdoor air often drier than indoor air | Ventilation with heating first; dehumidifier for humid periods |
Remember the rating condition: our refrigerant models are rated at 30 °C/80 % RH. At 16 °C and 75 % RH, a typical night setpoint, the same unit removes much less water. Ask for the capacity at your night condition. See how much water a dehumidifier collects in a day.
4. Get the dry air to the canopy
| Layout | Best for |
|---|---|
| Several units spread through the house | Most commercial greenhouses; each unit dries its own zone |
| Units with air tubes or ducts | Long bays; even delivery along rows |
| Ceiling-mounted units | Houses where floor space is used for crops or benches |
| Units plus horizontal airflow (HAF) fans | Dense canopies where air must move through the leaves |
Wet leaves for long periods raise the risk of Botrytis and other diseases; keeping leaves dry and air moving is a standard part of prevention (NC State Extension). Place sensors in the canopy, not only on a post by the door.
5–7. Drainage, corrosion and controls
- Drainage: route condensate to a drain or a collection tank; some growers collect it and reuse it for irrigation.
- Corrosion: ask for coated coils and corrosion-resistant casings where fertilizers, sulfur burners or coastal air are present.
- Controls: the climate computer should stage the dehumidifier with vents, screens and heating, so the unit runs when it is the cheapest way to dry the air.
Field example: calla lily greenhouse, Yunnan
In a calla lily greenhouse in Yunnan, China, night-time humidity was the main problem in July. The grower started with one RYCF-22S floor-standing dehumidifier and expanded to two. Average night-time relative humidity fell from 87.4 % to 77.1 % in July 2026. Read the full calla lily case study for the measurement points and lessons.
What to send for a greenhouse quote
- Crop, growth stage and planting density
- House size, height and number of bays
- Night temperature and humidity you see now, and the target
- Screen, vent and heating set-up
- Irrigation method and schedule
- Power supply and where condensate can drain
FAQ
Frequently asked questions
What size dehumidifier do I need for a greenhouse?
It depends on the night moisture load, not just the area. Dense crops such as tomatoes and cut flowers release far more water than seedlings. Send us the crop, area and night conditions and we will calculate it.
Is a refrigerant or desiccant dehumidifier better for a greenhouse?
Refrigerant for most heated greenhouses above about 15 °C. Desiccant for cold houses, frost-risk periods or very dry targets.
Can a greenhouse dehumidifier replace ventilation?
Not completely. Ventilation still controls heat and CO₂. The dehumidifier removes moisture when venting would cost too much heat or when outdoor air is too humid.
Where should dehumidifiers be placed in a greenhouse?
Spread through the house so each unit serves its own zone, with discharge directed along the rows or into air tubes, away from direct blast on plants.
Sources
- NC State Extension, Botrytis gray mold of tomato.
- Michigan State University Extension, Vapor pressure deficit and plant water loss.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
