Quick answer: Ventilation removes moisture only when the outdoor air holds less water (lower humidity ratio, g/kg) than the greenhouse air, and only as fast as the airflow allows. In cold weather it usually can, but every cubic metre of outdoor air must be heated and any CO2 enrichment is lost. In warm, humid weather the difference shrinks or reverses and ventilation stops working. Mechanical dehumidification removes water from recirculated air, with the heat staying in the greenhouse, and it works whatever the outdoor humidity. Many greenhouses combine the two: vent when it's cheap and effective, dehumidify when venting costs too much heat or CO2, or can't reach the target.
Quick answer
- Test: outdoor g/kg < indoor g/kg? If not, ventilation cannot dry the greenhouse.
- Cost of venting: heating the incoming air plus CO2 loss.
- Dehumidifier: works in any outdoor condition; keeps heat and CO2 inside.
Step 1: the humidity-ratio test
Compare water content, not RH:
| Greenhouse | Outdoor | Indoor g/kg | Outdoor g/kg | Ventilation dries? |
|---|---|---|---|---|
| 18 °C, 85 % RH | 5 °C, 90 % RH | 11.0 | 4.9 | Yes, strongly |
| 18 °C, 85 % RH | 15 °C, 95 % RH | 11.0 | 10.2 | Barely |
| 18 °C, 85 % RH | 25 °C, 80 % RH | 11.0 | 16.0 | No, it adds moisture |
(Sea-level pressure; calculated from standard psychrometric relations.) Cold air at 90 % RH is much drier than warm greenhouse air at 85 %. Warm humid outdoor air is wetter.
Step 2: can the airflow keep up?
Worked example (illustrative): 10,000 m² greenhouse, night moisture load (transpiration plus evaporation) about 150 kg/h; inside 18 °C / 85 % RH (11.0 g/kg); outside 5 °C / 90 % RH (4.9 g/kg).
- Each kg of dry air removes 11.0 − 4.9 = 6.1 g of water.
- Dry air needed: 150 ÷ 0.0061 ≈ 24,600 kg/h ≈ 20,500 m³/h (about 2 m³/h per m² of floor).
- Heating that air from 5 to 18 °C: 24,600 × 1.005 × 13 ÷ 3600 ≈ 89 kW of extra heat, every hour this runs.
When it's milder outside, the g/kg difference shrinks, so the airflow needed goes up steeply. At 15 °C / 95 % RH the difference is about 0.9 g/kg, so the same load would need about 176,000 kg/h. In practice that much airflow isn't there, and leaves get wet.
Step 3: the costs of venting
| Cost | Why |
|---|---|
| Heat | Incoming air must be warmed to the setpoint (89 kW in the example) |
| CO2 | Enriched air is exhausted; enrichment must be increased or stopped |
| Temperature stability | Cold air dumping on the crop; screens opened |
| Limited at humid times | Little or no drying when outdoor g/kg is close to indoor |
Wageningen research on production greenhouses at northern latitudes treats moisture removal as a major part of greenhouse heating energy and compares ventilation with mechanical dehumidification options (WUR, Heating and dehumidification in production greenhouses at northern latitudes: energy use).
What mechanical dehumidification changes
- Removes water from recirculated greenhouse air. It works when outdoor air is humid, and at night with screens closed.
- In an indoor recirculating unit, the latent heat released by condensation (about 0.68 kWh per kg of water) and the electrical input end up as sensible heat in the greenhouse. For 150 kg/h, that is roughly 100 kW of latent heat returned, plus the unit's power.
- Keeps CO2 inside.
- Costs: capital, electricity, drainage, maintenance; and the dried air must reach the crop.
See heat recovery vs dehumidification and our greenhouse dehumidifiers.
Combinations that work
| Condition | Route |
|---|---|
| Cold, dry outdoor air, no CO2 enrichment | Vent (often with heating) |
| Cold, with CO2 enrichment or tight energy budget | Dehumidify, minimum venting |
| Mild, humid nights | Dehumidify |
| Warm days | Vent for cooling; dehumidify only if humidity is still high |
| Dusk/dawn humidity spike | Start dehumidification before sunset; heat-and-vent briefly if outdoor air is drier |
Compare routes over the risk period (the nights and seasons when condensation and disease happen), not one design point.
Choose mechanical dehumidification when outdoor air holds as much or more water than greenhouse air, or when venting loses too much heat or CO2.
Information needed for a route comparison
- Floor area, height, covering, screens
- Crop, stage and density (moisture load)
- Indoor setpoints by night and day
- Outdoor conditions for the risk season (hourly if possible)
- Heating system and energy price; electricity price
- CO2 enrichment
- Current vent capacity
FAQ
Frequently asked questions
Is ventilation or a dehumidifier better for a greenhouse?
Ventilation is cheaper when outdoor air is much drier and heating or CO2 losses are acceptable; a dehumidifier works in any outdoor condition and keeps heat and CO2 inside. Many greenhouses combine both.
Why doesn't venting lower humidity on humid nights?
Because outdoor air holds almost as much (or more) water per kg as the greenhouse air.
How much heat does venting for humidity cost?
It depends on airflow and the temperature difference; in the example above, about 89 kW to remove 150 kg/h at 5 °C outside.
Sources
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- Wageningen University & Research, Heating and dehumidification in production greenhouses at northern latitudes: energy use.
