Quick answer: Good greenhouse ventilation needs enough opening or fan capacity, a clear air path from inlet to outlet across the crop, and controls that vent for the right reason at the right time. For natural ventilation, roof and side vents together are commonly sized at about 15–25 % of the floor area. For fan ventilation, summer cooling needs roughly one air change per minute (about 2.4–3 m³ per minute per m² of floor), with inlets large enough not to starve the fans. Vent for heat in the day; vent for humidity only when the outside air holds less water than the inside air, otherwise dehumidify.
Quick answer
- Natural vents: about 15–25 % of floor area.
- Fans: about 1 air change per minute in summer; much less in winter.
- Venting dries the house only if outside air has lower water content (g/kg).
1. Separate ventilation from circulation
Ventilation exchanges inside air with outside air. Circulation (horizontal airflow fans) mixes air inside. You need both: ventilation to remove heat and moisture, circulation to keep conditions even and leaves dry. UMass Extension explains how wind and buoyancy drive natural ventilation and how fans and inlets work (UMass Extension, Ventilation for greenhouses).
2. Check the vent area
| Ventilation type | Rule of thumb |
|---|---|
| Roof + side vents (natural) | Total vent area about 15–25 % of floor area; roof vents are more effective than side vents because warm air rises |
| Roll-up sides (high tunnels) | Full-length sides plus end doors; add roof vents in hot climates |
Too little vent area means high temperatures on calm, sunny days.
3. Size fans and inlets
| Season | Typical fan airflow |
|---|---|
| Summer cooling | About 1 air change per minute (roughly 2.4–3 m³/min per m² of floor) |
| Spring and autumn | About half |
| Winter (humidity and fresh air) | About a quarter, in short cycles |
Inlet louvres should be sized so air enters at low speed (fan maker's guidance), on the opposite side from the fans, and not blowing directly onto plants. Starved fans deliver much less than their rating.
4. Trace the air path
- Air should cross the crop, not short-circuit from an inlet straight to a fan.
- Benches, screens and dense crops block airflow; check with smoke or a simple anemometer.
- Keep fans and inlets clean; dusty shutters and guards cut airflow.
5. Vent for humidity correctly
Compare water content, not RH:
| Inside | Outside | Will venting dry the house? |
|---|---|---|
| 18 °C, 85 % RH (11.0 g/kg) | 10 °C, 90 % RH (6.9 g/kg) | Yes; heat the incoming air |
| 18 °C, 85 % RH (11.0 g/kg) | 16 °C, 95 % RH (10.8 g/kg) | Hardly at all |
| 26 °C, 85 % RH (18.0 g/kg) | 25 °C, 90 % RH (18.0 g/kg) | No |
When venting cannot dry the house, use dehumidification at night; see ventilation vs dehumidification and greenhouse humidity control methods.
6. Heat and vent at dusk and dawn
A short cycle of venting and heating at sunset and sunrise exchanges humid air for drier outside air before the crop cools to the dew point; UMass recommends two or three cycles an hour at those times (UMass Extension).
7. High tunnels
High tunnels are often closed at night and in cold weather, when humidity builds fastest. Open sides early in the morning as soon as temperatures allow, and close later in the afternoon so the crop dries before night. University of Nevada Extension covers high tunnel management (UNR Extension).
8. Control and measure
- Control vents and fans on temperature and humidity at canopy height, with a weather station for wind and rain.
- Log conditions before and after any change, under similar weather, to see whether it worked.
- Change one thing at a time.
For fan controllers, see temperature and humidity fan controllers; for solar options, solar greenhouse exhaust fans.
FAQ
Frequently asked questions
How much vent area does a greenhouse need?
About 15–25 % of the floor area in roof and side vents for natural ventilation.
How many air changes does a greenhouse need?
About one per minute for summer cooling; far fewer in winter.
Why is my greenhouse still humid with vents open?
Outside air may be just as humid, especially at night, in rain or in humid climates. Then ventilation cannot dry the house; use dehumidification.
Should I run circulation fans all the time?
Usually yes; they keep temperature and humidity even and leaves dry.
Sources
- UMass Extension, Ventilation for greenhouses; Reducing humidity in the greenhouse.
- University of Nevada, Reno Extension, publication 8280.
- FAO Irrigation and Drainage Paper 56 (vapour pressure).
