Quick answer: Greenhouse humidity rises at night for three reasons acting together: the air cools (so RH climbs even with the same water in it), the crop keeps transpiring while vents close, and leaves, fruit and glass cool below air temperature by radiating heat to the night sky. Water then condenses on the first surface that drops below the dew point, usually the roof first and then the crop. Act before sunset: start moisture removal while the air is still warm, keep air moving, hold a margin between the dew point and leaf temperature, and use heat-and-vent or dehumidifiers through the night.
Quick answer
- Cooling alone: 22 °C / 65 % RH air reaches saturation at about 15 °C.
- Clear nights: leaves and glass can be a few degrees colder than the air.
- Start control before sunset, not after the alarm.
Why humidity spikes after sunset
Example: at 18:00 the house is at 22 °C and 65 % RH (dew point about 15 °C). Overnight it cools to 16 °C:
- with no water added or removed, RH rises to about 95 %;
- the crop keeps transpiring, adding more water;
- leaves under a clear sky are often 1–3 °C colder than the air, so they reach the 15 °C dew point while the air sensor still reads 16 °C.
Result: wet leaves before midnight, even though the climate computer showed "only" about 95 %.
Find the first cold surface
| Surface | Why it is cold | Check |
|---|---|---|
| Roof glass or film | Radiates to the night sky | Drips from the roof; condensation lines |
| Gables and side walls | Wind and outside air | Wet walls, puddles at the edges |
| Top leaves and flowers | Radiate to the sky through the roof | Leaf temperature with an IR thermometer |
| Fruit at sunrise | Cold mass warms slowly | Wet fruit in the morning |
| Under screens | Trapped, cooling air | Humidity under the screen |
Compare each with the dew point of the air next to it. AHDB's guidance on avoiding condensation in glasshouses explains these mechanisms for growers (AHDB); UC ANR describes condensation on leaf and flower surfaces (UC ANR).
Night control sequence
| Time | Action |
|---|---|
| 1–2 h before sunset | Stop irrigation; start drying while air is still warm (vent if outside air is drier; start dehumidifiers if not) |
| Sunset | Close screens gradually, leaving a gap if humidity is high; keep HAF fans running |
| Evening | Heat-and-vent cycles if outside air is drier (UMass recommends two to three per hour at dusk and dawn); otherwise dehumidify |
| Night | Hold a humidity deficit or VPD margin (for example above about 0.2 kPa); keep leaves at air temperature with air movement and gentle heat |
| 1 h before sunrise | Start raising temperature slowly; keep moisture removal running |
| After sunrise | Ventilate as the house warms; avoid a sudden blast of cold air on wet fruit |
UMass Extension's guidance covers heat-and-vent timing (UMass); UF/IFAS covers greenhouse ventilation (UF/IFAS AE030).
When to add dehumidifiers
- Outside air at night is nearly as humid as inside air (humid climates, rain, fog).
- Venting at night costs too much heat or loses CO₂.
- Screens must stay closed.
- The night target is strict (cut flowers, tomatoes, strawberries, propagation).
Size for the night load at the night condition; see how to choose a greenhouse dehumidifier and our greenhouse dehumidifiers. For the disease side, see greenhouse humidity and disease.
Data for an equipment review
- Night air temperature and RH at canopy level, every 5–15 minutes
- Leaf and glass temperatures on clear and cloudy nights
- Outdoor temperature and RH
- Vent, screen, heating and irrigation states on the same timeline
- When and where condensation first appears
FAQ
Frequently asked questions
Why is my greenhouse humidity so high at night?
The air cools, the crop keeps transpiring and vents are closed. RH rises even with no extra water.
What humidity is too high in a greenhouse at night?
Any level that brings leaves to their dew point. As a guide, about 89 % RH at 16 °C or 83 % at 10 °C leaves a margin (UMass).
How do I reduce greenhouse humidity overnight?
Start before sunset, keep air moving, heat-and-vent when outside air is drier, and dehumidify when it is not.
Why do I get condensation even when RH is below 100 %?
Leaves and glass are colder than the air, so they reach the dew point first.
Sources
- AHDB, Avoiding condensation in glasshouses.
- UC ANR, Condensation on leaf and flower surfaces.
- UMass Extension, Reducing humidity in the greenhouse.
- UF/IFAS Extension, Greenhouse ventilation (AE030).
