Quick answer: Greenhouse leaves can get wet below 90 % air RH when their surface temperature reaches the dew point, including through radiative cooling to a cold sky. To prevent condensation, keep surfaces above the dew point (heating, screens, air movement) or lower the dew point (venting when outdoor air is drier, or dehumidifying).
Condensation forms on any greenhouse surface (roof film, glass, leaf, fruit, screen, pipe) at or below the dew point of the air touching it. At night the roof cools fastest, so water usually forms there first and drips onto the crop. Use anti-drip film so roof water runs off instead of dripping. First find which surface got wet first, because drips, leaks and irrigation look the same on a leaf.
Quick answer
- Condensation = surface temperature ≤ air dew point.
- Roof first at night; leaves next as they radiate heat away.
- Raise surface temperatures, lower the dew point, and keep drips off the crop.
Dew point by air condition
| Air temperature | 70 % RH | 80 % RH | 85 % RH | 90 % RH | 95 % RH |
|---|---|---|---|---|---|
| 16 °C | 10.5 °C | 12.6 °C | 13.5 °C | 14.4 °C | 15.2 °C |
| 18 °C | 12.5 °C | 14.5 °C | 15.4 °C | 16.3 °C | 17.2 °C |
| 20 °C | 14.4 °C | 16.4 °C | 17.4 °C | 18.3 °C | 19.2 °C |
| 24 °C | 18.2 °C | 20.3 °C | 21.3 °C | 22.3 °C | 23.1 °C |
(Calculated from standard psychrometric relations.) At 18 °C / 90 % RH, any surface below about 16.3 °C gets wet. A roof at 8 °C on a cold night is far below that. A leaf 2 K below air temperature is just above it.
Why air RH can look fine while surfaces are wet
- The sensor measures air; the roof and leaves are colder than the air.
- Leaves radiate heat to a clear night sky, often 1–3 K below air temperature.
- Stagnant zones (low in the canopy, near walls) are cooler and more humid than the sensor position.
Find where the water appeared first
| First wet location | Possible paths | What to check |
|---|---|---|
| Underside of roof film or glass | Condensation; rain through a seam | Surface temp vs dew point; seams after rain |
| Leaves under the roof | Roof drip, leaf condensation, irrigation | Which side is wet; match to roof position; irrigation time |
| Leaves in one still zone | Leaf condensation from poor mixing | Leaf temperature, local RH, fan pattern |
| Thermal screen | Condensation below; water lying on top | Both sides of the screen before it moves |
| Pipes, frames, ducts | Cold surface or a leak | Surface temperature; pressurise and check |
| Gutters | Underside condensation or overflow | Rain load; outlet blockages |
| Floor near walls or vents | Rain entry, runoff, drip | Weather and vent state |
UMass Extension describes how condensation on the cover drips onto plants and how to reduce humidity (UMass Extension). Virginia Tech explains why water beads on polyethylene film (Virginia Tech, CEA coverings). UC ANR covers condensation on leaf and flower surfaces (UC ANR).
Match the fix to the path
| Path | Fix |
|---|---|
| Roof condensation dripping | Anti-drip (anti-condensate) film; enough roof slope; gutters to collect runoff; thermal screen |
| Leaf condensation | Heat to keep leaves above dew point (pipe heating near the crop); HAF fans; lower the dew point |
| Humid air overall | Vent and heat when outdoor air is drier; dehumidify when not |
| Under a screen | Small gap, or dehumidify below the screen; see humidity under thermal screens |
| Leaks, overflows | Repair; air treatment can't fix liquid water entry |
| Cold pipes, frames | Insulate cold-water lines; reduce thermal bridges |
Related: humidity under thermal screens.
AHDB guidance shows how growers use plant temperature to avoid condensation in glasshouses (AHDB).
Dawn: the second risk
When the sun comes up, the air warms faster than the fruit and leaves. Warm, humid air can then condense on cold fruit. Start heating slightly before sunrise and keep the dew point low through the dawn transition; see night-time dew-point control.
Verify the fix
- Log air dew point and leaf or roof surface temperatures for several nights.
- Check for wet surfaces at the same time each dawn.
- Count wet hours (leaf wetness duration) rather than alarm counts; see greenhouse humidity and disease.
Our greenhouse dehumidifiers lower the dew point when venting would waste heat or can't dry the air.
FAQ
Frequently asked questions
Why does my greenhouse have condensation every morning?
Overnight the roof and plants cool below the dew point; at dawn, warming air condenses on cold fruit and leaves.
Is condensation in a greenhouse bad?
Drips and wet leaves drive Botrytis, downy mildew and other diseases; condensation on the cover also reduces light.
How do I stop condensation in a greenhouse at night?
Keep surfaces above the dew point (heat, screens, air movement), lower the dew point (vent when drier outside, or dehumidify), and use anti-drip film.
Sources
- UMass Extension, Reducing humidity in the greenhouse.
- Virginia Tech, Controlled environment agriculture coverings and glazing materials (SPES-821).
- UC ANR, Condensation on leaf and flower surfaces.
- AHDB, Avoiding condensation in the glasshouse using plant temperature.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
