Quick answer: Rainy weather raises greenhouse humidity in three ways: vents close (so moisture stays in), outside air is near saturation, and liquid water gets in through vents, gutters and leaks or drips off the roof. Venting still helps if the rainy outside air is cooler than the greenhouse, because cool saturated air holds less water than warm house air: heat it as it comes in. If outside air is as warm as inside, venting does little and a dehumidifier is the tool. First, stop liquid water entering; then keep air moving through the crop.
Quick answer
- Cool rain (outside colder than inside): crack vents and heat; it still dries the house.
- Warm rain (outside as warm as inside): venting won't dry; dehumidify.
- Fix gutters, vent seals and roof drips first.
Does rainy air still dry the greenhouse?
Compare water content, not RH:
| Inside | Outside (raining) | Water content in / out | Venting effect |
|---|---|---|---|
| 20 °C, 85 % RH | 12 °C, 100 % RH | 12.4 / 8.8 g/kg | Dries the house, if incoming air is heated |
| 20 °C, 85 % RH | 18 °C, 100 % RH | 12.4 / 12.9 g/kg | Adds moisture |
| 26 °C, 80 % RH | 24 °C, 100 % RH | 17.0 / 18.9 g/kg | Adds moisture |
UMass Extension notes that even rainy outside air can lower greenhouse humidity when it is cooler than inside air, because it holds less moisture once heated (UMass Extension).
Stop liquid water first
- Check gutters and downpipes for overflow back into the house.
- Inspect vent seals, roof joints and film for leaks.
- Use rain sensors to close windward vents, but keep a small leeward opening if venting is useful.
- Drain floors and paths; standing water evaporates into the house.
- Catch roof condensation and drips (anti-drip films, gutters under the glazing).
Control actions during rain
| Action | When |
|---|---|
| Crack leeward vents + heat | Outside cooler than inside |
| Keep HAF fans running | Always; keeps leaves dry and air even |
| Delay or reduce irrigation | Transpiration is low in dull, wet weather |
| Dehumidify | Outside air as warm and wet as inside, or vents must stay closed |
| Raise temperature slightly | Lowers RH and keeps leaves above dew point (combine with water removal) |
UF/IFAS, UGA and AHDB guidance covers ventilation and air movement in glasshouses (UGA Extension B792; AHDB).
After the rain
- Ventilate as soon as outside air dries or warms.
- Check for wet spots, drips and disease the following days, especially Botrytis on flowers; see greenhouse humidity and disease.
When to add dehumidifiers
- Warm, wet climates with frequent rainy periods.
- Crops sensitive to wet nights (cut flowers, tomatoes, strawberries).
- Houses where vents must stay closed in rain.
See greenhouse humidity by climate and our greenhouse dehumidifiers. Wageningen research covers dehumidification of greenhouses (WUR).
FAQ
Frequently asked questions
Should I open greenhouse vents when it rains?
If the rain is cooler than inside, crack leeward vents and heat; it still dries the house. If outside is as warm as inside, keep them closed and dehumidify.
Why is my greenhouse so humid when it rains?
Vents close, outside air is saturated, and water may be getting in through leaks, gutters or roof drips.
Does rain increase disease in greenhouses?
Long humid, dull periods with wet leaves raise the risk of Botrytis and mildews.
Sources
- UMass Extension, Reducing humidity in the greenhouse; Natural ventilation in greenhouses.
- University of Georgia Extension, Greenhouses: heating, ventilation and cooling (B792).
- AHDB, How to improve air movement in glasshouses.
- Wageningen University & Research, Dehumidification of greenhouses.
