Quick answer: Venting can only bring greenhouse air down to the water content of the outside air. In the tropics and in humid summer spells, outside air is almost as wet as inside air, so venting removes heat but little water, and dehumidification becomes the main tool at night. In arid climates the problem is reversed: fan-and-pad cooling adds water to cool the air, and humidity becomes a problem only when the pads stop in the evening. At high altitude, each cubic metre of air carries less mass, so ventilation removes less water than sea-level figures suggest.
Quick answer
- Tropical: outside dew point is often 22–25 °C; dehumidify at night, ventilate and shade by day.
- Arid with pads: a pad cools hot dry air by about 80 % of the wet-bulb depression, and adds water.
- High altitude: at 2,000 m, air density is about 20 % lower than at sea level.
The rule that decides it: compare water content, not RH
Venting removes water only when the outside air holds less water per kilogram than the inside air. RH alone cannot tell you this.
| Inside air | Outside air | Water content inside / outside | Can venting dry the house? |
|---|---|---|---|
| 18 °C, 85 % RH | 10 °C, 90 % RH | 11.0 / 6.9 g/kg | Yes, well |
| 18 °C, 85 % RH | 14 °C, 95 % RH | 11.0 / 9.5 g/kg | Weakly; large airflow and heat needed |
| 26 °C, 85 % RH | 25 °C, 90 % RH (tropical night) | 18.0 / 18.1 g/kg | No |
See greenhouse humidity control for the full venting vs dehumidifying calculation.
Tropical greenhouses
Tropical lowland climates have warm, humid air most of the year, with outside dew points often around 22–25 °C. Typical problems:
- Night: the house cools a few degrees, the crop keeps transpiring, and RH approaches saturation. Venting cannot help because outside air is just as wet.
- Rainy season: liquid water enters through vents and leaks, and wet floors add evaporation. Fix the water paths first: gutters, side curtains, drainage.
- Day: the main problem is heat; shading, natural ventilation and fans come first.
What works: refrigerant dehumidifiers at night, with vents closed or nearly closed and air moving through the crop. Tropical night conditions are close to the 30 °C / 80 % RH rating condition, so rated capacity is a fair guide there; in cooler houses it overstates removal. Allow for corrosion (coated coils) and continuous drainage of large condensate volumes.
Not every tropical crop needs drying. Some crops and propagation stages want high humidity. Define the crop's range first; then decide which hours need moisture removal.
Hot summers and humid spells
In summer, ventilation is mainly for heat. It removes water only when the incoming air is drier. Watch for:
- Humid spells: warm, wet weather when vents are wide open but the house stays humid. This is when dehumidifiers earn their keep in temperate greenhouses.
- The evening transition: after a hot day, vents close as the air cools, the crop is still transpiring and the canopy reaches the dew point. Start moisture removal before sunset.
- Fog and misting: they cool by adding water. Stop them early enough for the canopy to dry before night.
University of Florida IFAS guidance on greenhouse ventilation and cooling covers fan sizing and airflow (UF/IFAS AE030); UC Davis explains how crop evapotranspiration drives water loss in greenhouses (UC Davis).
Arid climates with fan-and-pad cooling
A wet pad cools air by evaporating water into it. A well-maintained pad cools the incoming air by roughly 80–85 % of the difference between the dry-bulb and wet-bulb temperatures (Alabama Extension, fan-and-pad systems).
Example: outside air at 35 °C and 20 % RH has a wet-bulb temperature of about 19 °C. After the pad: about 22 °C and about 70–75 % RH. The air gained about 5 g of water per kg, and it is wetter still at the fan end of the house after passing through the crop.
The same pad on a humid day (30 °C, 75 % RH, wet bulb about 26.5 °C) cools the air only to about 27 °C and pushes it above 90 % RH. Pads work in dry air and fail in humid air.
What to watch:
- Do not run a dehumidifier against an open fan-and-pad air stream; the two work against each other.
- Humidity problems usually come in the evening and at night, when pads and fans stop and the house closes up. That is when dehumidification fits.
- Keep pad water use separate from greenhouse humidity in your records; they answer different questions.
High-altitude greenhouses
Air pressure and density fall with altitude:
| Altitude | Air pressure (standard atmosphere) | Air density vs sea level |
|---|---|---|
| 0 m | 101.3 kPa | 100 % |
| 1,000 m | about 90 kPa | about 89 % |
| 2,000 m | about 79.5 kPa | about 79 % |
| 3,000 m | about 70 kPa | about 69 % |
Two effects follow:
- Ventilation removes less water per cubic metre. A fan rated in m³/h moves less mass of air, so the same airflow carries out less water. Correct sea-level calculations for site pressure.
- The same temperature and RH means more water per kg of air. The humidity ratio rises as pressure falls, so psychrometric charts and calculators must use the site pressure (ASHRAE Handbook—Fundamentals, Psychrometrics).
High-altitude sites also see strong night radiation to a clear sky, so leaves and glazing cool below air temperature quickly. Night condensation risk can be high even in a dry climate. Ask dehumidifier suppliers for capacity and airflow at your site altitude.
Choosing equipment by climate
| Climate | Main tool by day | Main tool at night | Equipment notes |
|---|---|---|---|
| Tropical lowland | Shade, ventilation, fans | Refrigerant dehumidifiers, air movement | Corrosion protection; large drains |
| Temperate with humid summers | Ventilation, shade | Vent + heat; dehumidifiers on humid nights | Stage with vents and heating |
| Arid with pads | Fan-and-pad cooling | Ventilation if outside air is dry; dehumidifiers when house is closed | Do not run against pad cooling |
| High altitude | Ventilation corrected for density | Vent + heat or dehumidify; watch radiative cooling | Ask for capacity at site altitude |
See our greenhouse dehumidifiers and how to choose a greenhouse dehumidifier.
FAQ
Frequently asked questions
How do you control humidity in a tropical greenhouse?
Fix rain entry and drainage, ventilate and shade during the day, and use dehumidifiers at night when outside air is too humid for venting to work.
Do fan-and-pad systems increase greenhouse humidity?
Yes. They cool by evaporating water, so the air leaves the pad more humid. They work well in dry air and poorly in humid air.
Why doesn't ventilation lower humidity in summer?
Because the outside air holds nearly as much water as the inside air. Venting then removes heat, not water.
Does altitude affect greenhouse humidity control?
Yes. Thinner air carries less water per cubic metre, so ventilation removes less, and psychrometric calculations must use site pressure.
Sources
- UF/IFAS Extension, Greenhouse ventilation (AE030).
- Alabama Cooperative Extension System, Greenhouse cooling: an overview of fan-and-pad systems.
- UC Davis, Greenhouse evapotranspiration.
- ASHRAE Handbook—Fundamentals, Psychrometrics.
- U.S. Standard Atmosphere, 1976.
