# Humidity Control

> Part of: [Climate](/climate.md) · Provider: [DutchGreenhouses](/)

Humidity control keeps the air dry enough for the crop to transpire steadily and wet enough that it never stops. Too damp and transpiration stalls, calcium stops moving and fungal disease follows; too dry and the plant shuts its stomata to protect itself, halting photosynthesis. Ventilation, screens and dehumidification hold the middle.

## Facts
| Property | Value |
|---|---|
| What decides whether venting dries the house | Outside absolute humidity, not outside relative humidity categorical |
| What the ventilation rate should be set from | The crop's required evaporation rate categorical |
| Relative humidity at which Dutch practice starts venting | 82–85 % RH |
| Humidity deficit below which Botrytis spores can germinate | 2.5 g/m³ |
| Night moisture removal required, unlit crop | 10–30 g water/m² per hour |
| Night moisture removal required, lit crop | 70–100 g water/m² per hour |
| Dehumidification efficiency, air handler with passive pre-cooler | 3 litres of water per kWh |
| Energy cost of removing moisture by ventilation | 8–10 m³ gas/m² per year |
| Energy saved by accepting higher humidity | 2.5 m³ gas-equivalent/m² per year |

Climate

# Humidity control.

    The invisible variable that decides crop health.

## The invisible variable that decides crop health

    Temperature and light get the attention, but humidity is often the variable that quietly decides whether a crop thrives or falls sick. What actually matters to the plant is not relative humidity on its own but the vapour pressure deficit (VPD) — the difference between how much moisture the air holds and how much it could hold at that temperature. VPD is the force that drives transpiration, the plant's process of pulling water and nutrients up from the roots and releasing it through the leaves. Get it right and the crop drinks steadily and grows evenly; get it wrong and everything downstream suffers.

The danger lies at both extremes. When the air is too humid, VPD collapses, transpiration slows or stops, and the plant can no longer move calcium and other nutrients to where they are needed — causing disorders even when the nutrient solution is perfect. Worse, high humidity and the condensation that follows create ideal conditions for fungal diseases like Botrytis and mildew to take hold. When the air is too dry, VPD spikes, the plant transpires faster than its roots can supply and closes its stomata to protect itself, shutting down photosynthesis and stalling growth. The healthy band sits between these two, and holding the crop inside it is the whole job.

Achieving that takes several tools working together rather than any single device. Ventilation exchanges humid inside air for drier outside air when the outdoor climate allows; screen strategy influences how heat and moisture are retained or released, particularly at night; and active dehumidification removes moisture directly when ventilation alone cannot cope — essential in sealed, high-humidity or hot-and-humid situations. The control system blends these continuously against the target VPD rather than reacting to a single reading.

Condensation management runs alongside, because the most vulnerable moment is when a cooling surface or a chilled crop drops below dew point and free water forms on the leaves — an open invitation to disease. Dew point is therefore monitored continuously and the climate steered to keep the crop just above it. We design the combined ventilation, screening and dehumidification strategy around your crop and climate so humidity becomes a controlled asset rather than a hidden risk. Get in touch to discuss humidity control for your greenhouse.

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## FAQ
**How do you control humidity in a greenhouse?**
By combining three tools against one target. Ventilation exchanges humid inside air for drier outside air, screen position governs how much moisture is held or released, and active dehumidification removes water directly when ventilation cannot cope.

**Can you vent to dry the greenhouse when it is foggy or raining outside?**
Usually yes, which surprises people. What matters is not the outside relative humidity but its absolute humidity — how many grams of water the air actually holds. Cold saturated air holds little, so while it is colder outside than in, exchanging air still removes moisture. Reading it off relative humidity leads growers to keep the vents shut exactly when opening them would help.

**How do you reduce humidity in a greenhouse?**
Export the moisture rather than heat the air. Raising pipe temperature is the traditional reflex and it works by forcing the vents open, which wastes heat. Venting above a closed screen, or dehumidifying mechanically, removes the same water at far lower cost.

**What humidity level should a greenhouse run?**
Dutch practice starts venting around eighty-two percent by day and eighty-five at night, and an energy-saving strategy deliberately runs wetter still. Growers steer on humidity deficit in grams per cubic metre rather than on relative humidity, because relative humidity confounds moisture content with temperature.

**Should ventilation be set from a humidity target at all?**
There is a strong argument that it should be set from the evaporation the crop needs instead. Humidity is a result of the plant's energy and water balances rather than an independent quantity, and at night the transpiration rate drives nutrient uptake — calcium above all. Holding a number can hold the crop below the evaporation it requires.

**Does a dehumidifier save energy in a greenhouse?**
Only if the control it provides is used to run the greenhouse wetter. Research is blunt on this: replacing screen gaps with blown-in outside air barely improves efficiency by itself, because the fans add electricity. The saving comes from accepting higher humidity, not from the machine.

**Why does high humidity cause calcium problems?**
Calcium travels with the transpiration stream and cannot be moved around the plant afterwards. When the air is too damp, transpiration slows, calcium stops arriving where it is needed, and disorders appear even where the nutrient solution is correct.

## Related
[Climate](/climate.md) · [Heating](/climate/heating.md) · [Cooling](/climate/cooling.md) · [CO2 Enrichment](/climate/co2-enrichment.md) · [Energy Screens](/screening/energy-screens.md) · [Tropical & Humid](/climate-zones/tropical-humid.md) · [Vapor Pressure Deficit (VPD)](/glossary/vapor-pressure-deficit-vpd.md) · [Humidity Deficit (HD)](/glossary/humidity-deficit-hd.md) · [Absolute Humidity (AH)](/glossary/absolute-humidity-ah.md) · [Dehumidification](/glossary/dehumidification.md) · [Transpiration (ET)](/glossary/transpiration-et.md) · [Plant Empowerment (GPE)](/glossary/plant-empowerment.md) · [Het Nieuwe Telen (HNT)](/glossary/het-nieuwe-telen.md) · [Tomatoes](/vegetables/tomatoes.md)