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Vapour pressure deficit (VPD) in the greenhouse

Relative humidity alone is misleading: 70 % does not mean the same at 15 °C as at 30 °C. Vapour pressure deficit combines temperature and humidity into one value that explains how much the plant transpires.

Domotica Valencia · Updated: 8 October 2026

In short

Vapour pressure deficit (VPD) is the difference between the water vapour the air could hold at that temperature and what it actually holds, in kilopascals (kPa). At low VPD the plant barely transpires and fungal risk rises; at high VPD it transpires too much and gets stressed. It is controlled with ventilation, heating and misting.

VPD chart by temperature and humidity

This is air VPD. Leaf VPD can differ somewhat, because the leaf is usually at a different temperature from the air.

Air VPD in kPa by temperature and relative humidity (Tetens formula)
Temperature	50 %	60 %	70 %	80 %	90 %
15 °C	0.85	0.68	0.51	0.34	0.17
20 °C	1.17	0.94	0.70	0.47	0.23
25 °C	1.58	1.27	0.95	0.63	0.32
30 °C	2.12	1.70	1.27	0.85	0.42
35 °C	2.81	2.25	1.69	1.12	0.56

Indicative ranges

These are reference values found in the literature and growing practice; each species and stage has its own, and setpoints should be set by your agronomist.

Commonly cited VPD ranges (indicative)
Situation	Indicative VPD	Aim
Germination and rooting cuttings	0.4 to 0.8 kPa	High humidity so the rootless plant does not dry out.
Vegetative growth	0.8 to 1.2 kPa	Active transpiration that draws water and nutrients.
Below about 0.4 kPa	Risk zone	Condensation and higher fungal risk.
Above about 1.5 kPa	Risk zone	Water stress: the plant closes its stomata and slows down.

How it is automated

High VPD (hot, dry air): short misting pulses, shading and cooling.
Low VPD (saturated air): ventilation, air circulation and, when cold, a little heating to bring relative humidity down.
The controller calculates VPD from each probe’s temperature and humidity and acts by zone.
Combine it with temperature limits: VPD does not replace heat and cold setpoints.

Frequently asked questions

Do I need a VPD sensor?

There is no such thing as such: it is calculated from a temperature and humidity probe. What matters is that the probe is reliable, ventilated and at crop height.

How is it calculated?

Take the saturation vapour pressure at the air temperature and multiply by (1 − relative humidity). For example, at 25 °C and 70 % humidity: 3.17 × 0.30 ≈ 0.95 kPa.

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