Vapour Pressure Deficit in a Greenhouse: How to Calculate VPD From Temperature and Humidity
TL;DR: Vapour pressure deficit is the difference between how much water vapour the air could hold at its temperature and how much it actually holds. It is calculated from air temperature and relative humidity with two FAO equations, and unlike humidity alone it means the same thing at any temperature. Ohio State University Extension notes that fungal pathogens survive best below 0.43 kPa and that infection is most damaging below 0.20 kPa. Logging temperature and humidity at crop height gives VPD for every reading.
Last updated: 14 September 2026
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Key takeaways
- Vapour pressure deficit (VPD) is the saturation vapour pressure of the air minus its actual vapour pressure, in kPa.
- It is calculated from air temperature and relative humidity using FAO-56 equations 11 and 19, so any logged pair of readings gives a VPD.
- The same humidity means a different VPD at a different temperature: 80% RH is about 0.25 kPa at 10°C but 0.50 kPa at 21°C.
- Ohio State University Extension notes that fungal pathogens survive best below 0.43 kPa, and infection is most damaging below 0.20 kPa.
- A lux sensor is not a PAR sensor. Use it to compare days and houses, not to measure the light plants use.
What is vapour pressure deficit?
Vapour pressure deficit is the gap between the water vapour air could hold at its temperature and the water vapour it actually holds. The FAO’s guidelines on crop evapotranspiration define it as the difference between the saturation and actual vapour pressure for a given time period, and it is measured in kilopascals.
For a grower, the size of that gap describes two things at once. A large vapour pressure deficit means dry air that pulls water from leaves faster. A small one means air close to saturation, which slows transpiration and brings the greenhouse closer to condensation on leaves and structures.
Ohio State University Extension’s fact sheet on greenhouse condensation control, by Jessica Prenger and Peter Ling, describes vapour pressure deficit as a way to evaluate the disease threat, condensation potential and irrigation needs of a greenhouse crop.
How do you calculate VPD from temperature and humidity?
Calculate VPD in three steps using the equations in chapter 3 of FAO Irrigation and Drainage Paper 56. You need only air temperature in °C and relative humidity in %.
- Saturation vapour pressure. es = 0.6108 × exp(17.27 × T ÷ (T + 237.3)), in kPa, where T is air temperature. This is FAO-56 equation 11.
- Actual vapour pressure. ea = es × RH ÷ 100, following equation 19.
- Vapour pressure deficit. VPD = es minus ea.
Take a greenhouse at 24°C and 70% relative humidity. Saturation vapour pressure is 2.98 kPa, actual vapour pressure is 2.09 kPa, and the vapour pressure deficit is about 0.90 kPa.
Now take the same house at night, cooled to 16°C with humidity at 90%. Saturation vapour pressure falls to 1.82 kPa, and the vapour pressure deficit drops to about 0.18 kPa. The air now holds less water vapour than it did in the afternoon; the fall in temperature alone has taken it close to saturation.
Why is relative humidity alone misleading?
Relative humidity alone misleads because it is relative to temperature. The FAO guidelines point out that actual vapour pressure can stay fairly constant through a day while relative humidity swings between a maximum near sunrise and a minimum in the early afternoon, simply because the temperature changes.
The Ohio State fact sheet gives a clear example. A greenhouse at 80% relative humidity holds about 14 lb of water in its air at 50°F, but about 28 lb at 70°F, and the vapour pressure deficit is 0.25 kPa and 0.50 kPa respectively. An alarm set on humidity would treat those two houses as identical.
That is why vapour pressure deficit is the better number to alarm on. It means the same thing at 10°C as at 25°C, so one threshold works through the whole day and the whole season.
What VPD should a greenhouse aim for?
A greenhouse should stay clear of the low values where disease thrives, and within the range your crop guidance recommends above that. The Ohio State fact sheet reports two critical values from studies of pathogen survival: fungal pathogens survive best below 0.43 kPa, and infection is most damaging below 0.20 kPa. It advises keeping greenhouse air above 0.20 kPa.
Upper targets depend on the crop and its stage, so take them from your crop adviser or grower guidance rather than a generic chart. Higher values increase the transpirational demand on the crop, which is why a band matters more than a single number.
The same fact sheet notes that using canopy temperature gives the best indication of condensation risk. Air sensors give air VPD, and leaves can sit at a different temperature from the air around them, so a margin above the thresholds is sensible.
Where should greenhouse sensors go?
Place air sensors where the crop is, not where it is convenient. Mount temperature and humidity sensors at crop height, shaded from direct sun, and away from heaters, vents and CO2 outlets. A sensor in direct sun reports its own temperature, not the air’s, and that error feeds straight into vapour pressure deficit.
Use at least two per house, one at each end, because glasshouses vary along their length. Long houses benefit from more.
Sensor accuracy matters near the thresholds. The EM300-TH reads to ±0.3°C and ±3% relative humidity between 10% and 90%. At 24°C, a 3% humidity error alone shifts vapour pressure deficit by about 0.09 kPa, so treat readings close to 0.20 kPa as a warning, not a line.
What else should greenhouse monitoring measure?
Beyond vapour pressure deficit, three readings round out a greenhouse picture.
- CO2. The EM500-CO2 measures 400 to 5,000 ppm with temperature, humidity and pressure, which shows whether ventilation or enrichment is working.
- Light. The EM500-LGT logs 0 to 100,000 lux on a 3 m cable. Lux is weighted to the human eye, so use it to compare days, houses and shading, not as a plant light measurement.
- Root zone. The EM500-SMTC reports soil moisture, temperature and electrical conductivity from a buried IP68 probe.
On light, the University of Arkansas greenhouse course explains that light between 400 and 700 nm is photosynthetically active radiation, and that meters mimicking the human eye are more sensitive to yellow and green light and not the most appropriate for plant requirements.
What are the limits?
Greenhouse sensors report conditions; they do not run the house. Four limits are worth planning around.
- Air, not leaf. Air temperature gives air VPD. Leaf temperature can differ from it.
- Lux, not PAR. A lux sensor cannot measure the light plants use. For that, a PAR sensor is the right instrument.
- Local readings. A soil probe measures the medium around it, and an air sensor its own spot. Place them where they represent the crop.
- No control. Monitoring alerts; it does not open vents, run heating or water the crop.
What does a greenhouse monitoring kit include?
A greenhouse monitoring kit needs a weatherproof gateway, air sensors at both ends and the extra readings you want. Our greenhouse monitoring kit combines a UG65 gateway, two EM300-TH sensors, an EM500-CO2, an EM500-LGT and an EM500-SMTC soil probe, with live prices and one click to the basket.
For more on the root zone, see our guide to soil moisture sensors. For outdoor conditions around the houses, read about IoT weather stations.
Frequently asked questions
How do you calculate VPD?
Work out saturation vapour pressure from air temperature with es = 0.6108 × exp(17.27T ÷ (T + 237.3)) kPa, multiply by relative humidity divided by 100 for actual vapour pressure, then subtract. These are FAO-56 equations 11 and 19.
What is a good VPD for a greenhouse?
Stay above the disease thresholds Ohio State University Extension reports: fungal pathogens survive best below 0.43 kPa, and infection is most damaging below 0.20 kPa. Take upper targets from guidance for your crop and growth stage.
Is VPD better than relative humidity?
For alarms, usually yes. Relative humidity changes with temperature even when the moisture in the air does not, while vapour pressure deficit means the same at any temperature.
Can a lux sensor measure light for plants?
Not properly. Lux is weighted to the human eye, while plants use photosynthetically active radiation between 400 and 700 nm. A lux sensor is still useful for comparing days, houses and shading.
Why does VPD fall at night?
Because the air cools. Cooler air can hold less water vapour, so the same moisture brings it closer to saturation, and vapour pressure deficit falls towards condensation.
Alarm on the number that means the same all day
Vapour pressure deficit turns two ordinary readings into one that works at any temperature. Log temperature and humidity at crop height, calculate VPD for every reading, and set alerts that keep the house clear of the values where disease thrives.
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