Greenhouse humidity control and VPD: how to calculate and manage it

Vapor pressure deficit (VPD) is the difference between the water vapor pressure air would have if it were saturated at its temperature and the vapor pressure it actually has. It measures the air's drying power, which is what pulls water out of leaves. Relative humidity tells you how full the air is as a percentage; VPD tells you how hard it pulls. That is why growers manage humidity by VPD, and why the same RH setting can be right in the morning and wrong at noon.

How do you calculate VPD?

Calculate saturation vapor pressure from temperature with the Tetens formula, then multiply by the fraction of "room" left in the air:

1. Saturation vapor pressure: es (kPa) = 0.6108 x exp(17.27 x T / (T + 237.3)), with T in degrees C.
2. Actual vapor pressure: ea = es x RH, with RH as a fraction (65% = 0.65).
3. VPD: VPD = es - ea = es x (1 - RH).

Convert Fahrenheit first: C = (F - 32) / 1.8. For example, at 77°F (25°C) and 70% RH: es = 0.6108 x exp(17.27 x 25 / 262.3) = 3.17 kPa, and VPD = 3.17 x 0.30 = 0.95 kPa. The VPD calculator runs this for any temperature and RH, including leaf temperature offsets.

Saturation vapor pressure by temperature (Tetens)

Temperature	es (kPa)	VPD at 60% RH	VPD at 80% RH
64°F (18°C)	2.06	0.83	0.41
68°F (20°C)	2.34	0.94	0.47
72°F (22°C)	2.64	1.06	0.53
75°F (24°C)	2.98	1.19	0.60
79°F (26°C)	3.36	1.34	0.67
82°F (28°C)	3.78	1.51	0.76
86°F (30°C)	4.24	1.70	0.85

The steep rise in es (about 6-7% per degree C near these temperatures) is the whole story of VPD management: warming air without adding water raises VPD quickly, and cooling it lowers VPD quickly.

Why does VPD matter to plants?

VPD drives transpiration. Inside a leaf, air spaces are essentially saturated at leaf temperature. Water vapor diffuses out through stomata at a rate proportional to the difference between that saturated vapor pressure and the vapor pressure of the surrounding air, which is the VPD (strictly, leaf-to-air VPD).

• VPD too low: transpiration slows, so calcium and other nutrients carried in the water stream move less (tip burn and calcium disorders in some crops), leaves stay wet longer, and condensation and fungal disease become more likely.
• VPD too high: plants lose water faster than roots supply it. Stomata partially close to protect the plant, which also restricts CO2 uptake and growth. Cuttings and seedlings with little or no root system wilt.
• In the target range: stomata stay open, water and nutrient flow is steady, and leaves dry after wetting.

What VPD should a greenhouse target?

Common targets rise with plant maturity, because larger root systems can supply more transpiration. Treat these as starting ranges: published recommendations vary by crop, cultivar, light level and source, and some crops are managed well outside them.

Common VPD targets and the RH that achieves them (ranges vary by crop and source)

Stage	Typical VPD	RH at 72°F (22°C)	RH at 79°F (26°C)	RH at 86°F (30°C)
Propagation, cuttings, germination	0.4-0.8 kPa	70-85%	76-88%	81-91%
Vegetative growth	0.8-1.2 kPa	55-70%	64-76%	72-81%
Flowering, fruiting	1.0-1.5 kPa	43-62%	55-70%	65-76%

Read across a row and the point becomes clear: to hold the same VPD, the RH setpoint must rise as temperature rises. A humidistat set to 70% is on target for vegetative growth at 26°C but gives only 0.62 kPa at 18°C (64°F), and 1.27 kPa at 30°C. That mismatch is why humidistats work for simple setups but VPD-based control works better where temperature swings.

Worked examples: VPD in practice

How do you raise or lower greenhouse VPD?

Move temperature, moisture or both. The table lists the usual levers and their side effects.

VPD control levers

To	Lever	Side effect
Lower VPD (air too dry)	High-pressure fog	Also cools; needs RO water and good control to avoid wetting
Lower VPD	Shade cloth or whitewash	Reduces heat load and leaf temperature; also cuts light
Lower VPD	Reduce ventilation (within temperature limits)	Keeps transpired moisture in; temperature rises
Lower VPD	Wet floors or pad-and-fan cooling	Pad-and-fan cools strongly but adds a humidity and temperature gradient
Raise VPD (air too humid)	Ventilate with drier outside air	Loses heat in cold weather
Raise VPD	Heat (often combined with venting)	Energy cost; standard night dehumidification strategy
Raise VPD	Stop or cycle fog and mist sooner	Less cooling on hot days
Raise VPD	Horizontal airflow fans	Does not change air VPD much but thins the humid boundary layer at the leaf and keeps leaf temperature closer to air

For hot-weather cooling that also moves VPD, compare methods in greenhouse cooling: fog vs pad and fan.

How does VPD change through a typical day?

VPD usually follows temperature: lowest before dawn, highest in early to mid afternoon. Outside air holds roughly the same amount of water vapor through a clear day while temperature swings, so the deficit opens up as the house warms. A typical sunny-day pattern and the usual response:

Typical daily VPD pattern in a vented greenhouse and control response

Time	Typical condition	Risk	Response
Pre-dawn	Cool, RH 85-95%, VPD often under 0.3 kPa	Condensation on leaves and glazing	Vent and heat briefly to purge moist air; no fog
Morning	Warming, RH falling	Usually in range	Let VPD rise into the target band; do not fog early
Midday to mid afternoon	Hottest, VPD may exceed 2 kPa	Stomatal closure, wilting of young plants	Shade, then fog or pad cooling with a humidity limit
Late afternoon	Cooling, RH rising	Leaves still wet at sunset if fog runs late	Stop fog early enough for foliage to dry
Evening	VPD dropping quickly	Condensation risk returns	Ventilate or heat as needed; no fog

The practical rule: fog is a midday tool. Morning and evening problems are almost always too little VPD, not too much, and the remedy there is air exchange and heat.

What are the most common VPD mistakes?

• Using a fixed RH setpoint all day. As the examples above show, a single RH number means a different VPD at every temperature. If you only have a humidistat, change its setpoint seasonally at least.
• Chasing afternoon VPD with coarse mist. Low-pressure mist lowers VPD by wetting everything, which trades a transpiration problem for a disease problem. Use fog-sized droplets (see the greenhouse misting guide).
• Treating targets as exact. The published ranges vary by source; a crop that looks healthy and grows well slightly outside a range does not need correcting.
• Ignoring leaf temperature under strong sun or clear night skies, when the leaf and the air can differ by several degrees.
• Mixing units. VPD is sometimes quoted in millibars (1 kPa = 10 mbar) or in grams per cubic meter as a humidity deficit. Confirm the unit before comparing with a target table.

How do you measure VPD accurately?

VPD is only as good as the temperature and RH readings behind it, and small temperature errors matter. At 30°C and 70% RH, a sensor reading 1°C too warm reports a VPD about 0.08 kPa too high, and one reading 5 percentage points too dry adds another 0.2 kPa.

• Shade and aspirate the sensor. A sensor in direct sun reads warm and dry, overstating VPD.
• Place at crop height, in the canopy's air, not at the ridge or the floor.
• Keep it out of fog plumes and away from heaters, vents and pads.
• Check RH sensors against a reference periodically; RH sensors drift, especially after exposure to condensation.
• Average over minutes, not seconds, before a controller acts, to stop fog cycling on every passing cloud.

For how fog and mist systems are sized and controlled to hit these targets, see the greenhouse misting guide. More greenhouse topics are on the greenhouses hub.