GREEN SPIRIT HORTICULTURE — GROW ROOM GUIDE

What Is VPD? The Complete Vapour Pressure Deficit Guide for Grow Rooms

If you've ever wondered why two grow tents can show the same relative humidity reading and still perform completely differently, the answer is VPD (Vapour Pressure Deficit). It's the single measurement serious growers use instead of — or alongside — Relative Humidity (RH), because it's what actually drives how your plants breathe, feed, and grow.

This guide covers what VPD is, why it matters more than RH on its own, how to measure it, and exactly what kit you need to keep it in the correct zone at every stage of your grow.

What Is VPD?

VPD (Vapour Pressure Deficit) is basically how thirsty the air is at any given temperature, and this thirst has a profound effect on your plant's transpiration.

To explain this in simple terms, think of the plant as a bank — but instead of being full of money, it's full of water. The air outside is like a thirsty customer walking in, desperate for a withdrawal. The plant obliges by opening its doors (the stomata) and handing over water to satisfy that thirst. But here's the clever bit — while those doors are open, the plant also lets in a delivery of CO2 at the same time, which it needs for photosynthesis. And as long as the plant has good access to water and nutrients at the roots, it keeps that water flowing and topping itself back up as fast as it's giving it away.

The problem starts when the customer gets too thirsty. If the air outside is exceptionally dry (high VPD), it doesn't just want a polite withdrawal — it wants to drain the whole vault. That's when the bank panics: it slams the doors shut (closes the stomata) to protect its reserves and stop itself running dry. But because CO2 comes through those same doors, shutting them to save water also cuts off the CO2 delivery — meaning photosynthesis grinds to a halt, growth slows, and nutrient uptake drops, even if everything else in the room is perfect.

So the goal as a grower is to keep that "customer" just thirsty enough to keep trade flowing steadily, without ever being so desperate that the bank has to shut up shop. That's the sweet spot VPD charts and calculators help you find.

How to read the chart

  1. Tap your plant's current stage above the table.
  2. Find your room temperature (or leaf temperature, in the second chart) down the left-hand side.
  3. Follow that row until you hit a cell with a bold outline — the column it's in is the humidity to aim for.
  4. Tap any cell to see exactly what it means.

Room temp or leaf temp?

Most growers work off room temperature, since that's what a standard thermometer gives you. But leaves run cooler than the surrounding air, so if you're using a leaf clip sensor or an IR thermometer, the second chart below lets you read your VPD straight off the leaf temperature instead — no offset maths needed.

Check your grow room right now

Enter what's happening in your tent today and we'll tell you exactly where you stand — and what to change if you're off.

SELECT YOUR PLANT'S STAGE

Showing every zone at once — pick a stage to spotlight your target range.

👆 Tap any cell above to see what that reading means for your plants.

Working from a leaf temperature reading? Use the chart below instead — it's built the same way, but the left-hand column is leaf temperature rather than room temperature, so there's no offset to work out yourself.

👆 Tap any cell above to see what that reading means for your plants.

Why VPD Matters More Than RH Alone

Plants, unlike humans, have only one hole where all the work gets done. This hole — or rather, these holes — are called Stomata, found on the underside of the leaf.

This single opening does double duty:

  • Vapour loss — it's where the plant loses water through transpiration
  • CO2 uptake — it's also where the all-important CO2 comes in, which drives photosynthesis

Essentially, the plant is sweating, breathing, and "peeing" all through the same hole, at the same time.

Here's the catch: if the stomata don't open to transpire and lose vapour, they can't absorb CO2 either — no matter how perfect your CO2 levels are. Same door, two jobs. One doesn't happen without the other.

And whether that hole opens or stays shut comes down to one thing: whether the pressure (kPa) is correct.

That's the whole reason VPD, measured in kPa, is a more important measurement than RH on its own — especially for growers running CO2 supplementation, since wasted CO2 with closed stomata is money down the drain.

How to Measure Your Grow Room VPD

As a grower, to know the correct VPD at the correct stage throughout your plant's life cycle, you only need two measurements:

  • Temperature
  • Humidity

With these two numbers, you can work out the kPa (kilopascals) — the value that tells you exactly where your VPD sits.

There is a calculation for this, but if maths isn't your strong point, don't worry. Instead, use the charts above, which map out the ideal kPa range for each stage of your plant's life cycle — or make it even easier on yourself with one of our VPD monitoring options:

Whichever you choose, you'll have everything you need to know exactly what your VPD is throughout every stage of the grow.

Image: AC Infinity VPD Thermometer handheld display showing kPa reading

How to Manage Your VPD: The 5 Areas of Grow Room Climate Control

Now you have the tools to measure your VPD, you need the tools to keep it in the correct zone — so your plants can thrive, especially if you're running CO2 supplementation, which can be an expensive exercise if your stomata aren't opening properly to make use of it.

We've broken this down into five areas:

  1. Temperature control (heating)
  2. Cooling
  3. Humidity control
  4. Air movement
  5. Automation

1. Heating

Heaters are essential for keeping your grow room warm enough when temperatures drop too low. Temperature also has a far more dramatic effect on VPD than the same-sized shift in humidity, so a heater isn't just making the room more comfortable for your plants — it's actively reshaping how thirsty the air is. This is why heaters are a crucial step in getting your VPD correct.

For growers who want VPD calculated and managed automatically, the AC Infinity THERMOFORGE T7 is built exactly for this — an AI-powered VPD temperature controller built into the heater itself, so it adjusts output automatically to hold your target range rather than you guessing at a dial.

For more standard setups, our Gorilla Tube Heaters, 2KW Ceramic Grow Room Gorilla Heater, and Ora 2KW Fan Heater all do the job reliably for tents and smaller rooms, while the Lighthouse Oil Radiator Heater gives a gentler, even heat without hot spots.

Whichever you choose, pairing it with a Day & Night Temperature Controller means it switches on and off automatically as conditions shift, so your VPD stays stable through both the lights-on and lights-off periods — rather than swinging out of range while you're not there to adjust it.

Image: AC Infinity THERMOFORGE T7 grow tent heater with VPD controller

2. Cooling

Extraction fans are the most common piece of kit for reducing heat and humidity in a grow tent, and they do it while pulling in fresh air from outside that's naturally rich in CO2. Options like the WhispAir Extreme Silenced EC Extraction Fan (from £259.95) and the Systemair Revolution Vector EC Extraction Fan (from £274.99) are popular EC fans for this job — quiet, efficient, and easy to speed-control.

For growers wanting VPD-aware smart control built directly into the fan, the AC Infinity CLOUDLINE PRO T12 (£572.95) is a heavy-duty 12" inline duct fan with a temperature, humidity, and VPD smart controller — it automates ventilation based on real conditions rather than a fixed speed. For a simpler all-in-one setup, the AC Infinity Air Filtration Kit 8" (£279.00) bundles an 8" inline fan, speed controller, carbon filter, and ducting together, so extraction and odour control are sorted in one purchase.

The catch is that an extraction fan is only ever as good at cooling the room as the temperature of the air it's pulling in. If it's summer and the air outside is hotter than the air inside, you as a grower will run into serious problems — the fan is just exchanging one lot of warm air for another.

So, the best kit to use in that scenario is an air conditioning unit, which cools the air already inside the room rather than removing it from the grow space. The AC Infinity Terraform All-in-One Grow Tent Climate Control A/C (£599.99) is built exactly for this, combining cooling with heating, dehumidifying, and circulation in one smart, VPD-aware unit.

Most grow rooms use both pieces of kit together, so that CO2, temperature, and odour are all controlled simultaneously.

Image: WhispAir EC extraction fan for grow tent ventilation

3. Humidity Control

Humidifiers help to increase the humidity in the grow room, lowering VPD. Because the humidifier is keeping the air in the room less thirsty, the drag on the water from the leaves is less aggressive, and this enables the stomata to stay open for longer, allowing better CO2 absorption, which means more photosynthesis.

For growers wanting VPD control built directly into the humidifier, the AC Infinity Cloudforge T5 (9L, £129.00) and the larger AC Infinity Cloudforge T7 (15L, £169.00) both offer smart controls with targeted vaporizing, so they hold your humidity — and therefore your VPD — exactly where you set it. For smaller tents or propagation setups, the AC Infinity Cloudforge T3 (4.5L, £72.99) gives you the same smart control in a more compact unit.

For a simpler, budget-friendly option, the RAM Ultrasonic Humidifiers (from £44.95) do the job reliably without the smart features, and the GAS Sonic Air Humidifiers (from £79.49) are another solid mid-range choice for tents and small rooms.

Dehumidifiers do the opposite job — they pull moisture out of the air, which raises your VPD. Because the air is now drier, it becomes thirstier, and that stronger pull drags water out of the leaf more aggressively. Up to a point this is a good thing, since it keeps the transpiration stream moving well and helps drive nutrient and calcium uptake. But push VPD too high and the plant senses it's losing water faster than the roots can replace it, so the stomata start to close to protect themselves — and that actually slows CO2 uptake rather than helping it. So it's a balancing act, not just "more dehumidifying = more growth."

Where a dehumidifier really earns its keep is in flower. As the buds get denser and tighter, moisture can get trapped inside the canopy with nowhere to go, and that's the perfect breeding ground for mould, mildew and bud rot. Pulling that excess moisture out of the air protects the crop right when it's most vulnerable and most valuable.

For growers wanting VPD control built directly into the unit, the AC Infinity Hydrone 7 (£279.00) comes with an AI-powered humidity VPD controller, so it reads the room and adjusts itself automatically rather than you guessing at a dial. The AC Infinity Hydrone 5 (£279.00) gives you 10 manual drying levels with all-climate performance if you'd rather set it yourself.

For bigger rooms or commercial setups, the ORA Dehumidifier 60L (£599.80) and the AlorAir Storm DP Commercial Dehumidifier (£779.95, with smart app control) can shift serious volumes of moisture, while the RAM Dehumidifiers (from £199.95) are a solid, no-frills option for smaller tents and rooms.

Image: AC Infinity Cloudforge plant humidifier with VPD smart controls

4. Air Movement

Even with your temperature and humidity sitting perfectly on your VPD chart, if the air inside the tent isn't moving, you can still run into problems. Still air lets a thin layer of humid, stagnant air build up right at the surface of the leaf — this is often called the boundary layer — and that layer can be far more humid than the rest of the room, giving the leaf a false low VPD reading regardless of what your hygrometer says. So the plant thinks the air around it is less thirsty than it actually is, and transpiration slows right down even though your kPa on the meter looks spot on.

Circulation fans fix this by constantly stripping that boundary layer away and mixing the air, so the leaf is always sat in the same conditions as the rest of the tent. This also helps stop hot spots and humidity pockets forming around dense canopy, which is exactly where mould and mildew like to take hold.

For a strong, no-nonsense option, the WhispAir 16" Oscillating Wall Fan (£23.95) covers a room reliably, and the WhispAir Extreme Heavy Duty Wall Fans (Twin Box) (from £39.95) gives you two fans to properly cross-circulate a larger space.

For inside the canopy itself, the AC Infinity Cloudray A9 Clip Fan (£61.99) is a strong performer for directing airflow exactly where it's needed, and for bigger rooms the AC Infinity Cloudlift S14 Floor & Wall Fan (£209.00) with wireless control moves serious volumes of air without a manual switch in sight.

Image: Grow tent circulation fan mounted on wall for airflow

5. Automation — The Piece That Ties It All Together

This is where most growers save themselves a lot of manual fiddling. Rather than you sat there adjusting dials throughout the day/night cycle, controllers let each piece of kit react automatically to sensor readings:

Many growers now skip juggling separate controllers altogether and run everything through a single smart unit like the AC Infinity Controller 69 Pro+, which reads dynamic VPD, temperature and humidity directly from a probe and automatically triggers up to eight devices — fans, humidifiers, heaters, the lot — to hold your target zone. This is the true "set it and forget it" end of the spectrum, and it comes with data logging via app so you can see exactly how your VPD has tracked over the grow.

Image: AC Infinity Controller 69 Pro+ VPD smart controller display

The Realistic Minimum VPD Setup

The realistic minimum setup for someone just starting to manage VPD properly:

  • A VPD thermometer or thermo-hygrometer
  • An extraction fan for basic temp/humidity exchange
  • A circulation fan
  • Either a humidifier or dehumidifier, depending on which direction your tent tends to drift

You add heaters, air conditioning, and automated controllers as the operation scales up or the margins for error get tighter — particularly in flowering, when humidity control matters most for mould prevention.

FAQs

What is a good VPD for a grow tent?

Ideal VPD varies by growth stage — generally lower for seedlings and clones (around 0.4–0.8 kPa) and higher through veg and flower (around 0.8–1.5 kPa). Use a VPD chart or a VPD thermometer to check your exact stage target.

Is VPD more important than humidity?

VPD accounts for both temperature and humidity together, which is why it's a more accurate predictor of plant stress and stomatal behaviour than relative humidity on its own.

What causes high VPD in a grow room?

High temperature combined with low humidity is the most common cause — often from grow lights heating the tent faster than the humidity can keep up, especially in summer.

Do I need a humidifier and a dehumidifier?

Not always — it depends on your local climate and grow stage. Many growers use a humidifier during propagation and early veg, then switch to a dehumidifier in flower to protect against mould.

Shop the full range of hygrometers and thermometers, heating, extraction fans, humidity control, and smart controllers at Green Spirit Horticulture.

How this is calculated: leaves typically run about 2.8°C (5°F) cooler than the surrounding air, because evaporation cools them down. This chart works out saturation vapour pressure at that estimated leaf temperature, and subtracts the actual vapour pressure in the room — which is what makes it more accurate than a plain humidity chart. Values are shown in kilopascals (kPa), the standard unit for VPD; 1 kPa ≈ 10 hPa ≈ 0.01 bar.