vpd humidifier

As a gardener, it’s up to you to ensure the plants have the nutrients they need. While there are many things to focus on, one is VPD (vapor pressure deficit).

Today, you’ll learn more about VPD, its importance, and how to control it. Let’s dive in.

Learn How to Control VPD with Aaron!

What is VPD?

Ultimately, VPD is one method for calculating the air’s thirst for moisture.

There are three components to consider here:

  • AVP (Actual/Ambient Vapor Pressure) – This is the current moisture level in the air.
  • SVP (Saturation Vapor Pressure) – This is the maximum amount of moisture the air could hold.
  • Deficit – This is the calculation between the current moisture level and the maximum the air can hold.

    In a sense, vapor pressure deficit controls how your plants
    photosynthesize and respirate.
VPD explained
A low VPD indicates that you have very moist air, which can lead to mold growth and plant
diseases because the transpiration rate is inefficient.

Alternatively, a high VPD indicates that the air is too dry. This might cause your plants to
become dehydrated and not get the nutrients needed to grow.

Overall, you’re looking for that VPD sweet spot, which offers many benefits, such as:

- Optimal Growth – You’re supporting efficient nutrient and water uptake.
- Disease Prevention
– This will help you lower the risk of fungal infections and plant
- Enhanced Photosynthesis – It’s easy to balance the stomatal openings, allowing for
more CO2 intake.
- Resource Efficiency – You can save water and energy, reducing the costs needed to
produce flowers, vegetables, and fruits.
- Improved Flavor, Health, and Quality – The right VPD for veg will improve the plant’s
health, quality, and flavor in the long run.

The VPD Sweet Spot

While you can learn how to lower VPD or raise it, the ultimate goal is to find the sweet spot.

This is a range of numbers we can hit to ensure our plants are getting the right amount of moisture throughout all stages of their life.

Since VPD is the relationship between humidity and air temperature, there are specific ranges we must try to hit at those different life stages.

We measure the vapor pressure deficit’s sweet spot using kilopascals (kPa). Here are a few things to keep in mind:

- VPD for Seedlings/Clones – Between .6 to .8 kPa
- VPD for Veg – Between .9 to 1.1 kPa
- VPD for Flower – Between 1.2 to 1.5 kPa

To get a better idea of what you require, check out this VPD chart:

How to Control VPD

The best way to control VPD is to get help!

And we think that the AC Infinity Controller 69 Pro is a great solution. It features a vapor pressure deficit setting, allowing you to calibrate everything to find the right numbers.

Overall, we realize it’s hard to figure out VPD, but we’re here to teach you this knowledge.

There are two trigger types in the VPD mode of the Controller 69 Pro: high and low. You go into that setting and can set your trigger points.

Though there are two triggers (low and high), they can simultaneously be activated if you’re not viewing specific ones.

Therefore, you should turn off the option that is not being used so it won’t interfere with the active one.

When you set the high VPD trigger, it will activate the tent devices you’ve installed, such as humidifiers and AC units, to either add cool air or moisture to your growing environment.

You’ll also find the low VPD trigger. This inversely activates your tent devices to heat the air or remove moisture to get you back down to your set point.

Ultimately, the types of devices you might have set up in your tent include dehumidifiers, heaters, air conditioners, and exhaust fans.

Since VPD is the ratio between relative humidity and temperature, we found that the trigger system works best when you have something keeping the relative humidity or temperature stable. Then, you can adjust the other one to avoid both of them fluctuating.

For example, we use an inline fan that’s set to manage the maximum temperature and a humidifier for the high vapor pressure deficit mode.

Additionally, we might also have a heater for the low trigger.

How to Lower VPD


When you have a high VPD, this means your air is likely too dry. For example, you’ve got a 44 percent relative humidity and are sitting at 82 degrees Fahrenheit. Your vapor pressure deficit is 1.9 kPa, which is a little excessive because the target range is 1.2 to 1.5 kilopascals.

If you want to know how to lower VPD in this situation, you’d hold the temperature consistent at 82 degrees Fahrenheit and increase the humidity to 60 percent. In time, you’d drop to 1.3 kPa.

A situation like this for the flowering stage indicates the goal is to have 1.2 to 1.5 kPa.
Therefore, you’d need a humidifier, and your trigger would be the high VPD. If you’re struggling to keep in that range, you might use a 1.4 trigger instead of a 1.5kPa trigger.

We recommend a 0.1 kPa buffer, but you can also raise the number if you simply can’t stay
within that range and the devices continuously turn on and off.

How to Raise VPD

There are a few solutions when you have a low VPD, including:

- Raise the Temperature – Warmer air tends to hold more moisture, so if you raise the temperature, you’ll decrease the relative humidity while increasing VPD. Set your heater to turn on at a lower vapor pressure deficit, such as 1.1 in bloom.

- Decrease Humidity – Lowering the humidity levels can increase VPD because you’re reducing the moisture content within the air without altering the temps. This raises the deficit between the wetness present and how much can be held.

Overall, you can use a dehumidifier or an exhaust fan to put cooler, drier air into the tent. Set your devices to turn on at a lower trigger number, such as 1.1 or 1 in bloom.

Let’s check out another example of low VPD. You’re flowering, and it’s 77 degrees Fahrenheit in the tent with a 64 percent relative humidity. However, your vapor pressure deficit is at 1.1 kPa, which is low.

If you have your heater on a low VPD trigger of 1.3, it would kick on to raise the temperature.

Once it hits 81 degrees Fahrenheit, it would shut off because the vapor pressure deficit number now meets the minimum range.

Here are the points to ponder:
- The device is the heater.
- The trigger is “low VPD.”
- The setting is 1.3 kPa.
- The buffer is 0.1 kPa.
- The transition is also 0.1 kPa.

Depending on the environment, you can use heaters for a low VPD trigger and exhaust fans set to auto mode for a high-temp prompt.

"VPD is a simply a ratio between temperature and humidity"

Aaron Woomer

A Word About Leaf Temp

It’s important for us to touch upon leaf temp briefly. Be aware that there will be a change in temperature from the air around your plants to the actual temp of them. 

You’ll find many calculators online that allow you to adjust the number based on the difference.

The only way to get the leaf temperature is to use an infrared thermometer. 


If you’re searching for the perfect VPD, consider using AC Infinity’s Controller 69 Pro. With it, you can precisely control your grow room’s vapor pressure deficit. This will help you enjoy healthier plants and improve their yield and quality.

Let’s recap what to do for low/high triggers:

- Turn an inline fan on auto if you have a high temp trigger.
- Turn on a humidifier for a high VPD trigger.
- Turn on the heater for a low VPD trigger.

Doing those three things at the right time can get you about 90 percent of the way to
controlling VPD.

There are ways to fine-tune things, as well. You must understand your growing environment, know the zone you live in, and more.

Do you need help controlling your VPD levels? 

Check out the AC Infinity Controllers at Happy Hydro today.

Author | Chris McDonald

With two decades of expertise, Chris leads Happy Hydro in redefining sustainable gardening and delights in backpacking adventures, mind-expanding journeys, and creating memories with his loved ones.

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Frequently Asked Questions

Does VPD really matter?

Yes, when VPD levels are incorrect your plants will not transpire and photosynthesize efficiently.

How do I make my VPD go down?

Make your VPD go down by decreasing your temperature or increasing your humidity.

Does VPD matter at night?

If your temperature swings are outside of your plants recommended range

What is the best VPD for late flowering?

In late flowering you'll want your VPD between 1.0 and 1.3 kPa.

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