How To Increase Humidity In Grow Tent

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Brad Smith
Written By Brad Smith

Whether you’re growing leafy greens, flowers, nightshades, flowers, or any other species of plants, grow tents are perfect for all gardening enthusiasts.

How to increase humidity in grow tent is one of the most critical issues you’re going to face as an indoor gardener. Thankfully, there are quite a few ways you can raise the humidity in grow tent.

In this article, we’re going to tackle all the ways you can raise the humidity in your grow tents and keep them at an ideal level.

But before we get into that, it’s important we answer the question what exactly is humidity, and how do humidity levels affect your plants?

As an indoor gardener, you need to have a keen understanding of what humidity can do to crop health and how it’s vital for a successful harvest.

Have you ever experienced those days when no matter what you do, you just can’t cool yourself down? That’s caused by high amounts of moisture in the air! When the air around you becomes saturated with water vapor, it’s almost impossible to cool yourself down since the sweat from your skin can’t evaporate.

Then there are days that are really hot but you can still sit around in the sun and stay relatively cool. That’s due to low levels of humidity in the air. When the air is dry, the perspiration on your skin can easily evaporate, thus cooling you down. However, excessively low humidity can have detrimental effects such as dry skin and itchy eyes.

The same is true for the plants in your grow tent or grow room. Humidity is one of the most underrated environmental aspects of indoor gardening – and it’s definitely something you should keep an eye on!

Before we get into the nitty-gritty of relative humidity, how it affects your plants, why it’s essential it’s stable, you need to keep in mind one simple thing. That one simple thing is that you should avoid extremes. Overly high humidity can be just as dangerous as extremely low humidity.

An ideal humidity range during vegetative growth is around 60 to 70 percent. This way you’re helping the plant will have an easier job maintaining the equilibrium of water uptake and water loss before it develops a strong root system.

For flowering plants, keeping a humidity level between 40 and 50 percent is good practice since it prevents mold or pathogens from forming on your plants. On top of that, it also ensures a good moment of water, nutrients, and minerals through the plant to the flowers or the fruit. Of course, there are a couple of other special circumstances, but we’ll talk about that later.

Talking about relative humidity levels is all well and good. But how to achieve them is another story. This is why we’ve decided to compile this article. Today you’ll learn all about humidity and indoor gardening. We’ll start with the basics and move on from there. After you’ve read through the entire article, you’ll have all the knowledge needed to ensure your plants get the right amount of air moisture through their entire development.

So, let’s start with the basics and move on from there!

Humidity In A Grow Tent: The Basics

vapor pressure and exchange of air in a grow tent

Before we get into ways you can raise the humidity in your indoor growing operation, there is a bit of basic science you need to learn.

Don’t worry, we’ll keep it as short as possible.

So, long story short, humidity is the amount of water vapor that air can hold. However, this is not how gardeners recognize it.

When it comes to gardening, there are three ways to address humidity: Absolute, Specific, and Relative. Since the most convenient of these three is relative humidity, we’ll stick to that.

What Is Relative Humidity?

Relative Humidity is a percentage that indicates the ratio of current moisture content in the air and the most possible moisture content of a certain amount of air.

You can calculate relative humidity using a simple formula: RH = (Actual Vapor Density / Saturation Vapor Density) X 100%

The higher the grow tent humidity percentage is, the more water there is that’s taking up space instead of air, and vice versa.

With that said, there are other environmental factors that also affect the humidity:

  • Temperature – RH raises as the temperature rises, and vice versa.
  • Light – RG rises as the light becomes more intense.
  • Ventilation – RH rises if there’s not a lot of air movement.
  • Irrigation: The more you irrigate the plants the more you raise humidity in a grow tent.

If this is all a bit confusing to you don’t worry, the next section will clarify things quite a bit.

Relative Humidity And Other Factors

Here are four factors that affect the humidity in a grow tent or grow room.


Humidity is inversely related to the temperature in your grow tent. As the temperature increases, the air’s ability to hold water vapor decreases. This means if you have a lower relative humidity level below the grow lights in your grow room or grow tent.


Light doesn’t impact RH directly, but grow lights give off quite a lot of heat which impacts the RH in the entire grow tent. If you don’t install a decent cooling system around the lights, you are going to have problems with low humidity levels in your grow tent.


Ventilation affects the air inflow and outflow of the grow room or tent. If you have too much ventilation, you won’t be able to get the tent or room humid enough.

On the other hand, if you don’t have enough ventilation, RH will rise and your plants will suffer for it.


If you have lots of water trays laying around your grow tent, you will have more humidity in the air. The more open water sources you have, the more water vapor there will be in the air. And if these water sources are close to the intake tubes or vent holes, it’s more likely they’ll let off vapor and make the air moister.

Why Humility Is So Important For Plants

depending on the size of a grow tent the vapor pressure can vary

There are quite a few reasons why it’s essential you keep the humidity level at an ideal level inside your grow tent. Here are the two most obvious answers:

Humidity Allows You To Control The Plant’s Nutrition

Plants in your grow tent need to keep taking in water through their leaves and roots. When the RH levels are sufficient, they will start absorbing water through their leaves. This lets them take up less water through the roots.

On the other hand, if the RH levels are low, the plant will start taking up more water through the roots. And as most gardeners put nutrients in the water, the more the plants drink it, the more nutrients they will take.

Long story short, learning how to control the humidity in the grow tent allows you to have control over the nutrition of your plants.

Humidity Affects The Health & Growth Of The Plants In The Grow Tent

Young plants love high humidity. As they mature, they’ll require a less humid environment. So, for growing plants it’s essential they have an abundance of humidity in their early stages. If there isn’t enough moisture in the air, that can put lots of stress on the root system of younger plants.

What Happens If The Humidity Is Too Low In The Grow Tent?

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The consequences of low humidity in a grow tent are many. Low moisture forces growing plants to exert more effort, which effectively shortens their lifespan and stunts their development.

Other consequences of low humidity include leaving the plants to desiccate, which exposes them to mold, mildew, as well as opens them up for insect infestations.

The lower the humidity the more the plant is vulnerable to infestations.

What Happens If the Humidity Is Too Hight In The Grow Tent

plants need high humidity of your grow tent

Although a high humidity level is essential for proper plant growth, it can also have some detrimental effects. If the levels of humidity are excessively high, you can do great harm to your plants.

It’s essential you control humidity in your grow tent if you don’t want it to become a breeding ground for bacteria and mold. What’s more, high humidity can also make flowering plants suffer from bud rot and botrytis.

High water vapor concentration can also clog the stomata and reduce carbon dioxide intake. This exchange is essential for the plant’s photosynthesis.

How To Measure Humidity

how to raise humidity in grow tent how to measure rh

The only way to effectively control humidity in a grow room or tent is to keep a track of it. The best way to measure the levels of humidity is by using an electric thermo-hygrometer combo.

There are two types of electric thermo-hygrometer combos. Here’s how they work.

Resistive Electric Hygrometer

Resistive electric hygrometers utilize a ceramic substance that’s exposed to air. As the RH increases, the ceramic contacts change their resistance. The degree of this change is converted into a precise measurement of water vapor in the air.

Capacitive Electric Hygrometer

Capacitive electric hygrometers use a capacitor-style setup – basically two metal plates with air circulating between them. The humidity of the air between these plates defines how much charge they can hold. This charge value is converted to the relative humidity level in the air.

How To Increase Humidity In Grow Tent Without Spending Money

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Keeping the RH at an ideal level is essential if you want to have any success with your plants. This is even more true for a confined space like a grow tent. However, most methods to increase humidity in grow tents require spending money.

Thankfully, there are a couple of methods that don’t require you to spend precious dollars on your plants. So, here are some suggestions on how you can increase the amount of water vapor in your grow tent without spending a dime.

#1 Turn Off Or Get Rid Of Half Of Your LED Lights

Turning off or getting rid of some of your LED lights is an easy and cheap way to increase RH.

The science behind this method is simple. Since you can’t have high RH and high temperature, getting rid of some of your lights will bring the temperature down. As the temperature goes down, the RH will rise – very simple.

#2 Lower Your Fan Speed

Having good ventilation is crucial to a successful harvest. Without proper ventilation, your grow tent won’t have much exchange of air.

Most people set the fan speed in their grow tent to moderate. This helps conserve the moisture at different phases of plant growth.

If you set your fan speed to high, two things will happen:

  1. The air inside the tent will dry up faster than normal.
  2. Water vapor won’t be able to gather on the leaves of the plant resulting in low moisturization

So, if your fan speed is set too high, that might be the reason why RH in your grow room or tent isn’t at an ideal level.

If you’re having trouble keeping the RH levels high, try lowering the fan speed as much as possible. This way, there’s less air circulation, resulting in RH levels rising.

#3 Modify Your AC

As we discussed at the begging, the temperature drops help in building up RH.

If your air conditioner fails to cool down the grow tent, you won’t be able to increase the RH level. So, check if your air conditioner can cool down the tent even more.

How To Increase Humidity In Grow Tent: 11 Ways You Can Increase Relative Humidity In Your Grow Tent

humidifier in a grow room pouring moisture on plants

Now that you know what are low-cost ways on how to increase humidity in grow tent, it’s time to learn what are some of the most common ways to raise the humidity in your grow tent.

#1 Use A Humidifier

The easiest way to increase humidity in a grow tent is by installing a humidifier.

A humidifier is specially designed to provide the space they operate in with the right amount of water vapor.

If you buy a humidifier, you will have to keep it under firm control. Otherwise, it will raise the RH to an intolerable level, which will negatively impact your plants.

Thankfully, you can control a humidifier if you get one with a built-in humidistat. A humidifier with this feature will allow you to set the RH level to your liking and let the machine work. Once the RH level is reached, the humidifier will turn itself off.

If you have a large grow room or tent, you should look for a humidifier with a larger water tank. For smaller spaces, a humidifier with a small tank will work.

If you want to learn more about humidifers for your grow room or tent, you can check out our article on the best humidifier for grow tent.

#2 Have Plenty Of Open Water Sources

Another easy way to raise the humidity in your grow tent is by keeping plenty of open water resources. This method works for both big and small tents.

By having lots of open water resources, you are ensuring the water molecules evaporate from the surface of the water. By placing a couple of water trays in your tent, you can increase the RH level by quite a bit.

You can place water trays next to passive air holes or air intake tubing so the vapor gets sucked in the tent.

#3 Combine Some Of Your Plants

Another easy way to raise humidity in grow tent is by combining your plants.

Because plants release moisture in the air as a part of their respiration, by increasing your plant density you can increase the RH level to an adequate level.

You can combine young plants with larger plants so they get the right amount of moisture during their most critical stages of development.

Just don’t overdo it because then the younger plants won’t be able to get their nutrition.

#4 Increase Watering

The trick with this method is to keep water in the drainage system.

With the excess water in your drainage system, the humidity in grow tent will rise. However, don’t get carried away. Increased watering might drown your plants.

#5 Hang Wet Towels

By now you should know that the more water surfaces you have the higher the RH will be.

So, an easy and inexpensive way of increasing humidity in a grow tent is by hanging wet towels inside it. For a 4×4 tent, 2 wet towels are more than enough – just make sure they don’t hamper the airflow.

Soak the towels in the morning and let them hang there until they dry up. If you want to keep them up longer, you can apply more water to them using a spray bottle or soaking them again. It takes around four hours for a wet towel to completely dry up, so plan accordingly.

#6 Set Up The LED System Correctly

If you have lots of LED lights set up in your tent, that will keep the temperature up. The higher the temperature, the less moisture there is in the air.

To ensure, your tent doesn’t get too toasty, make sure your LED system is set up correctly. This way, you’re not just ensuring the plants get enough moisture, but you also save on your electricity bill.

#7 Optimize Your Extractor Fans

An essential part of venting your tent is the proper use of the fan system. Without a fan system, you won’t get far in your indoor gardening endeavors.

With that being said, with too much ventilation, you won’t have lots of humidity. This is why it’s critical you set up your fan system so it circulates air just enough without compromising the RH levels.

#8 Use An Air Conditioner

Another way to keep the humidity at optimal levels is by using an air conditioner. Since the air gets dryer at low and high temperatures, you can use your AC to keep the temperature just right.

A good quality AC won’t have trouble keeping the temperature around 70 degrees Fahrenheit, which is ideal for growing systems.

#9 Lower The Temperature Around The Grow Tent

Another way of keeping the RH at the right level is by lowering the temperature inside your room. Once the room temperature drops, the humidity in the tent will start rising because of the temperature difference.

#10 Keep Up The Air Circulation Around The Grow Tent

Sometimes, the RH levels will fall in some parts of the tent, while other parts have plenty of moisture. This usually happens because of the lack of proper air circulation.

So, having ideal RH levels depends on proper ventilation. To ensure there’s enough air circulating you can use a stand-up fan, an extractor fan, or a clip-on fan.

The geometry of how you place the fans is essential. So make sure they’re all set up in the right places, with the right angles and right rotational speed.

#11 Use CO2 Sources

Since plants require CO2 to perform photosynthesis, and if there is no air circulation because that will lower the RH, there won’t be any CO2. In this case, you can use an artificial CO2 source. There are a couple of different CO2 systems you can use, and each depends on the type and size of the tent you’re using.

What’s The Ideal Humidity For Grow Tents

bigger plants sitting next to smaller plants next to an intake fan

Now that you know all about how humidity works and affects your indoor gardening, let’s talk about what are ideal humidity levels for different phases of plant growth.

Seedling Stage

During the seedling stage, plants are developing. The roots are still not developed to take up enough water for the plant. This is why it’s essential you keep the humidity level between 70% and 75%.

Having excessive humidity in the grow tent during the seedling stage will allow your plants to thrive and mature properly.

Vegetation Stage

Although the plants that are in the vegetation stage have their root system well developed, that doesn’t mean they don’t need to absorb water vapor. This is why you need to keep the humidity between 50% and 70%.

If the moisture in the air is too low during this stage, that can lead to stunted growth of your growing plants.

Flowering Stage

During the flowering stage, your plants need the humidity to be average so they have a conducive environment. Average humidity levels are between 40% and 50%. If you maintain it at this level, you won’t have trouble with mold and mildew forming around the flowers.

Harvesting Stage

When your plants reach the harvesting stage, it’s essential you keep the humidity in the grow tent between 45% and 50%. This way, you’re ensuring the crop doesn’t become brittle and harsh.

Raising Humidity Inside The Grow Tent In Different Seasons

a fan inside a ventilation hole in an indoor growing operation

Humidity levels don’t stay the same during all seasons. So, here are a few pointers on how to increase the humidity in the grow tent or grow room during different seasons.


In spring, both the temperature and RH start rising. So, if your grow tent RH starts dropping, try to use a humidifier or some wet towels to keep the RH up.


In summer the RH is relatively high. This means you won’t have to use any extreme methods to increase the grow tent humidity. Just make sure the temperature in your growing space is between 64 and 71 degrees Fahrenheit and the RH will adjust on its own.

If it doesn’t, you can try using open water resources or putting some wet sponges around your grow tent.


In the Fall the temperature starts dropping, and so do the RH levels. This is why it’s essential you keep track of humidity levels and correct it as soon as it starts dropping.


The winter is the driest season. If you’re not prepared, winter can mess up your whole operation. This is why it’s best you install a humidifier as well as an air conditioner so both temperature and RH levels are optimal.


This brings us to the end of our article. We’ve given our all to ensure this article covers everything you need to know about air humidity and how it affects the plants in your tent. We hope you learned something today and that you’re able to take your indoor growing operation to the next level thanks to the resources in this article.

Further Reading

  • For enthusiasts looking to optimize their grow tent setup further, understanding the role of efficient lighting is crucial. Dive into our comprehensive guide on the ‘Best 5×5 LED Grow Lights: My Top Picks for Maximum Yield‘, which explores top lighting options to complement your humidity control efforts, ensuring your plants thrive in the perfect growing conditions.
  • Explore our comprehensive guide on selecting the best humidifier for your grow tent to achieve the perfect humidity levels for optimal plant growth and health.
  • For enthusiasts looking to optimize their grow tent environment further, discover our comprehensive guide on the Best LED Lights for 4×4 Grow Tents, where we delve into top lighting solutions that promote plant growth and enhance yield.
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Written by Brad Smith

CEO & Lead Interior Designer

Brad Smith is an experienced interior designer and the founder of With a Master's degree in Interior Design from Pratt Institute and a passion for creating safe and healthy living spaces, Brad shares his expert insights and innovative design ideas with our readers. His work is driven by the belief that home is where every story begins.