How to Lower Humidity in Grow Tent When It’s too High

Humidity- the blessing and at the same time, the curse for an indoor grow system. Just like temperature, humidity has an important role to play when it’s about the quality and quantity of your indoor gardening yield. 

Having said that, when the grow tent humidity gets too high, it might bring on the worst nightmare for you as a grower. Indoor gardening rookies often underlook this and end up with problems like powdery mildew or bud root etc. 

We hope, you are not one of them who’d regret the entire money, time and effort just because they couldn’t deal with a situation like this.

This particular article is, however, the end result of our week-long research on EVERY possible way to keep the humidity of your grow room under control. We’ve provided both quick and long term fixes, with the list of reasons that might induce humid air anyway. 

Stay sticked until the end. 

Why? Why? Why?

So, we’ve got a situation where there is too much humidity in grow tent. But the whole incident was not a bolt from the blue. It must have some reasons why the Rh level had got high, and understanding the reasons is quite important to figure a way out.

Agreed? Here is a list of potential suspects-

Increased Transpiration

As plants grow bigger, so does the size of their leaves. And it’s a botany 101 that larger leaves will exhale more vapor in the air. If you don’t cope with your ventilation system up accordingly, the humidity will raise immediately.

This can also take place if you increase the number of plants all of a sudden.

Open Water Surfaces 

Open water surfaces are direct suppliers of water vapor into the air. It can be the nutrient reservoir, the wet floor of the room or just any other container of water. In case it’s near an oscillating fan, the airflow over the surface will turn more water into vapor, and bammm! Thermodynamics is in action. 

Too Humid Outside Environment

The average humidity in Alaska is 77.1%, and in Florida, that’s 74.5%. The list continues with states of Louisiana, Mississippi, Hawaii, Michigan, etc. (Here is the full report)

This quite explains how growing in a grow room in these kinds of places is hard. The slightest leak in the insulation system between the grow space and the outside environment will give you a surprising rise in the humidity. 


Overwatering the plants will make the soil soggy, and it will also force the plant leaves to leave more vapor in the air. Both wet soil and over-transpiring plant leaves are dangerously liable to raise the grow tent too humid. 

High Humidity is Bad, Here’s Why- 

There are a number of vulnerable effects of too low humidity in a grow room. But too much of it isn’t a blessing either. Here are some quick results when your grow room gets too humid- 

  • Unwanted biological growth like bud molds, powdery mildews, etc.
  • Too much moisture buildup will welcome bud rotting or flower rotting, which can’t be seen until you’re in the harvesting stage.  
  • Restricting the plant’s ability to intake CO2, followed by restricting photosynthesis.
  • A vapor-saturated air hinders further transpiration process of plants. Resultantly, the overall plant metabolism rate decreases.
  • Plants love humid air while they’re tender, and less humid air while they’re mature. If it still remains in high humidity while grown up, it will prohibit the growth rate and health quality of the plants.    

How High is ‘Too High’ for Grow Room Humidity?

As we know, humidity level varies from growth stage to stage. So, it’s not possible to assign a fixed ‘Rh Percentage’ as an indicator between ideal and too high humidity level.

But the rule of thumb is, the overall range of humidity in a grow room lies below 60%. A raise of humidity level above 60% can bring on several damages to the crop, which we’ve just discussed in the previous section.

However, to have a clear concept of what’s the strict Rh limit of each grow stages, here is a quick stage-wise humidity range chart- 

The Correct Humidity Levels for Different Grow Stages

Germination/Cloning: Rh 75-85%

At this stage, the initial growth of the plant’s roots takes place. So a high humidity level is mandatory. Humidity domes are often used to increase the Rh of the imminent air above the seedlings.

Vegetation: Rh 40-60%

Humidity has to be a bit high because grow plant leaves would require enough water in the air. A partial portion of the water consumption is done through roots as well. But as they’re not fully developed yet, leaves have to keep intaking water. 

Flowering: Rh 35-50%

This is the stage where high Rh starts to show it’s bad signs. So it’s very crucial from this point to ensure no raise in humidity. Otherwise, molds, rotten buds, etc will start to take place. 

At the beginning of the flowering stage, the Relative humidity should be kept within 40-50%. But as the buds start to produce a harvest, growers lower it down to 35-40%. And this has to be done gradually without any major, sudden drop in the Rh. 

Harvesting/Drying: 30-40%

At this stage, crops start to dry and result in harvest. But still, we have to keep a 30-40% humidity level not to let the buds dry too fast. Some growers love to stick to a bit higher humidity(50%) in order to let his buds dry slower. This is anyway, improves the quality of the cured buds. 

Germination/CloningVegetation Flowering Harvesting/Drying
75-85% Rh40-60% Rh35-50% Rh30-40% Rh

How to Decrease Humidity in Grow Tent/Room?

Active Ways

When you’re seeking steps on how to bring humidity down in grow room or room, things are just not easy to deal with. Any step will take enough time to take the Rh down. So you have to stick to the quickest possible ones.

The couple of methods that we’re about to discuss are, particularly for such situations. Here you go-

Use A Dehumidifier

It’s quite obvious that to turn humidity down, you’ve got to call our friend in need- the grow room dehumidifier

But before putting your money on just any dehumidifier, take a minute to think about the right size for your grow room setup. 

So, what defines a good dehumidifier that can take the exact humidity out that you need to? Here go some points of consideration- 

  1. It should be of the exact dehumidifying capacity that your grow system requires for. Technically, it’s supposed to take off the same amount of vapor that your plants transpire through their leaves. This capacity is measured in pints
  2. If you’re living in a humid area, make sure your dehumidifier can be attached to the drainage system. 
  3. It should have a large enough water tank. 
  4. It should sport an automatic timer or an automatic shut off feature(when the tank is full). 
  5. It should be gentle to the electricity bills. 

Once you’ve got the right size of a dehumidifier, you’ve got to put it at the right place anyway. Also, avoid plugging these dehumidifiers to the electrical extension leads.  

Our favorite dehumidifier for grow room: Ivation GDM30
Our favorite dehumidifier for grow room: hOmeLabs 1,500 Sq. Ft Energy Star Dehumidifier

Use An Air Conditioner

We know that the temperature and the humidity of a grow room go hand to hand. If the temperature rises, the ability to hold water vapor in it raises too, and you see a direct boost of the Rh in your room. 

On the other hand, if you manage to bring the temperature down, it will remove humidity from grow tent/room at a good extent. 

Having said that, using the right size of AC units is crucially important. Otherwise, it will either end up in condensation or will still be unable to control the temperature and humidity. 

Find more about this later in this post. 

Passive Ways

For smaller grow systems like 4×4, 5×5, 8×8 grow tents, using a dehumidifier or AC is not a convenient solution to take humidity down. There are several passive ways that are well enough to deal with this problem in such indoor grow systems. 

We’ve got a list of 3 actionable passive ways to do that. Have a look- 

Boost the Ventilation Up

Talking about passive ways of lowering humidity level down, boosting the air circulation tops the list. One of the root causes of excess humidity buildup in your grow room is the inability to vent humid air out at proper rate. 

Simply put- exchanging the humid air with the fresh, ambient outside air will put the Rh percentage down. 

Once you’ve got a high humidity situation, these are the things to do in order to boost the ventilation- 

  • Widen your intake hole, or create a new one. This will let more air in, and increase the efficiency of an exhaust fan. In case you use intake fan instead of holes, boost up the power(CFM) of it. 
  • Instead of using ‘just’ oscillating wall fans, using floor fans as air circulators throughout the grow room is an effective idea. These fans can pull air through the plant canopy and ensure a proper exchange of old, humid air with fresh ones. 
  • Use an atmospheric controller with humidity settings. It will automatically set the exhaust/inlet fan speed according to the humidity level of the room.
  • Put inline fans in charge of taking humid air out. More importantly, make sure that the fan is powerful enough, and is of the same diameter to your ducting. Keep an eye on the noise reduction of this buzzy thing as well.  

Use A Thermostatically Controlled Block Heater

Got a high humidity in grow tent at night?

Spikes in Rh level and condensation can take place more often during the dark hours comparing to the light hours. As you turn the lights off, the temperature goes down. With enough of a drop in temperature, you would see moisture buildup around the lights, which is termed as humidity fluctuation. 


When the lights go off in your grow room, using a thermostatically controlled block heater to keep the temperature close to the light-on level. 

Defoliate the Plant Leaves  

We’ve mentioned that when plant leaves grow bigger they become a bigger source of water transpiration into the nearby air. Result? Quick raise in grow room humidity. 

As a passive fix of getting humidity down in grow tent, you can defoliate some of the extremely large plant leaves. While removing leaves, you have to leave enough of them to the plants so that processes like photosynthesis still goes on normally. 

Comparing the expense, it’s a quite cheap way to lower humidity in grow tent as well.

Things to Do to Avoid Further Raise in Humidity

How can i lower the humidity in my grow tent for good? Well, here are some actions that you can take to prevent further raise in grow tent humidity

Don’t Depend on One-dimensional Ventilation

A humidity 101 is, if a proper ventilation goes on across the entire grow room, the air will never be saturated with water vapor. To make sure it happens, you can’t depend on the oscillating fans only. Because they can only reduce the temperature of the plant canopy. 

So, how do you induce a multi-dimensional ventilation system? Well, here are some suggestions- 

  1. Ensure multiple inlet holes if you’re growing in a tent. In case of room, source air through more than one intake fans.
  2. Ensure proper air circulation throughout both the bottom(floor-fans) and top part(wall-mount oscillating fans) of the plant canopy. Don’t let the CO2-rich, heavy air build up on the floor.  

Deduce Exposed Water Surfaces

It’s a quite simple and cheap hack to execute but turns out to be super effective in order to lower down the grow room humidity. And that is to deduce naked water surfaces in the grow room. 

Now, from where these exposed water surfaces come? 

  1. Stagnant water that doesn’t pool on the ground.
  2. Water reservoirs. 

Do deal with the first one, you need to ensure a proper drainage system throughout the entire grow system. And to terminate the second one, simply put covers on each and every water reservoirs you’ve got in the room.

Insulate Your Grow Space from Outside Humidity

Sometimes, you see the humidity level goes higher and higher no matter how much effort you put after putting it down.

That’s because, maybe, your indoor garden is in direct touch with a too-humid outside environment. The area where you live in might have a higher humidity than usual, just like New Mexico or Colorado. 

To fix this up, commercial growers often use barriers like foam insulators, etc to prevent this situation. When it’s about a grow tent instead of grow room, insulating the grow area becomes way easier though.

Use Soil That Absorbs Vapor

To get rid of humidity in grow tent, an effective step can be using soil that can absorb water and keep the Rh underneath. A good example of such soil is sandy soil. 

However, make sure your plants don’t have any issues in growing in such mediums. 

Use The Right Size of AC Units

Both oversized and undersized AC units may give a spike to the temperature and humidity level. 

For oversized AC units, the dead band turns out to be pretty short, which gives the nearby air a rise in humidity. And the undersized AC units, on the other hand, induces withering as a problem for indoor plants. Which might create a problematic climate for the plants, high humidity being one of them. 

Summary line? Always use the right size of AC units that can run consistent cycles for the longest period. 

Keep an Eye on Your Humidity Level

This is not a ‘prevention’ to reduce humidity in grow tent. But using a hygrometer can keep your eyes on the exact humidity level your tent/room is going through.

There are thermometer-hygrometer combos available in the market. With one of those hung up on your grow tent wall, you can kill two birds with the same stone. 

Here are some of our favorite picks of digital thermo-hygrometer combos not to let the humidity in grow tent too high– 

  1. ThermoPro TP65 Digital Wireless Hygrometer
  2. AcuRite 00613 Indoor Thermometer & Hygrometer

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