Grow Tent Ventilation Humidity and Heat: What You Need to Know

​When you grow plants indoors, you must protect your plants from excessive heat and humidity, by keeping the growing tent cool via ventilation. Your plants must benefit from adequate humidity, heat and carbon dioxide.




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GROWNEER 4 Inch 200 CFM Inline Duct Mixed Flow Fan
  • ​Low Noise and Variable Speed Controller


BloomGrow Inline Duct Flange Carbon Air Filter
  • ​Durable ceramic-coated
iPower 6 Inch 442 CFM Inline Fan
  • ​6" 1050+ IAV Australia Charcoal Filter 
  • ​6" 442 CFM Ducting Inline Fan Blower

How Will Ventilation in a Grow Tent Help?

​In my experience, ventilation helps rid the tent of excessive heat emitted by light. I have found that water vapor produced by plants can lead to excessive humidity. Using ventilation, I have been able to limit such humidity and ensure that my plants grow well and keep pests and diseases at bay. Ventilation has helped me access CO2 from outside to protect against the reduction of CO2 in the growing tent. Thanks to adequate ventilation, air is blown into the tent and the stems are stabilized. According to Epic Gardening, ventilation is key to healthy plant growth.

​How to Ventilate Your Grow Tent?

​How effective is the ventilation system in your grow room? The answer lies in whether you have proper air flow and circulation which in turn will depend on how well the fans and carbon filters work inside your tent. You must learn how to install a inline duct in your grow tent. You must connect the fans and the filters by using adequate ducting.

​​​​How to Enhance Air Circulation In Your Grow Tent?

​You can enhance the circulation of fresh air by using fans. Your growing room or tent needs a uniform distribution of temperature, humidity, and CO2. I have found that my plant yields could get smaller if I do not ensure such ventilation. That apart, excessive humidity and heat could destroy your crop. Fortunately, we can find fans that have been devised for every specific purpose of use.

​The Role of Oscillating Fans In a Grow Tent

We need to recognize that plants need a cool breeze to help them to transpire. I usually use an oscillating fan to ensure such a breeze while simultaneously neutralizing hot spots through the circulation of air. A medium sized fan should do unless you have a relatively large grow room, in which case you would need more fans.

How many oscillating fans do you need? To find out, count the number of 1000 watt lights you use in the room. Use one fan for every three such lights. You can decide on the optimal use of fans by the presence of air movement in the growing area as well as the simultaneous slight movement of your crops.

​How to Set Up the Fans?

I usually hang the fans on crossbars and use rope ratchets to manipulate the system while ensuring that my plants are minimally disturbed. You can use either an air extractor fan system or oscillating fans.

The latter is a cheap yet effective alternative you can use to provide continuous air circulation within your tent. Use enough of such fans depending on the size of the tent, while ensuring that the fans don’t blow directly on the plants as that could damage the plants through windburn.

You can eliminate oil from the atmosphere and bring in fresh air, thanks to the set-up of an air extractor fan system. After hanging them from the crossbar, I ensure good air circulation by running my extractor fan once every few minutes. If you set up the ventilation system simultaneously, you can minimize interference with your plants.

Ensure Air Entry Through Active and Passive Processes

My experience has taught me that air can enter my tent either through a passive process or via an active drawal. You can create a passive system by blowing out air and allowing re-entry of fresh air through a hole made for the purpose that should be four times the size of the hole for the exhaust to ensure optimal use of the fan and better suction for your ventilation system.

Erik Biksa of Grozine, an expert who has been writing on indoor gardening since 1999, underlines the importance of an exhaust outlet.

If you want to rely on active intake, you will need a fan to blow in the air and another to blow out air, thereby making the size of the intake hole immaterial.

Find the Optimal Level of CO2 In Your Grow Tent

I have discovered that the CO2 level within my tents could be less than ideal; to allow my plants to transpire, I inject carbon dioxide. Before doing so, I ensure that I seal the tent adequately to allow a high concentration of CO2 and more efficient use of CO2 by my plants. Such sealing also helps me prevent leakage of CO2. provides a useful method of calculating the optimal level of CO2 for your grow room.

How To Install Inline Duct In A Grow Tent

If you want help setting up the fan and filter system, look at this useful video tutorial:

How Often Should You Run the Exhaust in a Grow Tent?

According to Leafly’s Patrick Bennett, “Air replacement is just as important as airflow to the overall health of an indoor garden.” An exhaust fan provides a three-fold benefit in your grow room.

It helps cool the room, reduces the humidity level, and ensures fresh injection of CO2 to help your plants grow. I usually analyze the temperature, humidity and CO2 levels in the grow tent in both periods of darkness and light before deciding on a suitable exhaust plan.

How Lights Affect Temperature In A Grow Tent

You must know that a lot of heat can be generated by lights, regardless of whether you use fluorescent lamps or HID lights, especially in a relatively small enclosed space such as a grow tent.

Your plants can suffer if you allow the temperature to rise to 80 degrees under the lights. I have found that the ambient temperature in a grow tent housed within my house usually never drops below 70 degrees.

When I put on the lights in my grow room, the heat rises, in the forms of both convection heat and radiant heat. I am compelled to pump in air-conditioning, or else the temperature is likely to remain high, regardless of whether the exhaust is on or not, though when I keep the exhaust running continually, it helps control humidity and CO2 supply in my grow tent.

Importance of CO2 In A Grow Tent

When the lights are off, we no longer need to worry about the temperature level. However, you would want to ensure a high level of supply of CO2 during the night to help your plants grow.

Plant cells are made of carbon, and for plants to grow the CO2 level in the air must not fall below 250 parts per million; the normal CO2 level in the atmosphere is about 400 ppm. Thus, you must provide access to carbon, if you want your plant to grow. In a closed space such as you’re grow tent, the CO2 level can fall drastically.

Depending on how much air volume you have within the grow room, you can choose to keep the exhaust on for as long as possible to ensure that the air volume on the grow tent is completely renewed at least twice every sixty minutes.

The CFM (cubic feet per minute) rating of your fan will also help you decide the optimal time of its use depending on the size of the grow room. I usually work to renew air on the grow tent — hourly or sometimes more often – and I no longer worry about humidity when the lights are off.

Hydroponic enthusiast Jason has created this useful tool to help growers like you to measure the air volume you need in your grow tent.

How To Install Carbon Filter In A Grow Tent

You can avoid accumulation of odors in the grow room or tent by using a carbon filter attachment on the exhaust fan. From experience, I have found that using an activated carbon filter ensures that air is clean even as it flows efficiently within my grow tent.

Toxic contaminants and odors can emanate from the plant, the soil and any fertilizers we may use. According to GrowWeedEasy, an expert gardener, a carbon filter is more effective at controlling odors than any other method.

The Ideal Size of Carbon Filter

First, you need to get a carbon filter of the size best suited to your needs. You can choose from four-, six-, or eight-inch wide filters. You can connect the filter to your fan by using ducting made of aluminum or other material. Often, you get suitable ducting when you buy the carbon filter.

You can also obtain rope clip hangers and duct clips and duct tape that will ensure that the seal between the fan and the filter is air-tight. You should get an extractor fan to control air flow in the tent.

How To Hang Carbon Filter in Grow Tent

Carbon Filter for a grow tent

Mount the activated carbon filters to your exhaust fans. Hang the duct incline fans and filters using clip hangers; alternatively, attach them to a solid surface on the ceiling of the grow room.

Mount the filter on the intake side of your fan. That way you can rid the system of any contaminant before it reaches the housing of the fan.

To ensure that the denser air is taken in, we must place the intake fan on the floor of the grow room. The exhaust fan will control the warm and moist air at a higher level of the room.

I usually choose to place the carbon filter mounted on the intake side of the fan within the grow room. This allows the filtered air to return to the growing area. Thus, I can ensure that the air is continually cleansed and that odor in the growing area is removed.

​Use an adequate exhaust fan to replace air in the grow room every three minutes, on average. Before deciding on which one to buy, check the fan’s rating based on how many cubic feet it exhausts per minute (CFM).

You can calculate the cubic area of your grow room by multiplying its length by its breadth by its height. You must also consider how efficient your fan is.

You would need to double the cubic area if the path of the exhaust is short and straight. You would need to consider three times the area if the path is long or twisted. Ensure that the CFM of the fan you choose exceeds the area as calculated above. Addition of a filter will justify the need for a larger calculated area.

​You must facilitate air intake through ventilation, as I have already described. You would need a hole that is three or four times larger than the exhaust vent. Alternatively, you can also use an intake fan to go with your exhaust fan.

Find the Optimal Humidity Level for Your Plant

You would need to find out the optimal humidity level for your plant. Greater water vapor produced by larger plants could push the humidity level up. An exhaust fan can push out humid air and help with controlling humidity. You may also opt to invest in a dehumidifier. When I want to increase humidity, I simply seal off the growing tent and keep the exhaust fan switched off.

How to Increase Humidity In A Grow Tent?

humidity and ventilation in your grow tent

Tree Frog, an indoor gardening expert, says it is especially important to maintain proper humidity “in the early stages of plant growth.”

While many indoor gardeners pay a lot of attention to such factors as system cleaning, fertilizers, growth pH, or quality of seed/seedling, they often ignore the humidity within the grow room.

I know that water vapor in the air is very important for the healthy growth of my plants. You must learn how to increase humidity in your grow room or tent, whenever the level falls.

While the roots help plants to absorb water, other parts, such as the stem and the leaves also enable crops to absorb water.

A small opening on the surface of the leaf, called stomata, enables the absorption of water vapor. I have also used sealed propagators to concentrate moisture around my saplings — to allow the seedlings to grow.

Thus, high humidity allows my plants to grow and develop without the root system suffering any stress in absorbing moisture from the air: such stress could stunt the growth of my plants.

Owing to dehydration, stomata may close rendering it hard for the plant to absorb more water. Thus, you must nurture your infant plants by optimizing humidity.

​If there is low humidity in the grow room, your plant can desiccate, exposing it to harm from molds, mildew, and spider mites. In conditions of low humidity, harmful organisms and parasites thrive. Also, in such conditions, roots can work overtime and may absorb extraneous water, causing leaf tips to burn due to the redundant nutrients that can result in such conditions.

​When you set fans to rotate at high speed, air can dry quickly, and water vapor does not settle on leaves. Consequently, your plants can desiccate or dehydrate. By having fewer fans, you can increase the level of humidity in the grow room. You should set fan speed such that your plant enjoys at least 60% humidity.

Thus, high humidity allows my plants to grow and develop without the root system suffering any stress in absorbing moisture from the air. Thus, I can nurture my infant plants by optimizing humidity.

​You can also raise humidity levels by growing larger plants. Take care to avoid overcrowding whereby your younger plants could be robbed of light and nutrient. Larger plants respire, giving out carbon dioxide and water vapor. Water vapor is an especially useful source of nutrient for your younger and smaller crops.

By having more water sources in the tent, you can raise the humidity level. For instance, you can place water trays in the vicinity of the air intake tubing or the passive air holes. But do ensure that the water you use is clean and free of fungi and bacteria.

Options for installing your grow tent fan and filters

How to Lower Humidity In A Grow Tent?

Many indoor gardeners find it hard to reduce humidity in their grow room or tent. You can gain some advantages by growing your plants indoors. For instance, you can grow a variety of plants throughout the year, regardless of the season or climate zone, as a growing room allows you to cultivate your plant in a controlled environment.

Thus, you may even be able to replace conventional soil with hydroponics. Additionally, in a controlled environment, your plants may be better able to absorb nutrients. Further, a controlled environment would allow you to keep your plant safe from the harmful effects of various natural elements such as insect eggs, larvae, extra dirt, mildew, parasites or mold, among others.

​While high humidity allows your plants to remain healthy and grow well, too much humidity can harm the plants. Excessive water vapor in the air can encourage molds to grow, discourage plants from transpiring, cause bacteria to flourish, result in unnecessary biological growths, reduce CO2 intake by clogging the stomata, and reduce the ability of roots to absorb water.

​As I have already explained, the health of your plants will depend on the level of humidity, as much as on ventilation, PH, light, and temperature. There are various ways in which you can regulate the humidity level in your grow room.

​Venting a grow room is crucial to controlling or managing humidity, as fans help the circulation of air and its distribution to your crops. When air is exchanged in and out of the grow room, plants can access fresh air, rendering it impossible for fungi, bacteria, and other harmful microorganisms to stay or increase inside the growing area.

Fans set a moderate speed during the setting up of the grow tent can help in conserving moisture at various phases of the growth of your crop.

​Proper regulation of humidity in the grow room may require you to lower its level at times. You can accomplish this by using a dehumidifier. Air conditioners can also help dehumidify. Increasing the number of fans or raising the speed of rotation of fans will increase ventilation and lower humidity.

Also, you must take care not to overwater the plants, as that would lead to humidity beyond the needed levels. Plants with large leaves raise moisture. So, you can also defoliate in order to lower humidity levels. You can also lower humidity by increasing the absorption level of the soil you use. For instance, if you use sandy soil, it keeps water underneath: evaporation at the surface is reduced.

​You can use a humidifier to generate a cool mist that will, in turn, result in higher moisture in the tent. However, ensure that you exercise continual control to make sure that plants don’t suffer due to excessive humidity. Some humidifiers have a built-in humidistat that allows the devices to moisture in and out automatically; they simultaneously take the needs of a plant into account. Some humidifiers come accompanied by large tanks with a holding capacity of six liters of water.

grow tent fan and filter infographic

How to Increase Temperature Or Cool A Grow Tent

Ventilation, fan, and filter for grow tent

Maximum Yield emphasizes the impact of atmospheric conditions “on the quality and quantity of the finished crop.”

I have found it a challenge to regulate temperature. When the temperature is lower, there is higher moisture. While hot air rises, the relative high density of cold air causes it to descend to the bottom. So, if you wish to reduce the temperature in the grow tent, adjust the temperature on your heater or air conditioner.

Lower the intensity of the ultraviolet light, keeping it just high enough to allow water to evaporate. Fortunately, young plants do not crave a lot of light. A swamp cooler can help you in dual ways, reducing temperature and raising humidity, simultaneously.

The ambient temperature in your grow room can vary depending on various factors. Outdoor temperature may have a varying effect on your grow room temperature depending on whether it’s a grow tent or a room within your house. If you are using HID lights or do not provide much insulation to your grow room, the temperature within the grow room can be affected greatly by the temperature outside.

Because of excessive heat in summer, water could be lost; during winter, grow room temperatures can reduce to 40 degrees, which could lead to more moisture and consequently to stunted growth and the presence of mold due to humidity. Internal factors such as the heat from your grow light or the lack of it could also affect ambient temperature.

We must avoid too much heat as well as too little. We must aim for the ideal temperature level at which photosynthesis takes place in plants. That would allow absorption of nutrient and conversion of such moisture into the energy your plants need in order to grow well. The ideal temperature level can vary depending on the stage of growth of your plant.

For instance, a clone or seedling would need temperature ranging between 72 and 82 degrees; in the vegging or flowering stage, the temperature range would have to be between 68 and 78 degrees.

​If the canopy (leaf) level temperature is too high, photosynthesis cannot take place: you must ensure that the canopy temperature and ambient temperature are roughly the same. To avoid overheating the canopy, keep your HID grow lights at a reasonable distance.

But ensure that they are not so far away as to cause the canopy temperature to become cold as that could stunt the growth of your plant. If you are using LED grow lights, your canopy temperature may not be very high. So, depending on the type of light you use you can regulate the temperature via heaters or AC’s.

​Ambient temperature also affects the temperature of your grow medium (soil and water). To ensure the healthy growth of your crops, make sure that the temperature of the grow medium does not exceed the ambient temperature: in fact, you could make it a few degrees cooler.

If it’s too cold, your plant could see stunted growth, root rot, and waterlogging. If the temperature of your grow medium is too hot, mold and root rot could result, and the roots could get fried, leading to the death of your crop.


If you want your crop to grow healthily, you must ventilate your grow room adequately, as proper air circulation can help you to regulate humidity, temperature and CO2 levels within your grow room. You must set up an air extractor fan system that can provide both passive and active ways to ensure entry of air into your grow room.

You must find the optimal CO2 level for your plant and rely on a carbon filtration system to help avoid odors. You can use ducts to connect your fans and filters. You must also regulate the humidity level within the grow tent, as it is crucial to the health of your crop. Too much or little humidity can both be harmful.

You must also regulate the temperature inside your grow area. You must especially ensure that the canopy (leaf) level temperature is roughly equal to the ambient (room) temperature. Thus, your success as an indoor gardener could depend on how well you ventilate your grow room.

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Karen Miller

Karen Miller has been gardening and growing indoors since she was a child.Over the past few years, she has really started sharing her fast knowledge of indoor growing equipment.From grow lights, tents, ventilation, and nutrients you can count on Karen to have either used it or grown it.

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