Ventilation in weed grow tents: How much ventilation do I need?

If you’re setting up a weed grow tent, it’s important to consider how much ventilation you will need. We can help you determine the proper ventilation for grow tents so you can grow top-quality marijuana.

Grow tents require two types of fans. Ventilation fans remove humid, stale air to the outside of the tent and allow fresh air and CO2 to enter. Ventilation fans can even cool air-cooled light hoods as part of a passive exhaust system.

You’ll also need circulation fans within the tent to move air around, but that’s a separate topic — learn more about what size fans are needed air circulation or read grow tent fan reviews to find one that’s best for your needs.

Why is ventilation important in weed grow tents?

Proper exhaust ventilation in grow tents helps eliminate excess heat and moisture while supplying fresh air full of necessary carbon dioxide (CO2).

Too much heat will damage plants and affect growth. Too much moisture leads to mold and mildew problems, and increases the chances of other problems. The incoming fresh air allows the plants access to CO2, a basic requirement for the plants to process energy.

How much ventilation is needed in grow tents?

This grow tent setup uses two fans: a four inch fan attached to a carbon filter to exhaust air, and a second six inch fan attached to an air-cooled light fixture to remove excess heat from the 600 watt HPS bulb.
This grow tent setup uses two fans: a four inch fan attached to a carbon filter to exhaust air, and a second six inch fan attached to an air-cooled light fixture to remove excess heat from the 600 watt HPS bulb.

It’s important that you plan how much ventilation you need in your grow tent to create a growing environment where your marijuana plants can prosper.

Your goal is to be able to adjust the grow room conditions to as near perfect as possible for optimal marijuana growth and potency. A digital thermometer and hygrometer is a must. Look for models that track lows and highs for both so you can understand the range that your plants experience when the lights are off. To low or high humidity or temperatures can lead to problems.

How to calculate the amount of exhaust ventilation needed in grow tents

How to choose the correct inline fan size for your grow tent/room

Inline fans used in grow tents to provide exhaust are rated by the volume of air they can remove. Air volume is measured in Cubic Feet per Minute, or CFM’s. The bigger the fan, the more air it can remove.

You should always buy a fan that is bigger than you think you’ll need. It’s cheaper to spend a little more on a 6″ fan up front than to buy a 4″ fan and have to upgrade anyway.

Look for models with multiple fan speeds. Since most of the less expensive inline fans used by marijuana growers are a single speed (100%), using a fan controller can give you control to reduce fan speeds. Learn how to select the right fan controller. Fan controllers reduce vibration and noise, another benefit.

How to calculate the volume of air in your grow tent

The proper amount of exhausted air depends on the volume within your tent. To calculate the volume of air to remove, multiply the height, width, and length of your grow tent which tells you the tent’s volume.  Your goal is for your exhaust system to replace the air in your tent every one minuteto supply enough fresh carbon dioxide and to control moisture and temperature.

For example, if you use4x4x7 grow tent, you’d need to remove 112 CFM per minute. A 4x8x7 tent would need to remove 224 CFM per minute.  But there’s more to it than that.  You need to factor in how air moves through the exhaust vents.

Air resistance

But that doesn’t necessarily mean you’d only want a fan rated exactly for the volume of your tent. You also need to take into account the efficiency of your vents themselves. Your venting setup can have a big impact on the actual amount of air your fan can remove.

Air encounters resistance when being fed through venting ductwork. Short, straight venting runs are more efficient than long runs or those with lots of curves. We’re not going to get into the math for how to calculate this resistance. Instead as a rough calculation you can just double the volume of your grow tent for short vent runs.

If you have a long run or lots of curves, you should plan to exhaust three times your tent’s volume each minute. For very long runs, plan to add a small duct fan which attaches to the other end of your duct run to provide additional help for your inline fan.

Where to position exhaust fans in your grow tent

Your carbon filter should be placed at the top of the grow tent so that it can exhaust the hottest air in the grow environment.
Always place the carbon filter at the top of the tent so that it pulls out the hot air which rises to the top. Fresh air should enter the tent through a vent at the bottom to allow for good air circulation.

Since heat rises, exhaust fans should pull from the top of the tent. Air intake should come from tent openings at the bottom.

If you’re using a carbon filter, mount it on the ceiling with bungee cords.  Use flexible aluminum ducting to connect it to the fan, which should be mounted outside of the grow tent.

For tents using an air cooled light, you can then push the air coming out of the inline fan back into the tent and through the light hood. The hood then would be connected to vent ducting that leads outside of the tent. This requires tents with at least 3 duct openings at the top. Before you purchase a tent, make sure to fully consider the rest of your setup to ensure the tent meets your needs.

Never have your tent’s exhaust fans pull from the bottom of the tent, this will increase heat in the tent. You want to draw the hot air out at the top and pull in cool air at the bottom, which then will get pulled up to the top again. This cycle ensures a stream of fresh air across your plants, which is then pushed around by the circulation fans in your tent.

Automated ventilation controls

There are several ways that marijuana growers approach ventilation. Most small tent grows should keep exhaust fans running all the time to keep introducing fresh air and to prevent heat or moisture buildup.

You can also purchase fan controllers that turn on when certain temperature and humidity levels are reached. Be careful about carbon dioxide with this type of setup, since when the lights are off the temperatures may be cooler but plants still will need fresh air.

Advanced growers may use sealed rooms that supply carbon dioxide directly to the plants. These rooms also utilize air conditioning for heat control, and humidifiers/dehumidifiers for moisture control. These setups are often used with a series of high-powered 1000 watt lights that produce a lot of heat; the additional available CO2 allows plants to grow in higher temperatures.

Reducing noise from vent fans and vents

Exhaust systems using powerful inline fans can be quite noisy.

There are several approaches to quieting fans and the noise of moving air, including fan controllers, insulating your ducts, mounting equipment using bungee cords, and exhaust mufflers.

Learn how to pick the best fan controller, or get more details on how to reduce noise from inline fans within grow tents.


Proper ventilation in grow tents for marijuana is a big factor in the quantity, quality, and potency of your grow. With a little bit of effort your can provide the perfect conditions to grow great marijuana indoors.

Ventilation will let you control temperature, humidity, and the intake of fresh air while reducing the chances of unwanted mold, pests, or disease.

Ventilation in weed grow tents: How much ventilation do I need?
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Ventilation in weed grow tents: How much ventilation do I need?
We can help you determine the proper ventilation for your grow room or grow tent so you can grow top-quality marijuana.
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Happy Pot Farmer
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