In nature autoflowering cannabis plants get fresh air full with Co2 all the time. The wind blows and supplies cannabis with all the necessary Co2 and it also circulates the air around the leaves, dispersing heat and destroying insects. Outside air circulation also increases plant strength and helps male pollen to land on female cannabis flowers to produce seeds. Inside however growers need to supply all this artificially and the air circulation must be very close to what the plant would get outside so you need to install fans and produce the wind yourself.

Why is ventilation necessary?

Fast growing autoflowering cannabis will need a lot of water and light, but both of these aspects can generate negative things if the auto plant is grown indoors. First of all the light emits heat but cannabis plants don’t like temperatures above 32C (86F) and if the air is not vented out of the growing area then the temperatures will raise fast! Second thing is the water that we use for watering the plants. Autoflower cannabis plants evaporate almost 99% of the absorbed water in the air and the humidity can rise quickly. If the humidity gets too high the plant will struggle to grow and insects, mold and fungus risk will increase dramatically.

Final thing why air circulation is so crucial is the Co2 amount that is in the air. Ruderalis leafs will use all the Co2 around them really fast and if the air doesn’t circulate in the room they will struggle to produce their food from photosynthesis. Photosynthesis is the process where a plant uses water, Co2 and energy from light to produce sugars and oxygen and if the Co2 is not available to cannabis plant then this process is slowed down or even stopped and plant struggles to survive.

How to ventilate grow room?

It is really not a big difference if you are growing in a small closet, PC case, fridge or a large 100 square feet room, you need to properly ventilate the area and buy a fan that can manage that.

All the fans that you can find are measured by how much air either in cubic feet per minute (CFM) or in cubic meters per hour (m3/hr) they are able to move. A good practice for all growers is to put a vent fan that can blow out all the air from grow-room in less than 1 minute.

Before buying a fan you need to measure your growing space and calculate how much cubic feet of air it can hold – this calculation is done by multiplying the height , length and width of your growing area. After these calculations you need to find out how much air can your selected fan move and see if it can move all of the air inside your room in less than one minute (preferably 2 or 3 times per minute). So if you measure your grow room and it is 20 cubic feet then you need to buy a exhaust fan that can move at least 40 CFM.

Some growers can manage with only one fan that blows the air out from the growing room and pulls fresh air inside from cracks or holes. This system is very efficient and no air will get outside the room from those cracks and holes because the pressure will suck all the air in. If you are growing in small space and you have two fans the best solution is to use both fans for exhaust and cut a hole for intake because you will get twice as much airflow as you would get if one is blowing in and one out.

Some grow operations need another fan that pulls air inside the room but it has to be smaller and weaker than the exhaust fan in order to lower the pressure inside the grow room. Basically low pressure is created when an exhaust fan blows air outside the room and the intake fan or hole is smaller than the exhaust. When this occurs then inside air pressure becomes negative sucking in air through every hole. Low air pressure in the room is perfect for cannabis odor control because the only air that gets out of the room is the air that is exhausted through the vent and you can easily fit a carbon filter there to filter out the smell.

Where to put exhaust, intake fans?

Exhaust fan must be mounted as high as possible because hot air is lighter than cold air and raises to the top of your grow area. This hot air must be exhausted and the exhaust fan that is mounted right below the top of your grow room will perform that better.

Intake hole or fan must be at the very bottom of the grow room to take in the cold air and produce a better air circulation in the grow-room. The intake hole/vent performs the air circulation by letting the cold air inside the room and blowing it on the plants. This air then is heated up and exhausted at the top completing the air circulation chain.

Oscillating fan?

Oscillating fans provide the air flow in the growing room. Intake, exhaust fans pull air in and blow it out of the room but if there is nothing that mixes this air inside the room there can be some hot and cold layers forming and also the plants can’t get all the Co2 that they need. The oscillating fan blows fresh air on the plants and provides them with Co2 rich air and also strengthens them up by moving them in the wind.

If you are thinking of using some kind of a simple PC fan for your grow room air circulation then you must somehow let it spin or move or else it will blow in one direction and that can harm the autoflower cannabis plants.

Light, odor control

In small or big grow room autoflowering cannabis smell and light leaks can be very unpleasant so you need to think about these problems when installing the ventilation system.

Odor control usually happens at the exhaust fan and it is usually some kind of a tube mounted device. For small grow rooms the best way to control your cannabis smell is to put a carbon filter on the outside of the exhaust fan and let it force the air through the filter. If you will use a filter you must know that the ventilator will loose power and won’t be able to move as much air through the filter as it would normally so you need to install much stronger exhaust vent than you would normally need.

Light leak is also another problem for inside growers but it can easily be repaired.

To control light leaks you must construct some kind of a light blocker device. A good way to not let the light escape is to construct a Z shape (two 90 degree bend) tube at the inside or outside of the intake and exhaust holes to keep the light leaks to minimum. A 90 degree bend will unfortunately lower exhausted air amount and efficiency because these bends once again reduces the fans strength and you need to install more powerful fans if you want to fit a device like this.

If you put a carbon filter on your exhaust fan it will also fix the light problem so by fitting a filter you can hit two rabbits with one shot!


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  3. laurent godon on


    Great informative site, thank you !

    I don’t understand this though :
    “For small grow rooms the best way to control your cannabis smell is to put a carbon filter on the outside of the exhaust fan and let it force the air through the filter”

    If the exhaust fan forces air into the filter, then how does the filtered air is supposed to be extracted ?
    In other words, it’s like : air > exhaust in > filter > ?
    Because I’m left with the filter blowing filtered air int the tent, right ?

    Please help ?
    Thank you

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