Ventilation: getting good air flow for your cannabis grow

Basic plant botany tells us that plants absorb carbon dioxide and release oxygen. Roots need oxygen  so both must be present in the grow room for plant survival. In addition to this, leaves represent the plants lungs: if the stomata (pores) become clogged with poisons expelled by the plant during its transpiration, the leaves will die, and soon after so will your lovely marijuana plants. Therefore it is vital that there is an air exchange passing through your grow room, even if it  is only produced by the opening and shutting of the grow room door. However the more plants per cubic metre of space, combined with the need to extract heat produced by HID lights, means that even in the smallest of LED grows some kind of artificial ventilation will be necessary. Good ventilation also lessens the humidity of your grow room and the risk of disease to your marijuana crop.




Invest in an extractor fan system.

This needs to be large enough to exchange the total volume of air in the room every few minutes e.g., for a grow tent 1.2m by 1.2m by 2m, the total volume of air = 2.88m3. Assuming that this volume must be changed every 2 minutes, the hourly capacity of the ventilator is 2.88 x 30, which is 86.4m3/ hr.   The amount of air that can be moved by any particular extractor is known as its CFM rating.

HOWEVER this figure only applies to extraction of air. In order to remove heat (when using HPS grow light systems) you need to more than double this figure.

What size extractor do I need for my grow room?
  • 60cm  x 60cm (23″ x 23″) grow tent OR a 80cm (31″) x 80cm (31″) grow tent:  a 250 watt HID with this 4″ exhaust fan and a controller for the smaller tent (you probably won’t need a controller in the 31″ x 31″ area).




If you are growing cannabis using LEDs, then you only need an extractor to exhaust stale air and draw in fresh air as you will have no heat issues.  I’d recommend something like the

Premium 4″ Inline Air Blower Fan – Suitable for Hydroponics Ducting or other Exhaust System

for all grow sizes up to using 1000 watts of LED in a 50″ x 50″ area. You will still need to use a Speed Controller to regulate the volume of air extracted from the room.

Obviously you need to plan your extraction route when you choose and build your marijuana grow room. Hot air rises so it makes sense to place the extractor at the top of the room; air intake however should be nearer the floor. Make the air intake tubing slightly larger than the out-take, and that it is “light tight”, with a 90º bend at some point. Use flexible tubing to connect the extractor to a “safe” air outlet (i.e. not aiming at your neighbor’s bedroom), and you will need to consider odor control as well. Carbon filters fit onto the exhaust fan and filter out the smell.


Fans create movement of air

Oscillating fans do not create enough air movement to cool a room, but they do circulate it within the room, helping to minimise hot spots. They also serve a useful function to the marijuana plant, as they simulate fresh breezes and stimulate stronger stem growth. Fresh air movement also helps prevent mold and fungal disease, particularly to buds.  They

A wall mounted fan like this one is great: one will be sufficient for a smaller grow room, but for either of the larger areas, you will need 2.


Consider CO2

All experienced growers admit that the addition of extra C02 into a grow room will have significant effects on the yield of the marijuana crop. This is because whilst the levels of C02 in the air are on average 390 ppm (parts per million), marijuana can synthesize 1500-2000 ppm   C02 kits can be quite expensive, it is worth evaluating the additional benefits to your size of grow room against the start up cost.

Bear in mind that you will want your C02 added at differing times to your air extraction: no point just pumping it out onto your roof top, so you will need additional timers so that the extractor is off whilst the  C02  is on.
You will also need a thermostat as with  C02 the room temperature can be raised by 5ºC.


As many of the diseases capable of destroying cannabis are the result of a humid environment, its essential to use a humidity gauge (see vegetative growth). And if your grow room is too moist, then you should add a dehumidifier. This will not only protect your plants, but also helps them produce more resin. Resin production increases when the plant grows in a drier atmosphere: it acts like a moisturizer, keeping moisture in the leaf.


Ideal relative humidity levels are:

  • 80 – 90% during cloning stage
  • 60 – 70% during vegetative growth
  • 40 – 60% during flowering.


Also read Why airflow is so important for your grow room


If you’re one of our UK readers, then we suggest you look at this piece of kit here:

Rhino Pro Fan & Filter Kit-10″/250mm

A great all in one kit for an 18msq room. It will exchange the air in this size area 60 times an air and ensure no smells escape the grow area .. so you can grow securely!



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