Air circulation is one of those aspects of growing cannabis that is easy to misunderstand or not pay enough attention to, especially in the beginning when one isn't yet familiar with growing and the equipment. Setting up a functioning indoor grow room is the first step towards growing awesome cannabis, and ventilation is key for achieving that. Let's discuss why indoor cultivation spaces require proper ventilation and how to set up a circulating system that always provides your plants with enough fresh air.
Lighting and ventilation are the two most important things about setting up an indoor grow room. They tend to work together and allow the grower to control environmental aspects of the grow room such as temperature and humidity. Aside from strains, nutrients, pots and all those other necessities of a grow op, without the right lighting and ventilation it becomes a real challenge to yield decent weed.
A proper indoor grow room ventilation system makes good use of oscillating fans, as well as inline and exhaust fans fitted with carbon filters to draw fresh air into the grow space while controlling the strong smell as stale air is pulled out.
Cannabis plants need a constant supply of fresh air in order to function correctly. CO2 gets absorbed by the leaves and is converted into sugars for the plant through photosynthesis. Oxygen is released back through the pores (stomata) on the undersides of the leaves, and is finally drawn out to allow more CO2 to ventilate into the grow room.
Like for humans, air is critical for the health and survival of a cannabis plant. Without the right amounts the organism is likely to suffer and can display all sorts of different problems, such as slow growth, deficiencies, low yields, or even death.
As cannabis plants can 'breathe', so can they 'sweat'. As a cannabis plant drinks water, it is natural that some of that water gets lost through transpiration and evaporation. The process takes place through the leaves, but also through pores in the stems (called lenticels). Transpiration increases humidity in the space, which can eventually build up to dangerous levels if the moisture levels are not stabilised.
Transpiration happens mostly during the night when photosynthesis is not taking place. This is why it is common to notice moisture on your plants early in the morning right before the lights come on. As the sunlight intensifies and heat increases throughout the day, rates of photosynthesis speed up and the plant works to retain and use most of the moisture it has available.
Fresh air is necessary for plants to photosynthesise efficiently. The functionality of the gas exchange happening in the leaves is driven by the plant's photosynthetic health, which is also why light and water are just as important as air. Photosynthesis allows CO2 to be absorbed by the plant during the day, while at night it regulates itself primarily through transpiration.
Having control of the air circulation allows a grower to have more accurate control over humidity and temperature in the grow room. Optimum humidity and temperature levels are necessary for a cannabis plant to uptake water and transpire at the correct rate.
Get a hygrometer that measures both humidity and temperature and place the probe or device itself close to the canopy area. You may want to distribute 2 or 3 around the grow room to get a general reading and to see if air and moisture are being distributed evenly.
Having full control over these aspects of the environment is important because as the grow progresses we need to make adjustments. For most cannabis strains, the humidity levels for each stage of growth should fall within the following ranges:
| Stage | RH (Relative Humidity) | Day-Time Temperature |
| Clones | 60-80% | 22-30°C |
| Seedlings | 60-75% | 22-28°C |
| Vegetation | 60-65% | 21-27°C |
| Flowering | 45-60% | 19-26°C |
| Pre-Harvest | 40-50% | 18-25°C |
Tip: Lower temperatures at night are recommended. Try to gradually reduce it as you reach the end of flowering (as in winter outside), but avoid temperatures below 15-16°C if possible.
Stale, stagnant air is an inviting opportunity for pests and mold to settle. Growers can strategically place oscillating fans around the grow room, keeping in mind to cover crevices and corners where air may not be moving frequently enough.
Circulating the air thoroughly can help to reduce the chance of molds such as powdery mildew or botrytis, both of which can be extremely detrimental to cannabis plants in overly humid spaces.
The power of the inline and exhaust fans you need depends on the size of the grow space in question, but can be calculated using some basic multiplication. To measure the cubic feet of your available area, simply multiply its length by the width by the height. Here's an example for a 1.5 x 1.5 x 2m grow tent in feet (approx.):
5 feet (length) x 5 feet (width) x 6.5 feet (height) = 162.5ft³
You want to make sure you are replenishing the air in the grow space at least once every 1-2 minutes. This can be achieved measuring your grow space in cubic feet and choosing a suitable fan to match (measured in CFM or cubic feet per minute). In this example we would need a exhaust fan with at least 160CFM to be able to replace the air once each minute.
Aside from their power, extractor fans can be categorised based on their size. Most extractor fans for grow tents come in 4', 6' or 8' diameters. 6' tends to be the most commonly used size as it matches the size of most light fixtures with cooling hoods.
Passive air intakes refers to air drawn in naturally as air is extracted from a particular space. Through negative pressure caused by an exhaust fan pulling air out, air is pulled in through small holes or gaps to replace the 'empty' airspace. Negative pressure is caused when more air is being drawn out of space than is flowing in. With this in mind, we can adjust the air pressure using inline fans so our exhaust fans and carbon filter can work more efficiently together.
If you choose not to use an inline fan and work only with the extractor and passive intake, then there must be enough openings in the room to pull fresh air into the space fast enough in relation to how quickly it is being vented out, otherwise the fan has to work extremely hard.
To ensure this doesn't happen, make sure the total diameter of the inline holes is at least 3 times as much as the diameter of the extractor fan. For example, if you use a 6' fan, you need either one 18' hole, two 9' holes or three 6' holes.
Active air intakes works using inline fans to pull air from outside of the grow space to the inside. These fans can be set at a lower speed than the extractor fans to create a space with slightly negative pressure, which can help one manage aspects of the environment more easily. Although not entirely necessary, an inline fan is well recommended if you're looking for full control over the air circulation in your grow room.
Positive pressure in a grow room would be the result of pulling air in too quickly for the extractor fan to expel it. This takes away our ability to mess around with the conditions because the air is being removed before we, or our plants can do anything with it. Positive pressure results in a loss of fan efficiency and odor is likely to be pulled out of the grow room along with the air.
There are 4 main parts to a well ventilated space, each serving a different purpose. Air is pulled into the grow room using inline fans and is allowed to circulate around the plants using oscillating fans before being vented out through the carbon filter and extractor fan.
There are a few different ways these can be set up depending on the space or equipment, but for the purpose of this article we will be using the order of inline, carbon filter, extractor, followed by exhaust (ducting). Everything is kept inside the grow tent, rather than outside. This helps to minimise noise, which can get quite loud if you're sitting in the next room.
Using ventilation ducting, each part can be connected where needed to direct the air out of the space. Some lights even use a 'cool tube' system, which allows growers to vent the warm air produced by hot HID lights straight out of the space. Chains or roller hangers are usually used to hang the carbon filter and extraction fan securely above the plants.
The inline fan is placed low down in the grow tent (most grow tents have vent holes both at the top and bottom) to allow fresh air to come in underneath the plants. This fan needs to be around 10-15% less powerful than your extractor fan to create a negative pressure environment. As the interior of the grow tent is generally hotter than the exterior, the air begins to heat up.
The warm, smelly, stale air rises and is seamlessly drawn through the carbon filter and out by the extractor placed at the top of the grow room (preferably at the opposite end to the inline holes).
The oscillating fans circulate the air over the plants as it warms up from the lights and other equipment, picking up the odors and moisture being produced by the plants.
Oscillating fans have the added benefit of helping to strengthen the stems and branches on a cannabis plant by maintaining a breeze that continuously exercises the outer fibres. The slight movement stimulates encourages growth hormones in plant tissues, allowing them to grow thicker in preparation for supporting big buds.
Tip: Place one small oscillating fan low down and one around the canopy area to circulate fresh air up and through the plants towards the exhaust.
Depending on where you live, the smell of your grow operation can be a big issue and most growers choose to take control by adding a carbon filter to their ventilation system. Using carbon filters allows a grower to remove smells from the air before they have even left the grow room. Carbon filters generally use activated carbon to strip odor particles from the air as it is pulled out of the space.
Carbon filter tends to decrease the efficiency of the extractor fan by around 25%, so take this into consideration when calculating the size of the fan for your grow room and opt for around 25% more power than you need to compensate for the loss. Likewise, the carbon filters need to be big enough so that the extractor fan is not sucking more air through it than it can handle, otherwise we can also experience smells escaping.
So, going back to our example earlier with a 5 x 5 x 6.5 foot space. The addition of a carbon filter requires us to make the following calculation:
160x25% = 200CFM
You would need at least a 200CFM fan with an inline carbon filter to replace the air at least once per minute and allow the filter to properly pick up the odors.
As we briefly mentioned earlier, ducting is the material used to connect the ventilation equipment. The ducting creates the channel and allows us to direct the extracted air outside. The length of the ducting, as well as the angles or bends, make a difference in terms of the fans efficiency. The longer the ducting, and the more angles there are in the ducting, the more powerful the fan needs to be.
To keep it simple, add around 10% to the CFM output of your fan for every 10 feet of ducting, and for every 90 degree angle in the ducting, add another 15%.
The only time you would turn off your extractor fan is if you are having serious problems with low humidity and if that is the case your extractor fan may be too powerful for the size of the space. We recommend investing in a humidifier or using a fan with a lower speed (don't forget that you still need to replenish the air at least once every 1-3 minutes). It is not recommended to leave the fans off for more than an hour at a time.
Depending on the size of the grow room more or less ventilation needs to be added, but we recommend having at least one extractor fan and two oscillating fans, as a minimum. If you're not on a tight budget then inline fans and carbon filters are worth the investment. Most grow tents have enough holes to allow for passive air movement with just an extractor fan, but the more control you can achieve, the better.
Sealing your grow tent to create a separate air space makes a lot of sense for a few reasons:
Greenhouses need multiple openings or ventilation holes to allow for passive air movement within the space. The vents can be opened when humidity or temperature increases too much and can be closed in the case of the opposite.
Advanced or permanent greenhouse structures can have equipment fitted to further control the conditions, meaning fans can be added. Greenhouses can get extremely hot and humid, so having the ability to efficiently vent air in or out when you need to is worth the investment considering how cannabis plants (both indoor and outdoor) can get seriously ruined by mold or pests. A few oscillating fans may be enough avoid stale air building up in the greenhouse.
Getting the air circulation right in your grow space takes a bit of practice but once you know your equipment and how cannabis plants respond to it, you can make improvements to fine tune it exactly the way it needs to be for the time of year or climate.
Here are some extra tips to consider when setting up an efficient ventilation system for your cannabis plants:
Well circulated grow room by suppadupp from GrowDiaries.
If you found this article useful or have any tips regarding ventilation in indoor grow spaces, why not share your thoughts down in the comments section?
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This article was updated January 2021.
