Cannabis grow room ventilation guide

Grow room ventilation is one of the most important aspects of your grow. Your cannabis plants need fresh air to breathe and photosynthesise. Stale, moist and hot air needs to be continually removed to prevent humidity reaching mould/problem-inducing levels. Cool, fresh air is needed to replace warm air and maintain optimised grow room temperatures. Our complete guide to grow room ventilation explains all you need to know, the rookie mistakes to avoid and the best practices to follow.

Why is proper ventilation vital for cannabis plants?

Having reliable, effective, grow room ventilation ensures that your cannabis plants grow in an optimised environment with temperatures and humidity where they should be, allowing you to grow the most potent buds. Insufficient air-flow through your tent/grow room really does reduce both yield and quality as the plant biochemistry is adversely, and permanently, affected.

Having a good quality carbon filter gives you maximum peace-of-mind that all the air removed from your grow room is scrubbed clean of pungent cannabis smells. It’s always good advice to plan your grow room well and ensure that your fans, filters, clips, ducting etc are well matched to the size of your grow room. Your grow room, when planned well and treated to good quality cannabis seeds, LED lights and good ventilation will more than repay the grower with superior quality crops enjoying maximised cannabinoid/terpene levels.

Most serious growers also keep a spare carbon filter ready, just in case. As they would keep spare pH and E.C. meters for the inevitable day when they too fail and start to give erroneous readings. Whilst it is expensive keeping spare carbon filters, pH/EC meters etc it could be argued that the value of each crop from a typical home growers tent more than covers the costs of these essential spare items.

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The basics of air extractor fan systems

In simple terms good grow room ventilation removes stale, warm and moist air and replaces it with fresh air which your cannabis plant can use to grow better quality buds. In nature, a simple breeze would do the job just as effectively. But in an enclosed grow room or grow tent it is the growers responsibility to ensure sufficient flow of fresh air.

In a typical tent the grower would have a carbon filter connected to an extraction fan. The fan draws stale air through the carbon filter, removing smells, before pushing the clean air out of the tent through ducting. This stale air can be vented outside, although many urban growers simply vent the air into their apartment/house – even though this perhaps isn’t ideal. Some growers install dehumidifiers near/inside their grow room if they are concerned about humidity levels.

When venting grow room air outdoors in winter, the cautious grower should be aware of clouds of condensation forming as the exhausted grow room air (warm/moist) hits the cold outdoor air during winter months. If a tell-tale plume of condensation is seen, then a re-think may be required on how/where the stale air is removed.

Grow room ventilation diagram

Cannabis grow room ventilation diagram

How many CFM is needed for your grow room?

Many growers aim to replace the air in their grow room once every minute. However, that is often considered an absolute bare minimum and many growers feel that 2-3 complete air changes per minute is necessary especially when heat-producing HPS lights (or hot grow rooms in general) are used.

When growing in summer, for example, growers may be inclined to run the extraction fans at higher settings than they would in winter to help ensure temperatures don’t rise too far. Likewise, those growing in a loft may well find that temperatures are far hotter than a comparable grow room situated in a basement or bedroom and will need to run extraction fans at higher settings.

The skilled grower will consider several aspects, not just the tent size, when selecting an extraction fan and choosing the setting to run it. The ambient external temperatures, the season you are growing in and the precise location of the grow room in your property will all play a major role.

First of all, you need to know the volume of your grow room. Once you know this you can select the correct fan size. A grow room of e.g. 8m³ would need a fan that could extract at least 8m³ per minute and possibly 2-3 times that if you grow in summer or a warm grow room. A grow room of 100 cubic feet (100 ft³) would need an extraction fan with at least a 100CFM rating (100 Cubic Feet per Minute) and possibly 2-3 times that, depending on the specific conditions/location of your grow room.

Metric: A grow room which is 1m wide, 1m deep and 2m tall has a volume of 1m x1m x 2m = 2m³

Imperial: A grow room which is 4 feet wide by 4 feet deep by 6 feet tall has a total volume of  4ft x 4ft x 6ft = 96 ft³

Suggested extraction fan sizes for common tent sizes

A 150/180m3/h extraction if often used in 60x60cm tents. A 250-350m3/h extraction fan is often recommended in tents of 80x80cm, 120x60cm & 100x50cm. A 300-450m3/h extraction fan is well suited to 100×100 or 120x120cm tents. When you use more power/wattage in your lights, you will need a bigger fan too. Especially when you need to keep temps and humidity under control.

Extraction fans with built-in temperature sensors are worth consideration

Many serious growers use an extraction fan with a built-in temperature sensor, allowing air extraction to be increased if a temperature increase has been detected. This allows you to keep your grow room at a stead temperature. Fans with built-in temperature sensors automatically adjust the speed/power of the unit in small steps. They also have a setting for a minimum RPM setting (and a maximum setting) so you won’t lose the vacuum/pressure that is created and needed in the tent.

Many grow shops, and their websites, explain grow room ventilation equipment in great detail. Note also that there are a wide array of extraction fans, carbon filters and related equipment available.

Pro tip

Some of the more expensive grow room ventilation options are worthy of consideration. For example those, growing in apartments or shared houses may favour professional silenced extraction fans and acoustic ducting to minimise sound from the grow room ventilation system. These products are specially designed with sound insulation to minimise noise and vibration – perfect for those growers that prefer to be a little more discreet about their indoor gardening hobby. Power-adjustable extraction fans are also very popular, allowing growers to use ultra-quiet low level extraction settings while the plants are small.

How to set up ventilation in your groom

If buying your grow tent/equipment from a competent grow shop you can find them to be a great source of experienced advice. They can help specify the right type/size of fan, filter and ducting for you.

Proper air circulation is vital for your cannabis plants

How to ventilate a cannabis grow tent

Stick to the tried and tested principles. A good quality carbon filter is connected to an extraction fan. The fan pulls air through the carbon filter, as this happens the activated carbon inside the filter chemically absorbs the terpenes (and other aromatic compounds) locking them inside the carbon filter. The fan then pushes the cleaned air outside the tent through ducting.

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How to ventilate a micro cannabis grow

To maximise the amount of space for your plants you may keep only the carbon filter inside your micro cannabis grow. The carbon filter is then connected to ducting that leads to your extraction fan outside the micro grow area. The fan draws stale air through the carbon filter, cleaning it as it does so. The air is then vented away safely.

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How to ventilate a greenhouse

Most growers rely of auto-opening roof-shutters that open in hot weather releasing warm air from the top of the greenhouse. But some professional greenhouse cannabis growers prefer to use very large fans to force clean, filtered air into the greenhouse (filtered incoming air ensures no bugs/pests). Equally large fans are also often used to remove stale air from the greenhouse.

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How to ventilate a basement grow

Many basement cannabis growers will invest in slightly more powerful extraction fans that will allow them to pump the stale air up a storey or two so that the extracted air can be dumped outside via e.g. a suitable wall/roof vent.

Note that grow room air flow can also be improved by careful defoliation of your cannabis plant. As well as improving air flow, the resulting improved light penetration can also allow heavier yields.

How to defoliate cannabis plants for bigger yields

Cannabis grow room ventilation and odour control

Those growing in countries/states where cannabis is still illegal have extra motivation to ensure that their grow room ventilation is up to the required standards! Even slight odour leaks from your grow room could cause disaster for your grow. So, it really pays to take grow room ventilation seriously.

As well as planning/designing your grow room to the best standards it’s also vital to pay extra attention to the small details. Many growers have run into trouble for failing to ensure proper air-tight connections between e.g. carbon filter and fan. Even a small leak could allow unfiltered air to be sucked out of the grow room via a poor-quality seal. You may wish to regularly check all connections between fans/filters & ducting, possibly using duct-tape to ensure genuine air-tight seals.

Pro tip

Many serious growers, especially those with large crops, also add an ozone generator to the exhausted air. The ozone chemically reacts with, and permanently removes, any trace amounts of odour which may have escaped the carbon filter. Anyone growing cannabis in countries where it’s still illegal to do so generally prefers to pay extra for the higher quality carbon filters. These have extra durability and last longer. Many serious growers also keep a spare carbon filter to ensure an immediate fix… if required.

Use carbon filters to avoid cannabis odors

Grow room ventilation FAQ

Just as the selection of the grow room LED light is a critical decision for your crop quality, the design of your grow room ventilation is equally important. In order to optimise the results from your autoflower seeds or feminised seeds you have to ensure a steady flow of fresh air and a continuous removal of stale, warm and moist air.

How important is air flow for cannabis?

Adequate air flow is absolutely critical for the optimised growth of cannabis. Without fresh air you deprive your cannabis plants of the oxygen and carbon dioxide required to create new growth and sticky buds. Without these fundamental chemical building blocks your plant growth will be stunted. In the worst cases your plants will die.

Pro tip

Many growers use clip-on fans to improve grow room ventilation and air circulation, both above and below the canopy of buds. These can be directed at the canopy (and below it) to ensure plenty of oxygen and carbon dioxide is available and eliminate pockets of stale, uncirculated air.

How often should air be exchanged in a grow room?

Many growers aim to exchange the entire air volume of their grow tent approximately once every minute and will fit an extraction fan of the appropriate capacity to match their grow volume.

What are the essential tools for an efficient grow room ventilation?

Aim to design your grow room ventilation around the best quality components you can afford. Remember, you’re growing a high value crop and want to exceed the quality levels from alternative sources. Ensure that the fan size is appropriate for the grow room size and select a high-quality carbon filter for maximum peace of mind. Where possible use professional (rather than budget) clips to seal the connections between ducting, carbon filter and fans. The cost difference is minimal and the difference in the quality of the seal could prevent odour issues.

Pro tip

Ensure all seals are air-tight, if in any doubt use copious amount of duct tape to ensure no unfiltered air can escape into the exhaust air stream.

How to choose between passive and active intake?

Many hobby growers with average 1.2m x 1.2m grow tents (4ft x 4ft) use passive air intake. This means that they don’t have a fan pumping air into their grow room. Instead, air is allowed to flow in naturally thanks to the negative pressure created inside the grow room by the extraction fan. Grow tents have several adjustable fabric ports which allow passive air to flow into the grow room.

Pro tip

It’s recommended to cover passive air intakes with a fine net/mesh cover to prevent insects/pests entering. The more serious growers, or those using larger scale grow rooms often prefer to have an active air intake. This is achieved by a fan that blows air into the grow room from outside. Usually, the intake fan size is deliberately smaller than the exhaust fan size to ensure a negative air pressure inside the grow room.

Should I install my extractor fan inside or outside of the tent?

Most growers tend to use the space at the top of the grow tent (or grow room) to position the extraction fan, carbon filter and ducting. This allows the grower to remove the warmest air from their tent. However, some growers, especially those that specialise in micro-grows (growing cannabis seeds in small places) prefer to keep the grow space as uncluttered as possible and will site the extraction fan outside the grow room.

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Do cannabis seedlings need air flow?

Cannabis seedlings are small with minimal levels of photosynthesis and respiration. At this stage of their life they can manage without significant air flow and can grow in small incubators etc. However, as the cannabis seedling grows and increases the amount of leaf area, so it’s need for proper ventilation also increases.

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