Air Circulation & Ventilation: How to Calculate CFM for Your Cannabis Grow Room


Cannabis plants love fresh air and a nice gentle breeze when grown outdoors. However, we don’t really have much control over the wind and the weather, which is why most cannabis growers and hobbyists prefer cultivating their crops in indoor tents and grow rooms.

Growing cannabis in an indoor environment gives the grower much greater control over the vast number of variables that can affect a plant’s health and growth, particularly common cannabis pests like mites, aphids, fungus, mold and mildew. You have the ability to adjust temperature and humidity, or even control air circulation to mimic the best parts of nature and create the perfect indoor atmosphere for your cannabis plants.

Why airflow is important in indoor cannabis cultivation

Movement of air through an indoor cannabis grow space is vital in maintaining a healthy crop. Without proper ventilation, air is stagnant and the grow room becomes susceptible to a host of issues such as carbon dioxide depletion and the proliferation of marijuana pests and diseases. CO2 depletion can lead to nutrient lockout, and areas of high humidity are prone to pest infestation, mold, or mildew.

To mitigate these potential issues which can harm plant growth and yield, creating sufficient air movement is imperative. The ultimate goal of using air circulation and ventilation is to create homogenous airflow throughout the entire grow space. This helps regulate CO2, relative humidity, and air temperature, while preventing fungal disease and cannabis pest infestation at the same time.

Indoor growers use many tools to help control airflow throughout a garden. These include oscillating fans, intake and exhaust systems, and even filters. A good combination of all these elements can create the perfect airflow for a grow space of any size.

Creating the perfect airflow to prevent common cannabis pests

Most novice growers think that having a fan blowing directly on their cannabis plants is good enough to provide airflow in a grow room. But that isn’t the case—you also need to exhaust heat that causes the grow area to become hot and humid. Otherwise, you’ll have to deal with problems like bud rot, fungus gnats, spider mites, or white powdery mildew. Venting old air out within the grow area is just as important as air circulation. Having both an exhaust fan and small oscillating fans in the grow area will give you the best results.

Great air circulation combined with an exhaust system helps disperse water vapor and heat evenly throughout the grow space. To ensure there are no hot or humid spots in your grow space, have small oscillating fans blowing gently above and below the canopy to help equalize the air. An exhaust system ensures that all the air in the grow space is replaced regularly, so plants stay cool, get a fresh supply of CO2, and live in the right humidity. It’s important for plants to be exposed to fresh, moving air for the best growth rates.

Air blowing over the leaves of your cannabis plants also help carry away the moisture released during transpiration. As a result, you plants pull in more water and nutrients at the roots, greatly lowering your chances of running into issues like molds, white powdery mildew, and bud rot. Additionally, air constantly flowing over both plants and the soil helps fight against nasty marijuana pests like fungus gnats and spider mites, both of which thrive in stagnant air.  Airflow also dries out the top layer of the soil, making it harder for fungus gnats to breed (moist top soil creates the perfect breeding ground for fungus and molds).

Placing fans in your grow room

Some of the best tools to control airflow, humidity and temperature in the grow room are fans along with a way to vent out used air. Let’s cover what you need to know about fans in your grow room and as part of your exhaust system.

Indoor cannabis growers mostly use oscillating fans to create a nice breezy environment within the grow area. Small oscillating fans are great since they’re cheap and can be used to provide a nice gentle breeze to a relatively wide area without blowing on any one part too long. Ideally, a nice breeze should surround the main canopy so you’ll want air blowing above and below your plants. Make sure all parts of your grow area get a slight breeze and see to it that fans do not point directly at a plant as this can cause wind burn and damage the leaves and stems. Wind-burned leaves often recede into a claw-like deformation and they can look like they’re droopy from overwatering, underwatering, or possibly a nitrogen toxicity. If leaves nearer to the fan are clawing while those further away look good, this is a surefire sign of wind burn in your cannabis plants.

Now that you have oscillating fans mimicking the gentle breeze that cannabis plants love in the outdoors, you’ll also need to choose the right fan for your exhaust system. Even if heat isn’t an issue in your grow room, you will still have to vent out old, stagnant air to maintain an optimum environment. An exhaust system uses fans (and often ducting) to move hot and humid air out of the grow space. To know what fan to use for your exhaust system, you will need to calculate for CFM or cubic feet per minute rating.

Calculating exhaust fan strength (CFM)

The whole idea of setting up an exhaust system in your grow space is to create negative airflow so that any hot air is regularly replaced with new, fresh cool air. All the air in your grow room should be replaced by new air every 1-3 minutes and unless you live in a cooler place, seasoned growers recommend replacing the air every one minute for best results.  

Exhaust fans come in different shapes and sizes, but there’s one thing you should check on when purchasing them for your indoor cannabis grow room—CFM rating. Exhaust fans (or most fans for that matter) are classified by how much air they can circulate and those sold in the US are measured in cubic feet per minute or CFM (other countries may use cubic meters per minute).

To figure out the CFM rating of the fan you need, first determine the volume of your grow space using this formula.

Air Volume = Length x Width x Height (in feet)

For example, if your grow room is 10’ x 6’ x 8’, your air volume will be 480 cubic feet. An efficient air exhaust system must have the capacity to circulate at least this much air in your grow space every minute. However, every exhaust system will have some level of efficiency which can lower the amount of fresh air being circulated, especially if you’re using duct tubing and carbon filters to scrub air as it leaves the space. For this reason, most growers account for a 25% loss in efficiency. To calculate this, multiply the air volume by 1.25.

Accounting for a 25% loss in efficiency, the CFM computation for your 10’ x 6’ x 8’ grow room will look like this:

128 cubic feet x 1.25 = 600 CFM

This means you need to have an exhaust fan with a 600 CFM rating, at the very least. You’ll generally need a higher CFM rating if your grow space tends to get hot or humid, and it’s usually better to get an exhaust fan that’s a little too big than one that is too little!

Using CFM ratings is a good way to figure out how much power your exhaust system will need to create negative pressure and achieve optimum air circulation and ventilation in your grow area. For a small investment, a proper ventilation system in an indoor grow space will greatly improve the overall health of your cannabis plants. Even a basic air circulation system composed of oscillating fans and an exhaust fan will ensure healthy and productive growth, while also reducing the presence of common cannabis pests and other environmental ailments like fungus, mold and mildew.

To help you calculate CFM rating for your particular grow space, Ant & Garden Organic Pest Control in Beaverton Oregon recommends you use this exhaust CFM calculator

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