Balmy Breezes: How Air Circulation Affects Temperature, Humidity and CO2 in Your Growroom

By Shane Hutto
Published: July 23, 2019 | Last updated: April 21, 2021 11:09:57
Key Takeaways

We all know that circulating air inside a growroom is essential for replacing the CO2 used by plants. However, other factors are also affected by these breezes, including humidity and temperature.

We’ve all heard that moving and exchanging the air inside a growroomis essential. Why? When growing indoors, you are artificially creating a microclimate in which the CO2 in the air immediately around the plants’ leaves depletes rapidly when plants feed during daytime hours.


This depleted atmosphere must be exchanged with the higher-CO2 air in the rest of the growroom in order to improve the conditions for the plants. (Additionally, this circulation of air deters pests and disease outbreaks, and aids plant health by keeping transpiration rates high.) Proper circulation is as simple as adding an oscillating fan to the room. Then, as the overall CO2 levels in the growroom diminish, this air must also be replaced.

Air inside the growroom also affects humidity and temperature. There are several different methods and tools available for purchase that will help you to control these sectors, but the main trick for new to intermediate growers to understand is how it should work.


The main goals of ventilation systems are to remove heat, correct humidity and circulate the air. Your budget will determine what sort of system you can install. The components of a high-end ventilation system can run into thousands of dollars, but many low-budget systems can also perform satisfactorily when designed properly.

The best ventilation systems are planned well in advanced and are designed to be effective beyond what is necessary. The reason they need to be oversized is because summer heat and winter cold are often more drastic than expected.

In order to determine what you need in your growroom, first monitor and record your existing conditions. In particular, you should have a good idea of the range between nighttime lows to daytime highs, and how this changes throughout the growing season.


Next you will need to evaluate how much air needs to be moved. To estimate your ventilation requirements, simply measure the length, height and width (in feet, as this is how most components are rated) and multiply these three numbers together. The resulting figure will be cubic feet (CF).

Now, keep in mind that if you have a 600 CF room, a 600-cubic-feet-per-minute (CFM) fan will likely take longer than one minute to exchange the air because it is not perfect. Expect a certain amount of pressure loss throughout the system—which is another reason why your components should be oversized for your space (if your budget allows it, of course).


In order to affect temperature, you will either have to cool the air that is already in the room or bring in fresh, cooler air. The latter is easiest and less expensive compared to cooling the air and leaving it in the room. The biggest problem associated with bringing in new air, however, is that the old hot air must go somewhere.

Read also: Mastering CO2: Ideal CO2 Levels for Growing Marijuana

Most often, it is vented directly outside through a high-placed outlet (the higher it is, the less likely the outlet is to cause discomfort or problems outside). These outlets also often have a carbon filters attached to aid with any odors that may be present in the exhaust. Keep in mind though that the carbon filter creates a loss of pressure, thus lowering the volume rating of your components.

Air conditioners work well in a sealed system—but not so much so in a system that brings in outside air. Sealed systems can be tricky to build and maintain, but often are more effective in holding temperatures at the desired point. The major problem when using a sealed system for production growing is the lack of CO2 input.

A properly constructed sealed system will need some sort of CO2 injection system. CO2 injection can be achieved with bottled CO2 or by using a burner, both of which can be purchased at most hydroponic stores. Bottled CO2 can be cumbersome, but it does not add heat to the room as a burner would. CO2 burners are a good choice, however, if you have extremely oversized air conditioners or if you grow in a colder climate where heat is not a problem.

Humidity correction is another constant battle in most growrooms because every time you water, turn on a fan or air conditioner, or even open a door to outside air, the humidity in the room changes.

Even the plants themselves can drastically affect humidity levels—we know that approximately 90% of the water absorbed by plants is evaporated through their leaves, meaning 90% of the water you feed your plants goes into the air as humidity.

Also, since air conditioners (by design) cause the humidity to drop, air conditioners operating in humid climates have a greater tendency to wear out because they will run nonstop to lower the humidity. In dry climates, conversely, air conditioners are often fighting a humidifier. The key to humidity control is an overall environmental controller. If you can afford the normally expensive set-up, environmental controllers are a highly effective weapon.

In the end, the components you select for your system should be of good quality because your plants’ survival depends on it—it only takes one day of your controls failing to fry your plants.

This is also goes back to why the first step of monitoring is also the most important. By simply checking your thermometer, you can usually tell a few days in advance if your components are experiencing problems because most part will not fail immediately, but rather over time.

Remember, plants don’t care how fancy your components are as long as they effectively provide adequate air circulation, and control the humidity and temperature.


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Written by Shane Hutto | Owner

Profile Picture of Shane Hutto

Shane Hutto is the owner of Horticultural Solutions, a cannabis cultivation consulting company. He earned a bachelor’s degree in horticulture at Oklahoma State University and received a research assistantship for his master’s degree. During his graduate studies he researched production and extraction of surface waxes on horticultural commodities. His passion for growing is complemented by his experience in many types of controlled environment operations and design.

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