Grow Room Ventilation 101


Indoor growers enjoy an endless growing season, complete control over growing conditions, and the convenience of growing inside. This all sounds great, but too often we forget about grow room ventilation.

My first attempts at indoor growing were plagued with problems. I had a fan set up, which I thought was sufficient to provide air circulation for my plants. I was wrong. Getting grow room ventilation right requires more than a corner fan.

If you’re still asking yourself questions like:

  • Why do grow rooms need ventilation?
  • How to vent a grow room?
  • How do you know what kind of fan to buy?

Don’t panic. I’ve made all of these mistakes (and more)…so you don’t have to. Grow room ventilation is actually pretty simple as long as you understand the fundamentals. So let’s get into it.

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Why Do You Need to Ventilate Your Grow Room?

So why do grow rooms and grow tents need to be ventilated? As it turns out, there are quite a few reasons.

Plants grown outdoors enjoy the mitigating effects of light breezes, sunshine, evaporation, and precipitation. Indoors, we substitute fans for breezes and evaporation, grow lights for sunshine, and watering systems for precipitation. What we often forget about is that outdoors, air circulates more naturally than in your grow room.

You need both a fan and an air extraction system to keep the air in your grow room fresh. Here’s why.

Remove Excess Heat

Grow lights emit heat. It might not seem like much to you, but a few degrees in temperature can mean the difference between a successful crop and a less successful one. Certain grow lights emit more heat than others, but all produce enough heat to merit ventilation.

Control Humidity

Humidity is another factor that can contribute to problems with indoor growing systems. Plants emit water vapor throughout the day which increases the humidity of your grow room. Uncontrolled humidity leads to poor growing conditions and can result in the development of pests and diseases.

A good ventilation system also wicks away some of the water released during transpiration, allowing your plants to absorb more water and pull up nutrients through their root systems, boosting growth.

Prevent Pests and Diseases

Air circulation helps prevents pests and diseases. Mold, powdery mildew, spider mites and fungus gnats prefer stagnant, humid conditions. Pests lay their eggs in damp topsoil, so having a fan to keep the surface layer of soil dry can slow their reproductive cycle, and a steady breeze makes it harder for them to establish themselves on your plants in the first place.

Control CO2

Plants need outside air for one very basic reason: CO2. Plants absorb CO2 throughout the day as part of their nutrient cycle. If your grow tent setup is sealed, this means that the level of CO2 in the room gradually decreases, limiting your plants’ growth. Ventilation brings in fresh air and fresh CO2 from outside and removes the old air from your grow room, boosting growth and yields.

Manage Wind Stress

Outdoor plants are constantly buffeted by the wind. This makes the stems of the plants stronger, which serves them well when the time comes to bear fruit. Stronger plants produce and grow better than weaker plants, which can break under the weight of their own yields.

So why do you need a grow room ventilation system? Let’s review. A good ventilation system:

  • Reduces the chances of diseases like mold and powdery mildew from developing
  • Helps protect your plants from pests like spider mites and fungus gnats
  • Helps you control the temperature and humidity of your grow room
  • Helps strengthen the stems of your plants

If those reasons still don’t convince you that you need a grow room ventilation system, then consider cost.

Grow tent setup is not cheap. If you are going to invest in a grow tent or grow room, you might as well do it right.

How to Ventilate Your Grow Room

Place Oscillating Fans in the Grow Room

As you learn how to vent a grow room, keep in mind that your grow room ventilation set up needs two types of fans.

  1. Air extractor fan system
  2. Oscillating fans
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We will get to the air extractor fan system later, so for now let’s focus on oscillating fans. These fans are both cheap and effective, which are two of my favorite qualities in any system. They blow air constantly above, under, and around your plants, helping to keep the grow room cool and improve air circulation.

Oscillating fans are also easy to set up and move around, which means you can easily experiment with placement to make sure you find the optimum location for your fans. You want your oscillating fans to reach all areas of the growing room, which might mean investing in a few fans, but you also want to prevent your fans from pointing directly at your plants. This can result in ‘wind burn’ and can damage your plants (think gentle breeze vs tropical storm).

Setting up oscillating fans will help moderate the temperature and humidity in your grow room, but fans alone are not enough. You need an air extractor fan system to keep the air in your grow room or grow tent fresh.

Air Extractor Fan System

Basic air ventilation
A basic ventilation setup. source

An air extractor fan system cycles out old air and brings in fresh air, which your plants need if you want them to thrive. Ideally, your extractor fan system should replace the air in your grow room every minute, or at least every three to five minutes. This means that you need to choose the right size fan for the job.

Combining grow tent set up with grow tent ventilation set up was one of the best decisions I ever made. Before you get invested in grow tent set up, take a minute to read about grow tent ventilation. The best time to install a ventilation system in your grow tent is during set up, as you won’t have to work around existing plants or alter your system.

Calculating Fan Strength

Calculating the fan strength you need requires a little bit of math, but it is relatively simple. Fans are rated by CFM, or cubic feet per minute. To figure out the CFM you need, you need to know:

  • The dimensions of your grow room or tent
  • The exhaust efficiency

Calculate the total volume of your grow room by multiplying the length x width x height. This is the cubic feet measurement of your room. Hold on to this number. Next, we are going to multiply it by the efficiency drop.

This efficiency varies based on the age and make of the filter and the length of the duct between the fan and filter, on top of an endless list of other factors. At the minimum, you want to multiply the volume of your room by 25%, but here is a good rule of thumb: if you have a long duct path, multiply it by three, and if you a short path multiply it by 2.

Your formula should look like this:

  • Volume of room x efficiency drop = CFM

For example, if the volume of your room is 320 cubic feet and you have a short duct, then you want to multiply this number by 2.

  • 320 x 2 = 640

Now that you have your CFM, look for a fan that is higher than this number. If this is confusing, check out this video.

Passive Intake vs Active Intake

There are two ways that you can bring air into your grow room, referred to as passive intake or active intake.

Passive intake relies on passive airflow and negative pressure. With this grow room ventilation system, you have a fan blowing air out but no fan blowing air in. Instead you have an intake hole that passively allows new air to enter, thanks to the pressure differences between the inside of the room and the outside.

To make a passive system work, you need to make sure that your intake hole is three to four times larger than the exhaust hole. This will prevent your fan from over-working and will help your ventilation system achieve proper suction. You can also have more than one passive intake hole.

Active intake systems actively draw air into your grow room with a fan. All that this means is that you have one fan blowing air out and another blowing air in. The size of the intake hole doesn’t matter as much in this system, so if your intake hole is the same size as your out take hole then you might want to consider using an active intake system.

Controlling Humidity

Humidity plays a huge role in plant health. Too much humidity can cause disease in your grow room and too little can negatively impact growth, so we need to be monitoring humidity at all times.

Humidity in the grow room is largely a result of transpiration. Transpiration is the process by which the leaves of the plant give off water to the atmosphere. Much like a straw, the suction created by transpiration pulls nutrients up through the roots as the plant produces water vapor.

This is why plants stop growing when they dry out. You are not only depriving them of water, you are effectively starving them of all nutrients. On the flip side, too much humidity in the air creates the perfect environment for diseases and pests to develop and also prevents evaporation, limiting the plant’s ability to perform transpiration, so you need to be able to raise and lower the humidity in your grow room easily.

Lowering humidity is simple. All you need to do is vent the humid air out using an exhaust system, as long as the air from the intake is less humid than the air inside the room. You can also install a dehumidifier.

There are times during the growing process when you might need to raise the humidity. Sealing up your grow tent or temporarily turning off your exhaust system will raise the humidity of the room fairly quickly.

Do a little research into the humidity levels preferred by your crops and do your best to keep the levels within range. Remember also that larger plants produce more water vapor than smaller plants, so as your plants grow you may need to pay more attention to humidity levels.

CO2 Injection

If plants require CO2 for transpiration, then it seems logical that all we need to do to boost plant growth is inject CO2 into your grow tent. While in theory this is true, there are a few things you need to know before you use a CO2 injection in your grow room.

CO2 injections require a sealed grow room for two very important reasons. The first is efficiency. CO2 only works at very high levels, which means you need a sealed grow room or grow tent setup in order to get the concentration high enough to be worthwhile.

The second is safety. The high levels of CO2 recommended for plants are dangerous for humans. You don’t want that much CO2 leaking out into your home and potentially putting your loved ones at risk.

Before you add extra CO2, also keep in mind that you need bright grow lights in order for your plants to take full advantage of the CO2. MH/HPS lights or strong LED lights produce enough light, but FLs and T5s are not bright enough to get really good results.

So should you add CO2 to your grow space? Adding CO2 is something you should consider once you have optimized all other aspects of your system, as it is most effective in an optimized environment. You also need to weigh the cost. If you are happy with your yields and have a limited budget, then a CO2 injection is not necessary.

If, on the other hand, you have some wiggle room in your budget and you want to experiment with increasing yields with CO2, then do your research and give it a try.

Grow Room Ventilation FAQ

Let’s get some commonly answered questions out of the way before we wrap up this tutorial.

Q. What kind of fan do I need?

Choosing a fan for your grow room ventilation simple is not easy. There are a lot of choices out there. For starters, you need an in-line duct fan that is greater than your grow room’s calculated CFM.

Once you have the specifications for the fan, you have to choose a make and model. Fans range in price, so make sure you read lots of product reviews to make sure you are getting the best fan for your money. Also check to see if the filter comes with the fan or if you need to buy it separately.

Q. How big of a fan do I need for my bulb?

One factor that can help narrow down your fan search is the size of your bulb. Most fans come in 4-inch, 6-inch, and 8-inch diameters. You probably want to get at least a 6-inch diameter fan due to the size of the light hood’s opening. This opening is usually 6 inches, but measure just to be sure.

Q. How big of a fan do I need for my room?

To find out how big a fan you need for your room you need to calculate the CFM of your room using the formula we went over earlier. This number can help you find the range of fans that will work best for your set up.

In addition, you should also invest in some cheap oscillating fans to help regulate temperature and control airflow in your grow room.

Q. How big does my intake fan need to be?

The size of your intake fan depends on two factors: the system you use, and the size of your out take fan. If you use a passive intake system, then the answer is simple. You don’t need an intake fan. If you use an active intake system, then choose a fan that is the same size as your out take fan.

Q. What is negative pressure?

Negative pressure is when the pressure within your grow room is less than the pressure outside of the room. This principle comes into play during passive intake, as the negative pressure in the room pulls clean air in from the outside through your intake hole.

Confused? Think of negative pressure as a journey beneath the water in a plywood box. As you sink, water comes rushing into the box because the pressure inside the box is less than the pressure of the water outside. Now pretend that the water rushing in is clean air and the water leaving in bubbles is dirty air. As you sink, congratulate yourself on understanding negative pressure.

Q. What kind of ducting do I need?

Flexible aluminum ducting works well for most grow systems. It is affordable and easy to install, but you can also use insulated aluminium ducting or a heavier duty duct, depending on your preferences and budget.

The important thing to keep in mind when selecting ducting is size. Smaller ducts create a greater airflow resistance, as do more bends in the duct and the length of the duct, as airflow reduces the further it has to travel.

Q. How can I improve the performance of my ducting?

If you have flexible aluminum ducting, the first thing you can do is smooth out the wrinkles. This will increase the airflow and improve the performance of your ducting. You can also try to reroute the duct so that there are fewer turns in its course, and when possible shorten the length of the duct system.

Go Forth and Ventilate!

I hope you enjoyed this tutorial on grow room ventilation systems. Choosing the right ventilation system for your grow room is complicated, which is why I think it is important for indoor growers to understand how ventilation systems work and the benefits of grow room ventilation systems. Once you know the basics, you are in a much better position to choose the right system for your operation.

Do you have a grow room ventilation system that you love?

Do you have something to add to the discussion?

Please feel free to comment below or share this article with other gardeners, and good luck growing!

Last update on 2024-05-07 / Affiliate links / Images from Amazon Product Advertising API