Growing Tips

Grow Room Ventilation Guide

grow tent setup

Article sections: Benefits | How To | Faq’s

One of the key benefits of indoor growing is that the season goes on and on and on – but there are some drawbacks too, if you’re not careful. A few simple tips and tricks will help to eliminate these though, and help you maximise the advantages.

Among the most important things to consider is airflow – achieved through proper ventilation. This is more than just popping a fan in a corner; your ventilation system should be tailored to your particular grow room, and should function according to the differing needs throughout the year. This may sound daunting, but it really isn’t that difficult.

Below we’re going to go through the reasons you need proper ventilation, how to set up the right system for your situation, and some tips on buying the right things without breaking the bank.

Benifits of Grow Room Ventilation

There are several reasons why grow rooms need good ventilation. Outside, plants are in the midst of natural breezes, sunshine, changes in temperature, natural evaporation, and precipitation – rain and/or snow. Some of these conditions can be a strain on the plant, and indoor growing removes that strain. Some of these conditions also help the plant though, and proper ventilation (along with appropriate lighting and space) will provide these vital elements to your grow room.

Ventilation comes in two main parts: the intake, and the extraction. Both are important to your indoor grow room conditions, and each of them needs to work in balance with the other.

So what will the right system do?

Control Heat

As well as the ambient temperatures in your house, the grow lights you use will also add a lot of heat. A few degrees might not feel very different to a human being, but to a plant it can mean a huge difference in moisture loss, crop volume, and overall health. The ventilation system removes excess heat.

Control Wind

A little breeze can toughen up plants, but it can also stress them in the process. A grow room situation removes the heavy winds, but it also removes the lighter ones that can cause a plant to toughen up stems – and tougher stems can bear more fruit without breaking.


Some humidity is good for plants of course, though the amount varies from species to species. Plants emit water of their own, too, as part of a healthy circuit of water up from the nutrient-rich roots to the shoots and leaves. But this humidity can build up around them in an enclosed space. Too much of it and plant diseases can develop, pests can move in on weakened plants, and mould and mildew can become a real problem. The right ventilation system will remove enough of this humidity to prevent these issues, but not so much that it endangers the health of your plants.

C02 Management

Plants need CO2 like human beings need oxygen. They absorb it through the daylight hours and it helps to fuel their nutrient cycle. A sealed grow room (or tent) can mean a steady depletion of CO2 and subsequent slowing of growth and productivity – basically it starves the plant of needed nutrition. Bringing in fresh air replenished the CO2 levels and allows the plant to continue its regular cycle – producing better growth and yields.


Many pests do well in stagnant, humid conditions. Such places are favourite spots for the laying of eggs and the growth of mould and mildew. A dry topsoil layer discourages the laying of eggs and turns off pests like spider mites and fungus gnats. Even if such pests are present, controlled conditions can slow the growth cycle and give plants more of a fighting chance against them. A balanced ventilation system is a good defensive measure against these issues.

Buying a Ventilation System

If you are going to go to the cost of setting up a grow room (or tent), it doesn’t make sense to stop short of the line and spend your money on a system that starves your plants, reduces yields, or encourages pest infestations. Finish strong with the right ventilation system and the value of the money you spend will increase – as will your enjoyment when you bring in those big, satisfying yields.

You can buy everything you need to control your grow room environment on our website here.

How to Ventilate Your Grow Room

There are two types of fans you’ll need – think of them as the two halves of a full system, that need to be in balance for the whole system to work.

1. Oscillating Fans

Oscillating fans are not expensive, and give great value for the money spent on them. The constantly move air around the room, keeping all areas in and around each plant’s growing space well-ventilated.

They can easily be moved from one location to another, so most people experiment a little with where they are most effective in a particular grow room configuration, or with particular plant species and placement. It isn’t too complex though – simply make sure that the air flow reaches all areas of the room. This is often best achieved by using more than one fan, especially in larger rooms or among denser plant types.

A word of caution though: Don’t aim the fans directly at your plants. This can cause ‘wind burn,’ an unhealthy drying out of the plant tissue, causing damage and stress to the afflicted plant.

Once you have decided on your oscillating fan needs, it is time to turn attention to the extractor fan part of the system.

2. Air Extractor Fans

Extractor fans pull old air out of the growing room, which creates vacuum pressure which pulls in fresh air. This fresh air, as mentioned above, is vital for supplying your plants with enough CO2 for their nutritional system needs.

The goal is to replace the air in the grow room or tent, once every minute or so.

That means that it is relatively easy to figure out what size fan you need for a certain size of room. Take the volume moved by the fan (this will be listed in the product information for the fan – we’ll get into it in a bit more detail below) and compare it to the volume of the grow room. The fan should move roughly the same amount of air in a minute that the room can hold. As long as it is moving that amount of air, and the oscillating fans are circulating the air within the room, you can be confident that you are providing your plants with what they need to thrive.

A little tip for new growers: Set up your fans and light systems before adding any plants to the room – that way you won’t have to work around them or risk damaging them.

NOTE: Understanding Fan Strength

Fans are given a rating based on how many cubic feet of air they move in a minute. This is the CFM rating (Cubic Feet per Minute).

Before you can choose the right CFM level, you’ll need to know the volume of your grow room or tent, and the exhaust efficiency.

Measure the length, width and height of your grow room. Multiply them together (L x W x H). If your room is an irregular shape, then figure out the volume of each portion of it separately, then add them together. Many grow tents have the volume listed in the product information.

Note this number – you’ll need it later.

Next you’ll need to determine the efficiency drop. This is the loss of fan efficiency due to the resistance of filters, and in some cases, the volume of the room. The length of duct between the fan and the filter is also a factor. Don’t panic! Here is a good rule of thumb (well, two rules of thumb!): The first is, take the volume of the room and multiply it by 125%. Use this as your room volume. The second is that, if you have a long duct path, multiply it by three; a short duct path, and only multiply it by two.

So, it looks like this:

(Volume of room x 125%) x (2 or 3, depending on duct length) = needed CFM

Let’s plug in some easy numbers as an example. Say your grow room volume is 100 cubic feet, and your duct length between the fan and the filter is short. Multiply the grow room volume by 125%, then by 2, and you have your needed CFM. In this case, that’s 125 x 2 = 250.

So for that example, you’d need a fan with a CFM of at least 250 (but not a huge amount higher, or you risk over-drying the room).

Intake Types (Passive vs Active)

As mentioned above, pulling air out of the grow room will suck fresh air into it (this is known as passive intake), but sometimes more help is needed to ensure a steady, easy flow – in these cases you might want to consider a, active intake system, one in which air is also pushed into the room by an intake fan.

A passive intake system needs an intake hole at least three or four times larger than the exhaust hole. This aids in easy flow and helps to prevent over-working your fan. Using more than one intake hole can also make up this total, and can help even out the fresh air when ventilating rooms of irregular shape.

An active intake system blows air in with a fan. This reduces the workload on the extractor fan(s) and you’ll be fine with an intake hole the same size as the exhaust hole.


Plants need humidity for proper growth, but too much of it can cause disease, aid in pest infestation, and can hamper growth and yield volumes and quality. Part of a healthy indoor grow room requires constant monitoring of humidity levels.

The leaves of plants will give off their own humidity through a process known as transpiration. This loss of moisture from the ends of the branches of a plant causes suction from the roots and lower portions of the structure, drawing up new moisture that is rich in nutrients to replenish the plant.

If a plant dries out, this process stalls and growth ceases – sometimes to a fatal degree. If ambient humidity is too high, evaporation ceases and the process is stalled from the top end, causing similar end results.

Adjusting humidity is, however, quite simple. To lower it, vent humid air out of the room and take in dryer air through the intake vents. If the outside air is also very humid in your area, you may require a dehumidifier to do the job. To raise humidity, slow the exhaust system temporarily to allow the humidity given off by transpiration to build up a little. You may even want to seal up the grow room for a temporary period to allow humidity to increase.

Monitor each situation closely, as humidity can change quite quickly, and you won’t want to over-adjust in either direction.

Some species of plants, and larger plants in general, need more humidity, so you may need to adjust the grow room conditions as your plants move through their growth cycles.

CO2 (It’s a fine balance)

In some cases, you may want to increase CO2 levels in your grow room by injecting more CO2 artificially. This is similar to giving a human being oxygen when the person is injured or needs to recover from a period of high exertion.  In a grow room situation, however, there are some factors to consider.

First of all, the level of CO2 necessary to benefit plants is dangerous for human beings. CO2 leaking from the augmented grow room could cause health problems in your home.

Secondly, your grow room will need to be sealed. CO2 injection needs very high levels to be effective, and that concentration will not be reached in a ventilated grow room, even if the fans are turned off. In addition to this, you’ll want bright grow lights to enable your plants to take full advantage of the added CO2.

In most cases, CO2 injection is not necessary for an otherwise optimised grow room setup.


Below are some frequently asked questions around grow rooms and grow room ventilation:

How large should my fan be?

You will need to calculate the CFM of your room using the formula we explained in the article above. This is not difficult to do, and can mean the difference between strong success, or total failure. A few inexpensive oscillating fans will also make your ventilation system more effective.

Does my bulb size matter compared to the fan?

You should get at least a 6-inch diameter fan, as this is the usual diameter of the light hood’s opening. It is a good idea to measure first, as they are also available in 4-inch and 8-inch sizes.

How do I know which type of fan to choose?

There is a wide variety of fans out there. Look for an in-line duct fan with a CFM higher than your calculated need for your grow room (see above article). Choose a make and model that fits with your budget and has good reviews. Check whether the fan comes with a filter included, or if it is an additional purchase and factor that into the cost.

What is Negative Pressure?

When the exhaust (out take) fan pulls air from the room, it creates a vacuum within the room. That vacuum is negative pressure. Air from outside the room rushes in to even out the pressure.

What’s the best type of ducting?

For most systems, the best choice is flexible aluminium. It is not expensive and is by far the easiest to install. Other options include heavy duty rectangular ducting or insulated ducting. The key factor is size – the smaller the duct size, the greater the air resistance. Bigger ducts are better for longer distances and to reduce the workload on fans.

Can I make my ducting more efficient?

There are a few things that can improve the performance of your flexible aluminium ducting. The first is to smooth out the curves and corners as much as possible, and then to smooth out the wrinkles in the tube itself. The shorter the length, and the fewer the curves, the lower the resistance.

What size intake fan do I need?

If you are using a passive intake system, you don’t need an intake fan. If you want an active intake system however, you should choose a fan that is the same size as your out take (exhaust) fan.

You can shop a range of Hydroponic supplies here online at Acorn Horticulture, including a fantastic range of propagation and grow tents.