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Grow Room Ventilation

Grow Room Ventilation

Good ventilation is essential for healthy plant growth. In fact, it is one of the most important factors you need to consider when growing, because if there is not enough ventilation in your grow room, your plants will struggle to live. Read on to learn more about ventilation.

Ventilation is about controlling the quality of air, CO², heat and humidity in the grow room. To achieve a high quality of air in your grow room, you need to ensure that there is a sufficient amount of air exchange. The amount of air exchange required will vary according to the temperature outside. But as a general rule, maximum winter ventilation rates rarely exceed 12-20 air changes per hour; however, maximum summer air exchange rates can go up to 60 air changes per hour.

To achieve sufficient air exchange, we recommend that you purchase a good extractor fan. Basically, an extractor fan will help you to achieve the ideal growing environment in your grow room. By circulating and exchanging air, an extractor fan will give you a greater degree of control over the following:


One of the main functions of an extractor fan is to remove heat that accumulates rapidly in indoor growing situations. Ideally, the temperature in your grow room should range between 72-82°F when the lights are on. You should aim to reduce this temperature by 10°F at night. You may seriously damage the health of your plants if you allow the temperature to exceed or drop below this range.

Tip - A heater is essential to keep the temperature around 70°F during cold winter nights.


You should aim for 50-75% humidity in your grow room when your lights are on. A high temperature and too much air flow can both affect the relative humidity (RH) in a grow room. Generally, when growing indoors, a high temperature lowers humidity - creating a poor environment for plant growth. This is a common problem in summer, so additional moisture may need to be introduced via a humidification system.

Tip - Continually check the humidity level of your grow room with a hygrometer.

If humidity in your grow room drops very low (below 40%), it may cause the young leaves of your plants to become smaller, whilst older leaves may simply curl at the margins, appear burnt and drop off. It is important to note that such stresses tend to stunt plant growth. This is more apparent when plants are very young. Large, mature plants naturally raise humidity. 

Tip - Use a humidifier when the humidity level in your grow room is too low.

During the winter months, you are more likely to need an extractor fan to replace fresh CO² rich air and for dehumidification (removing moisture), rather than reducing temperature. Grow room air is not exchanged as frequently in winter, and this enables plants to naturally maintain higher grow room humidity without the air being exhausted.

Tip - A ventilation fan helps to promote healthy growth by constantly replenishing air.

However, too much humidity can create a perfect environment for fungal diseases. Your plants are particularly vulnerable when you switch your lights off and the temperatures drop when the humidity increases. In fact, inadequate ventilation is the root cause of most fungal diseases; where fungal spores thrive in humid, stagnant air and jeopardize entire crops. But if you keep fresh air flowing, then mold can be a thing of the past!


Good ventilation greatly aids CO², which is vital for photosynthesis. In fact, photosynthesis will not take place without CO². Remember, photosynthesis is the process by which all plants make food - the key ingredient for healthy growth.

Tip - An intake fan will bring fresh CO² rich air into your grow room.

Air Circulation

Installing an additional air circulator in your grow room is one way of achieving a more even distribution of carbon dioxide, humidity and temperature - especially during the winter months. We, therefore, advise you to use an air circulation fan with your ventilation system. This will encourage the cold air brought in by your ventilation system to mix evenly with the warm inside air. Circulating fans are relatively inexpensive to operate, and are located to move air along the length of the grow room.


There are many different types of fans available, but here we talk in-depth about the extractor fan. We also touch upon other equipment you can use to improve ventilation.

Extractor fan

The extractor fan is the most critical fan in the growing area. It is capable of removing heat and moisture from the grow room. Remember, both heat and moisture accumulate rapidly in indoor growing situations.

Tip – An extractor fan is also known as an exhaust or ventilation fan.

However, you must make sure you have the correct size fan to get the most out it. Most growers also require a ventilation system capable of dealing with outside summer temperatures.

Each fan has a rating that tells you how many cubic feet of air per minute (CFM) it will move. When dealing with above average temperatures you will want your fan to exchange the grow room air 3-5 times in one minute, so for a room that is 40 cubic feet, a fan that is capable of moving 120-200 cfm (cubic feet per minute) is recommended. If you only want to replace depleted levels of co2 and are growing in a closet using fluorescents, one room change per five minutes (divide room size by 5) will be adequate.

To calculate your room size, multiply Width by Length by Height, this will give you the CFM rating required for one room change per minute.

If is often not feasible to have openings for intake air. We recommend that you use an intake fan as well as an extractor fan. An intake fan is, quite simply, an extractor fan turned around so that it forces air in (rather than expelling it outside). This is referred to as ‘active ventilation’.

When sizing an intake fan you should input 15% less than the actual air flow rate to maintain a negative air pressure. This ensures all air departs the room through the carbon filter. Follow the steps below to work out your intake fan size.

Other ways to improve ventilation

  1. You can reduce ventilation requirements by making some small modifications to your grow room. For example, using air cooled lights will help to prevent heat generated by H.I.D lighting from entering your grow room. They'll also reduce "hotspots", allowing for closer light to plant tolerances. Your plants will also be able to use C02 much more efficiently.
  2. Position your intake fan low down, diagonally opposite the extractor. Remember, you should mount your extractor high up. This will allow a fresh current of cool air to flow across your grow room, whilst removing any hot, humid air at the same time. When mounting your fans, use the proper brackets to avoid excess vibration.
  3. To achieve accurate climate control in hot and cold weather it is worth investing in a temperature regulated fan speed controller. Regular air changes are critical for good growth; however, fans on full power can create noise problems. A combined fan speed controller and thermostat will allow you to regulate the number of air changes per hour. It will also compensate for hot and cold weather by increasing or decreasing the air flow if necessary. Fans will run at a reduced speed continuously, only firing up to full speed when triggered by the thermostat.
  4. If noise is a major concern, consider using insulated ducting. This can remove up to about a third of the noise generated by air turbulence. It is necessary to use at least three feet on both ends of the fan to get the full benefits. Using silencers is also a good way to reduce noise. They can make it possible to run a ventilation system in a small domestic setting, very useful indeed when you have a lot of lights running. Again, for full effect, fix a silencer on each end of your fan. This will help to reduce noise levels by up to a third.
  5. Certain types of crop can cause bad odor. In a domestic setting, the scent of your vibrant crop of garlic may smell wonderful to you, but your guests may think differently! A professional carbon filter attached to the exhaust fan will help you to achieve 90-95% odor removal, meaning that just about any crop can be grown without comment.

(Note - it is important to match the correct carbon filter to your extractor fan. A 600 CFM capacity extractor fan needs to be matched with a carbon filter capable of treating 650 CFM of air or more. Do not try to use a lower capacity filter, it may be able to treat the odors but it will significantly reduce the efficiency of the extractor fan, resulting in high grow room temperatures and a short life expectancy for the fan.) 


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