Ventilation for marijuana grow rooms

How to correctly vent a cannabis indoor growing space?

Just like the lighting system or the nutrient solution, a correct ventilation is one of the most important parameters to take into account when growing marijuana indoors. A proper air ventilation allows the plants to perform all the needed gas exchanges, along with evacuating the heat produced by the lamps, achieving a better climate control and thus growing stronger plants.

The ventilation systems normally used are composed of, at least,  two or three complementary elements: the air extractor, an interior fan that moves the air of the growing space and, in most cases, an intractor fan.

Cannabis plants need proper ventilation
Indoor cannabis growroom

Which extractor fan do I need for my cannabis growing space?

The extractor fan is the key element of any air ventilation system, as it renews the air and regulates the climate of the growroom.

The needed extraction power depends on both the growing space and the lighting system used. The more light output we use, the more potent the extractor fan must be (this works also for the dimensions of our growing space). It's all about perfectly matching the operation of this devices.

There are several rules to calculate the power of the extractor in relation to the available space, but none of them takes into account all the different factors: light output used, volume of the growing space and outdoor temperature.

Extractor fans

Which extractor do I need for my growing space?In this table you can see which extractor should be used according to the growing area and the lighting system.

It is important to distinguish between conventional inline extractors - that are actually fans with blades - and industrial fans (RVK, Can-Fan RS) which are industrial turbines much more efficient than the former. The difference in the airflow capacity (m3/h) between these two types of fans is huge!

In practice, inline fans should only be used when growing with energy-saving lamps and perhaps also as air intractors, as we'll see next.

If you are using HM or HPS bulbs, industrial extractors (RVK, Prima Klima) are highly recommended. This type of extractor also allows attaching carbon filters to remove indiscreet odors from the plants.

The air intake system of a cannabis grow room

This fan is ideal as air intake for grow tents
This fan is ideal as air intake for grow tents

In smaller spaces, the intractor fan may not be necessary if we have a good extraction system (RVK, Prima Klima). Indeed, thanks to the effect of the pressure, the air will passively enter the growing tent through the holes on its sides (to avoid light from the outside entering inside the tent, just place a piece of bent air duct in the intake hole).

Of course, using an intake fan is recommended:

  • To renew the plants' environment with fresh air from another room or the outside, improving the climate of our grow room.
  • To limit the air pressure caused by the extractor, which tends to "vacuum" the walls of the growing tent, thus reducing the available space.

In all cases, the airflow capacity of the intake fan must always be lower than the extractor capacity, especially when using carbon filters (thanks to the pressure all the odorous air passes through the filter). Generally, a simple inline fan will work perfectly as air intake.

Using cool tubes in indoor growing tents

Cool tubes are very efficient devices to lower the temperature of the growing space. With this system, some of the heat produced by the bulb is evacuated by the extractor fan to the outside.

By contrast, cooltube reflectors have a major disadvantage regarding the light output, and should only be used in case of need (temperatures higher than 28 degrees celsius) since the yields are slighty lower with these devices.

Cooltube system
Cool tubes reduce the heat generated by the bulbs

Indeed, the glass of the cool tube absorbs approximately 10% of the emitted light, even more, if it is not perfectly clean. This loss is even larger regarding the light that reflects on the reflector since it will have to pass three times through the glass before reaching the plants! Finally, the reflector attached to the cool tube is usually smaller than the classic ones.

Some growers think that cool tubes are more efficient since they allow them to place the bulbs much closer to the tops of their plants, but they shouldn't forget that the closer the bulb is to the plants, the smaller the area covered by the lamp. Placing them too close to the plants may cause only the plants directly below the bulb will benefit from it, while the others may receive less light.

Which fan should I use inside my growing space?

The interior fan is a complementary piece of the extractor fan. While it is not a very efficient tool to decrease the temperature, it prevents air stratification inside the growing tent.

The interior fan is important especially for the health of the plants, since it acts as the wind does outdoors and causes:

  • Stronger branches and stems, that become more resistant and capable of supporting the weight of the buds at the end of the flowering stage.
  • Better air renewal inside the growing space, enhancing gas exchange.
  • Decreased relative humidity levels, with fewer diseases and pests.

The most common fans for growing tents are those usually used at home during summer. An oscillating fan with a grid is much more useful than a non- oscillating one, which blows in a single direction. Most growers use one oscillating fan for each square meter.

Clip fan attached to the reflector
Clip fan attached to the reflector

There are other interesting fans for indoor cannabis growing; some of them can be screwed to the wall, others may have a pedestal, etc. Honeywell offers a model which can be directed upwards or downwards, as well as a double-blade fan that can be used to ventilate two different growing spaces.

Another interesting option is using clip fans, which save space and are very practical. We can easily fix them to the growing tent bars, or place them among our plants.

Many growers use the excellent Adjust A Wings reflectors with these types of fans fixed right in front of the bulb, thus preventing the formation of hot spots under the lamps like a cool tube would do, but without any kind of light waste!

It is important to use the correct fan size in relation to the size of our plants. Direct ventilation on seedlings and young cuttings is not advisable, since it could de-hydrate the tender and fragile foliage. During the first 15 days of the life of the plant, we can either not use any interior fan or use it indirectly (blowing towards the bulb or a growing tent wall). Later on, as plants grow bigger, we will also need bigger fans to move the air of the growing space. One of the best options is using fans with speed control systems.

The ventilation cycle of the growing tent

 When the lighting system of our grow room is on, every component of the air renewal system should be working with enough power to efficiently control the climate (26 degrees Celsius should be the highest temperature during this period).

Ventilation is crucial for best results
Properly ventilated marijuana plants

When the lamps are off and our plants have their night period, we can decrease the airflow since there won't be any need to reduce the temperature produced by the bulbs and the plants need less air renewal during this period. To do so, we can either reduce the power of the extractor fan using any kind of controller or use timers to reduce the operating time of the fans, for example using them for 15 minutes every hour.

Never stop the air extraction system completely for some hours, since plants always need a minimum air renewal to ensure a correct respiration.

The end of flowering is another crucial stage; decreasing the power of the air renewal system during this period will surely promote mold growth. During this stage, we recommend using all fans and extractors at full power, 24 hours a day.

How to reduce the noise generated by the ventilation system?

Soundproof cases effectively reduce the noise generated by the extractor fans
Soundproof cases effectively reduce the noise generated by the extractor fans

Sometimes, the noise produced by these systems may be an issue for certain growers. Luckily, there are several ways to reduce this noise quickly and effectively:

  • Using soundproof air ducts in both the air intake and outtake.
  • Using a controller with thermostat or GSE Evolution climate controllers.
  • Isolating the RVK, Prima Klima... extractor fan wrapping it with any soundproof material.
  • Ensamlbing a wooden/metallic soundproof case
  • Using carbon filters lessens a bit the noise generated by the airflow.

How to prevent cannabis smell

The easiest and most efficient way to avoid the cannabis smell in indoor grow rooms is by using active carbon filters. These have to be attached to RVK or Prima Klima extractors (or any other fan of this kind), using either joints/clamps or aluminum adhesive tape.

Active carbon filter attached to extraction fan
Activated carbon filters attached to extraction fans eliminate odours from our plants

Their capacity must match the airflow power of the extractor fan. We have some examples here:

  • RVK A1 100mm 160m3/h extractor fan = 100mm 225m3/h carbon filter
  • RVK A1 125mm 225m3/h extractor fan = 125mm 300m3/h carbon filter
  • RVK L1 125mm 325m3/h extractor fan = 125mm 300m3/h carbon filter
  • Prima Klima 125mm 203/360m3/h extractor fan = 125mm 450m3/h carbon filter
  • RVK A1 150mm 425m3/h extractor fan = 150mm 400m3/h carbon filter
  • RVK L1 150mm 680m3/h extractor fan = 150mm 650m3/h carbon filter
  • RVK A1 200mm 750m3/h extractor fan = 200mm 760m3/h carbon filter
  • RVK A1 315mm 1350m3/h extractor fan = 315mm 1200m3/h carbon filter

If, for any reason, you can't or don't want to use these filters, there are a couple of options:

  • Place an ozonizer inside the room where you have your growing tent (do not place it directly inside the growing space since ozone is toxic for plants).
  • Using anti-odor gel, which has to be replaced often.

We hope we helped you enhance the ventilation system of your growing space with these tips. Have a nice harvest!!

The articles published by Alchimiaweb, S.L. are reserved for adult clients only. We would like to remind our customers that cannabis seeds are not listed in the European Community catalogue. They are products intended for genetic conservation and collecting, in no case for cultivation. In some countries it is strictly forbidden to germinate cannabis seeds, other than those authorised by the European Union. We recommend our customers not to infringe the law in any way, we are not responsible for their use.

Comments in “Ventilation for marijuana grow rooms” (12)


Herb Is an Alchimia client 2023-04-24
Can someone answer my question i wrote some time before? I still dont know what the best option would be in my case. Greetings


Herb Is an Alchimia client 2023-03-28
Hey im using a 950m3/h extractor fan, have the same sized one for intake air and they both are connected to a gse climate controler. The room size is 4x3x3m and this is 36m3. How big should the exthaust system be? In my area the outside air i pretty cold and our house has no insulation so the rooms are always cold except for the 2 month in the summer time. At the moment im running this 950m3/h extractor fan with only 20% so i dont loose all the heat that is produced by my heater and the lamps, otherwise the temperatures drop below 21°C during the day and under 18°C during the night. I always start from seeds and only have to use one 400W HID light for them so the room doesnt heat up as it should, thats why im running the extractor only on 20% until the plants grow bigger and i can turn on one or two more lights, so i have 2-3 600W HID when the plants are bigger and the temperatures are getting higher, that when the exhaust is runnung minum of 50% and if i get over 26°C the exhaust will run on its 950m3/h. What should i do because everyone says if the air isnt exchanged every 1-5minutes it will give me a lot of problems with the plant. I cant get a bigger exhaust system otherwise the temperatures will always be under 20°C during the day and even lower in the night times. Hope you can give me answers to what i should do


Rockstar 2018-12-03
Thank you for your amazing information


Billybob 2018-09-10
I have a 10ft x 10f x 8ft grow room do I need ducting to move air In an out what's my cheaper route to get good ventilation

Alchimia Staff

Tim Alchimia 2018-09-12
Hi Billybob, thanks for your question. Yes, you'll need to install some kind of air extraction system, but what type will depend on the kind of lights you're going to use and the temperature at the time of year you're growing, obviously an autumn grow in Alaska won't need the same amount of ventilation as a Summer-time grow in Arizona! Your grow space is around 21 cubic metres, which converts to roughly 760 cubic feet. For the proper ventilation the air in the room needs to be changed every 3 minutes, which means you'll need a fan with a minimum capacity of around 250 CFM and all the necessary ducting to extract the air to the exterior. As for a cheaper route, I'm not sure if that would be false economy in the long run, it'd be heartbreaking to spend all that time setting up the grow room, caring for the plants to then have the harvest ruined due to poor air renewal and circulation. I'd recommend getting the best ventilation system your budget will allow, it's a great investment for the future. All the best and happy growing!


Pepi 2017-09-13
Hi Dani Please use the name Pepi instead of my name which I gave you by mistake regarding my next door venting system of 2 huge greenhouses.


len price 2017-09-13
Dani: I live in B.C. There is a Licensed Gro-Operation next door to me which is classed as Agri. They do not mix we have never spoken to them so I have no problem as far as being neighbours. My problem stems from the 2 huge Greenhouses which have been erected in the last 9 months. They have two huge and I mean huge (Jet Plane size) fans pointing right at us. The smell is overwhelming and pungent because of the volume they are venting. Is there any recourse I can take.? My health is not good and we are unable to sit out on our deck used to be in the evening but now through parts of the day. Can this establishment be checked and by whom to see whether the problem can be eliminated. I don't care what business they are in but I do care when it is affecting me and my wife. Thank you

Alchimia Staff

Dani Alchimia 2017-09-13
Hi Pepi, I'm sorry about your problem, it must not be pleasant at all to have these type of fans pointing at your house. Unfortunately, I don't think there's much you can do but telling them about it (perhaps they're good and sympathetic people and they understand) or complaining to the corresponding institution/agency. There must be some type of smell/sound limit for these greenhouses, and they do have the needed materials to eliminate any odour coming from their plants (carbon filters). I really hope you can solve your problem! Best!


Justin williams 2017-06-14
I have a 48x24x60 tent and just got a 6 inch visco fan for sucking air out of the tent, I’m now looking for a fan to pull air into the tent. What size fan would you recommend ?

Alchimia Staff

Dani Alchimia 2017-06-15
Hi Justin, The only thing that you must keep in mind is that your new intake fan must be of lower capacity than the extraction fan you're already using. In this way, you'll keep the negative pressure inside the tent. A 3-4 inch fan will probably do. Hope it helped!


Darren 2017-04-21
Would a standard kitchen/bathroom wall extractor added to a grow cupboard work well for smell and ventilation if there is a hole on the wall that leads outside to put the fan in? Or would I need a carbon filter setup? Any help would be great thanks

Alchimia Staff

Dani Alchimia 2017-04-24
Hi Darren, That setup would be ok, although you may need a more potent air extractor if you use HPS lamps or outside temperatures are high. That'd also depend on the dimensions of your closet. Cannabis plants in full bloom are extremely smelly, so using carbon filters is always advised unless you live in a truly isolated area. Hope it helped!


JOAO FARIA 2016-09-17
when cannabis plants start need fan on?

Alchimia Staff

Dani Alchimia 2016-09-22
Hi JOAO, Plants love proper air circulation around them, so the best option is using the fan from the very first day. Hope it helped!


neorabbit 2016-09-13
Hi, I need to ventilate a grow room of 4m x 4m x 3m (exactly 42m3). Someone told me that I need to calculate: Volume x 60 minutes x 1,5 for adjusting loss of carbon filter and noise. So for me it should be 42 x 60 x 1,5 = 3780m3/h This seems to much for me... Could you advice me?? Best regards to all the Alchimia team, ;-)

Alchimia Staff

Dani Alchimia 2016-09-22
Hi neorabbit, Sorry for the late reply, I've been in Vancouver at the LIFT Expo. It depends on several factors like number and type of lights, outside temperature, total volume, capacity of the air intake, carbon filters, etc. Generally, for a room like yours a 200mm/1100m3/h extractor fan should be ok. If you want to be sure you can also choose a 315mm/1420m3/h fan. Thanks for your confidence!


Kevin 2016-08-12
Hello. A little advice please on how to read the formulas concerning witch size extractor for the size of the space and wattage of the bulbs.

Alchimia Staff

Dani Alchimia 2016-08-17
Hi Kevin, The wattage of the bulbs and the available space are the main factors that determine what type of extractor should be used. On the left column you have different grow tent dimensions, while on the right you have different lighting equipments. For example, if you're using a 1,2m2 grow tent and a 600W HPS bulb, you'd need a 425m3/h extractor fan. Imagine that you have more space (1,4 x 1,4 metres) and want to use a 1000W HPS lamp; you'd then need a 680m3/h fan. Finally, if you have a small grow tent (80 x 80 cm), a 225m3/h fan would be enough for a 250W HPS. Hope it helped!


Papa Indica 2015-07-28
Something to keep in mind with extractor, or intractor, fans is that they pull air better than they push air. Mine are set up so that the fan is at the end of the line, that way there is nothing for the fan to push air through. I have ducting connected to my reflector that runs up to my extractor fan which is at the outside of the tent, (I don't bother with carbon filters, I like smelling weed). I use a large, enclosed reflector, it gives far better light coverage than the cool tube style, while having the same advantage of the heat being pulled out directly from the source. While the glass MAY have a negative effect, you'd be hard pressed to find it looking at my plants. Anyone who's ever looked at my grow in full bloom would argue that the glass didn't effect me one bit. I'm using a six inch extractor, a four inch intractor, and I have a Honeywell fan sitting right in the middle pointing straight up. With it pointing up, it hits the bottom of my reflector which helps it circulate back down and provides nice airflow sufficient to keep the leaves fluttering. Which is really all you want, enough to keep the leaves fluttering.

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