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Create The Best Climate For Marijuana Grow Room

Create The Best Climate For Marijuana Grow Room

Once you’ve decided to grow your marijuana plants indoors, your next step is to figure out how exactly you are going to do it. There are a number of factors to consider when setting up your growing environment indoors.

Whether it’s the temperature or humidity of the room, the carbon dioxide level, the ventilation or the smell, you are going to have to arrange everything perfectly so you create the ideal environment for your marijuana plants.

Although this may seem like a huge amount of effort at first, it has a huge payoff once you harvest. The only thing you might regret is not researching or preparing enough ahead of time; if you put in the work ahead of time, your result will be well worth it. This article will cover the following key elements of creating and maintaining a proper growing environment for your Cannabis.


Although it can vary somewhat between varieties and strains, the best temperature for Cannabis to grow in is usually somewhere between 68-77 degrees (20-25 Celsius).

If the ambient temperature around the plant drops much below this, the growth of the plant will slow and its potential yield will be inhibited or possibly even stopped entirely, if the plant never matures. It’s good to note that this temperature is most important during a “day” cycle when you are letting the plant get light.

That’s when photosynthesis and the potential for growth occur. Still, you don’t want large temperature swings between day and night if you can avoid it.

Marijuana temperature

Marijuana growing temperature

If the temperature of your plant rises up above 77 dergees (25 Celsius), the metabolism of the plant will accelerate, and it will require additional inputs: more light, more water, more carbon dioxide, and more fertilizer.

Make sure you plan accordingly for changes in temperature, whether intentional or otherwise. It’s wise to invest not just in a thermometer, but a thermometer which is attached to a ventilation and heating system, so that it can automatically manage the temperature of your grow room. A working automatic system can also provide you with excellent ventilation for fresh air and avoid carbon dioxide shortages.

Read the article Marijuana & Temperature for more info


The ideal humidity in the environment of the Cannabis plant lies somewhere between 40-70%. To measure the humidity, you need a hygrometer.

Although any tool that measures humidity will do, an electric hygrometer is probably a better choice for most growers. It is more precise and often has some automatic functioning that gives you some measure of control over the humidity, which is always good for indoor growing.

marijuana humidity

Check your humidity while growing

If the humidity of your plant drops below 40%, then the plant will experience a faster rate of transpiration. There should be no huge concern. It will simply cause your plant to use up water at a higher rate. As long as there is plenty of water in reserve, you won’t have any issues.

On the opposite end of the spectrum, if the humidity is too high, you put the plant at risk for fungus, especially during the flowering period, where things can get rotten really quickly. You’ll almost certainly want a dehumidifier on hand if you don’t have one automatically set up so that you can fix moisture issues as they arise.

Read the article Marijuana & Humidity for more info for more info

Carbon dioxide

Carbon dioxide as a foundational piece of the photosynthesis process, and one of the major building blocks for how plants work. Out in nature, the air will contain a lot of carbon dioxide naturally, and plants will automatically and constantly be supplied by a reserve of between 300-400 ppm CO2 present in the atmosphere.

Your plants use up carbon dioxide very quickly, however, something you need to be mindful of when working in a closed indoor system. If there is too little carbon dioxide for a plant to use, photosynthesis will not occur at all, and a plant will not grow.

Carbon dioxide

Carbon dioxide

The volume and ratio of carbon dioxide in the environment of your grow room is also something that benefits from having a well-controlled exhaust and ventilation system. CO2 is denser and heavier than oxygen, so a fan will mix it up and make sure that it reaches your plants.

Remember that if you aren’t adding extra carbon dioxide directly into the environment, you need to make sure you let the exhaust and ventilation run the whole time you have lights on. Photosynthesis uses up carbon dioxide very quickly!

Raise CO2 levels

You’ll want to keep a close watch on all the factors described above. Missing the right levels of water, fertilizer, carbon dioxide or heat could severely inhibit the growth and eventual yield of your plant, in worst-case scenarios ruining a crop entirely.

You can bring up the CO2 levels in the air to raise the metabolism of the plant, up to 1500 ppm CO2. Then you want to make the temperature is stable between 77-86 (25-30 Celsius). This also means you want to bring the humidity up from 40% to ~60% (keep a close eye here, that’s quite humid).

You’ll also need to provide extra fertilizer. If you manage all of these factors extremely carefully, you will end up with a significantly higher yield within the exact same grow time, but you have to put in a little bit of extra work.

co2 marijuana graph

1200 ppm of CO2 can double your yield!

If you want a strong cannabis plant with very tight internodes and buds and a short turnover for harvesting, chances are good that you’ll want to add CO2. Beware, however, this technique is very difficult so only experienced growers should try it.

Also, if you have a heat problem and the exhaust system doesn’t work properly you could try using artificial CO2 as an ameliorative option. Things happen faster when you do this, so if you don’t need to, skip it (especially if you are a new grower).

Tip: make sure to download my free Grow Bible for more information about growing with CO2

Once you know what you are doing, you can try it to save a little time. Remember to be careful with adding CO2 though, because although plants benefit from it, it can make the air unbreathable for humans. Plus, it’s unfortunately fairly difficult to measure.


Check this video by Mycoheadgrower about integrating supplemental CO2
You can measure the CO2- levels, but the necessary tools are very expensive. While it does involve a chemical process, it is a one time test.  Measuring involves a glass tube that has been divided into degrees and also a syringe.

The tube will include a reactive substance for CO2, it is then filled with the sample air. Activate the test by breaking the tube at both ends and attaching it to the syringe. Once the air flows through it, the reactive substance will change color. If it does, it indicates the CO2 level.

Cannabis absorbs carbon dioxide and exhales oxygen through stoma in the leaves. Without enough fresh air, those stoma will eventually close, and the plant itself will die. Not so dissimilar from what happens if there’s no fresh air for humans to breath. Make sure you have a good ventilation and exhaust system, whatever it is and however you set it up. It helps you manage a lot of different facets of growth.

Ventilation and air flow

Proper aeration and movement of air in a room can be difficult to control completely, but it’s just as important as getting the right nutrients, light, and water. Proper air flow will give you a stable humidity, temperature, CO2 level and also control the aromatic nature of an indoor grow room.

If you’re growing indoors, you want the best possible environment for your plant. Out in a natural setting, you’d have wind as a natural source to strengthen the structural integrity of your cannabis plant, and rain to help keep it free from dust and troublesome parasites. Try to recreate this sort of environment to the best of your ability. Read on to learn how!

Ventilation and air flow

Proper ventilation and air flow

Make sure that your ventilators are spaced out to different locations around your grow room if possible because you want to move all of the air around, not just the air in one section of the room. You’ll get the best results if you maximize the efficiency of your ventilation units.

Generally, you want two ventilators facing each other in a room, and the exhaust system installed on another side entirely. This sort of setup gives you a more stable atmosphere in the room, helping to balance carbon dioxide ratios, humidity, and temperature.

If it seems like you’re having a lot of trouble with too much humidity in your grow room, consider having the ventilators face the ceiling. That way you will limit any inhibiting factors caused by humidity or moisture. Another option is hanging ventilators right at the same height as your grow lamps. This will help blow the hot air around the lamps out of the room and reduce the temperature in your grow room.

Control temperature

In case you haven’t got it yet, it’s best for your cannabis plant to get lots of fresh air, and especially good to get lots of CO2. The easiest way to make this happen is just to let the exhaust system run all day long. However, it’s very easy to run into problems with this sort of method.

Control temperature equipment

Control temperature equipment

Outside air will not necessarily be  the right temperature for your plants, especially if you are growing at a latitude that experiences winter (which is a lot of indoor growers, frankly), or if you just happen to have cold nights. Although a marijuana plant is hardy, it’s still a warm-weather plant. It thrives in relative heat.

Additionally, you can run into trouble when the humidity of the outside air you are circulating through your grow room is too low. Dry air can damage a plant and inhibit development. All of these issues, fortunately, are fixed by setting up an automated system which gives you greater and more precise control over the environment of your grow room. Buy the best fan controllers at this link

The thermostat is a device that regulates the temperature by automatically turning the heat on and off. You probably are already familiar with this device. What you will need to do is set the right temperature for your grow room.

If you connect a controller with a thermostat to the exhaust system, it can turn the exhaust on when it gets too hot. It can also switch it off when it is too cold. The device can also be connected to a central heating, radiator or climate control.

You have options, but at the end of the day, you’re just looking for something that can sense heat and activate/deactivate heating accordingly. More about temperature in my free marijuana grow guide.

Control humidity

To control humidity, use an automatic moisture meter. It works the same as the thermostat. You can link it to the exhaust system and turn it on or off based on the amount of humidity present.

Optimal humidity

Optimal humidity

One problem with a setup like this is that if the exhaust system gets messed up at all, the carbon dioxide will not be refreshed in the room. Then again, that’s not so different from where you’d be without it, right? The automatic moisture meter can even be connected to a humidifier or dehumidifier, which will really fine-tune the humidity levels in your grow room.

Earlier in this grow guide, we offered a couple hypothetical spots in which you might place your personal indoor grow room. Now we’ll offer a couple extra hints for those small spaces (some of which are still applicable for larger locations).

For example, if your grow room is a cupboard space, you’ll want to be using a cupboard that is close to a window or some sort of exhaust/vent system. That way you’ll ensure that you have a way to cycle air through your grow room.

Measure twice and cut once - when it comes to sawing holes for tubes and pipes, especially in cupboards or rooms of a house. You can fill up any leaks you develop around the edges with silicon, but if you make a lot of holes, you’re just making a lot more work for yourself. Buy the best grow gear at this link

air growroom

Add air into your growroom

For most indoor growers, you’ll want to take discretion into consideration. One of the biggest ways to help keep your grow room private is to manage the odor, which we’ll discuss just ahead. In addition to the smell, however, you’ll want to take some simple steps to camouflage your set-up.

First of all, you’ll want something that hides the exhaust vent you have outside somewhat, without interfering with its ability to put out all that air. You can use plants, or bits of plastic, or those funny fake rocks. Use your imagination!

Another tip is to find any rattling surfaces and apply silicon or plastic to those, too. That way you’ll minimize any loud vibrations or clattering you could have from all those fans and motors.

Also, ask around when you’re checking out various ventilation systems. Some are noticeably louder than others and getting some help from a shop-owner or someone with personal experience can be helpful.

Control smell

There’s a reason people refer to the marijuana plant as “skunky”. It has an extremely powerful odor, especially during the flowering stage. It gets intense.

Your exhaust system will obviously take care of this, whisking the smell away from the grow room for you, but that isn’t the end of it. That smell is going somewhere, but you don’t always want to be pumping the air around your house with the scent of rich flowering cannabis.

carbon filter

Carbon filter, great to get rid off the smell

For this, you can use an odor filter, usually some sort of carbon binding contraption that traps molecules in it. There are a number of different options for this sort of device, and they work far better than trying to spray the air with some other powerful scent in an attempt to mask the odor. Buy the best carbon filters at this link

Most odor filters are cylinders made out of metal with carbon in them. Filters are placed inside outlet propellers so that the air that leaves the growing area is clean. This works because the carbon sucks up the marijuana scent as it passes over it.

To help the carbon last longer, use cotton. Wrap the cotton around the cylinder – it will filter out extra dust particles. This will extend the life of your filter, but it's not a permanent solution.  You will need to replace the carbon every 25 weeks, otherwise it may start to fail.

Thanks for reading. Please leave comments or questions below and don't forget to download my free grow bible.


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Comment Section

31 thoughts on “Create The Best Climate For Marijuana Grow Room

By dan on 12 June 2015

I enjoy your site but while go through your plant nutrients you go into deficiency and did not mention nutrition toxicity and it's cause ie: ph excited newbie

By latewood on 14 July 2015

You should bring this up in the support forum. 🙂

By clint brooke on 30 June 2015

if plant is budding indoors can I take it outside will it keep budding? july 1st and full moon.

By latewood on 14 July 2015

Of course. As long as you area in a region that has a Summer solstice in June. It is all about the length of days, which should be shortening each day In July 🙂

By latewood on 10 July 2015

Yes. You can move a budding plant outside as long as the photo period is no longer than 2 hours of what you had sett inside 14 hours max.

In most cases this may prolong the flowering and finishing period

By hi4life on 16 January 2016

ive been trying to find a hobby, and this site was my deciding factor. every question I had was answered

By Mark on 12 February 2016

I'm wanting to start small grow so I can concentrate on just one maybe two plants my first go . I was either gonna use my closet or I have I nice metal cabinet about six foot tall by about four feet wide and maybe two and a half feet deep. Wanted to know what you think would work best and the best way to ventilate it. Any response would greatly be much appreciated. Thanks for your time.

By latewood.ILGM on 16 February 2016


Both spaces would work well for you. Choose the one that is easiest to modify to include ports for exhaust, and, access for privacy. The metal cabinet has perfect dimensions for a 2 plant grow. The closet might be nice for a small propagation/seed starting/cloning area.

Happy growing

By Mark on 12 March 2016

Oops should have pointed out different Mark

By Leah B on 13 July 2016

Im a rookie just testing my thumb, I have 3 plants in a metal locker about 6x4 with 3-60 watt CFL's on them. The locker is in my basement where its about 73 degrees F. They've only been growing for about 4 days. I wanted to know what else i need, if its too cold for them. Thanks

By gg on 27 November 2016

I was just reading your comments on getting rid of the smell in the grow room. Well i understand that there is some kind of solution in a can that can kill the smell in the grow. Don,t laugh but this stuff is something to do with dead bodys. Is somebody pulling my leg here, but i tell ya the smell i have is so heavy something must be done and fast. can anyone help me with this wonderfull problem i have? PLEASE

By gg on 27 November 2016

Leah B...You need to do alot of reading. theres alot to do when growing and iam sorry to say but your going about things all the wrong way ok. Just keep those plants in the locker and throw away the keys. No pun meat ok.

By Keez on 26 January 2017

I Have a 4x4 tent and using LED 1200 watt 10wchip etc....the younglings are looking good however I'm only using an outtake duct and on opposite side I'm using a fan over he top of a humidifier(budget stuff, taped box etc...)
Any tips on where the LEDs should be placed? Can i top em and i really need a good video
of exactly how to top and when to feed properly. I really love your expertise on this thanks for helping so many!!

By latewood_ILGM on 28 January 2017


We can help you learn to grow successfully but, you need to join our support forum at

We have many knowledgeable members and experts to show you what you need to do in order to have a great harvest. 🙂

By lawrence purdon on 9 February 2017

how many lights for a 5x5 tent ?

By latewood_ILGM on 14 February 2017

Lawrence, That all depends on what size lamps you are using. I would go with a 1000 watt digital lamp.

Perhaps you would benefit from joining out support forum.

By 80s Bush on 23 June 2018

1300 Watts per square meter is ideal, 5x5ft is 2.3m Square so 3000 watts of output is the target. Of course you can use less, but yield will lower and growth times will increase.

By Chris Baker on 10 August 2017

I'm building a completely closed facility with CO2 supplementation, 2 flower rooms, one veg room and one drying room. I plan to alternate the "lights-on" in the two flower rooms to spread my power usage. The problem I have is maintaining temperatures. During the early spring and late fall, temperatures drop here in Oregon to the point where I'll need to heat the "lights-off" room while cooling the "lights-on" room. I've found no one that deals with this situation. I can't just "turn on the ventilators" since its all sealed.

By Cryoguy on 3 October 2017

I have a similar setup and the way did it was to do an air exchange between the flower rooms that way your using the heat from the lights to supplement heat to the lights off room. You'll still need the cooling system but it will be spread between both rooms, also you'll need to provide additional dehumidification.

By latewood_ILGM on 18 December 2017

Chris Baker,

You have to have an intake and exhaust. Even if you want a sealed room. How do you cool the room down? I advise you to join our support forum and Mention "latewood" in the title of your topic and I will be able to help you here.

One major reason for having an exhaust system, even though attempting to acheive a sealed environment is that if the Co2 gets too high (above 1500 ppm is toxic to life) you need a way to draft the air to lower the Co2 levels. Should have been mentioned in the Video.

See you in the forum. latewood

By latewood.ILGM on 18 December 2017

Sorry about "typos" I am legally blind and posted before I edited for sp.

By explorer20276 on 28 August 2017

I have build an enclosure to grow inside. it is 4.1 ft long x 2.5 ft wide x 6.6 ft or 67.6 cu ft. lighting is 4' t-5 4 bulbs about 216 watts and a 250watt cfl for a total of +/- 466 watts. in an enclosure this size 1) is this enough light? and 2) what would be enough of a co2 generating system for this size room? in one of your columns you talked about vinegar and baking soda. would that be enough co2 for this enclosure?

By latewood_ILGM on 18 December 2017


Your lighting is inadequate for flowering plants. Not enough light intensity. You can grow nice plants with a quality yield with your T5's, but must temper expectations of yield. CFL is pretty much only good for a small space during propagation. Some high wattage models are available, but, otherwisem, inadequate for any flowering. Join our support forum for more ideas and guidance on adequate lighting.

Vinegar/baking soda would be fine for a small enclosure. I would want to make sure that I totally understood and could measure Co2 levels. Above 1500 ppm is toxic.

Join our support forum for expert guidance. Place "latewood" in title if you want my direct support.

By Oldpuffer on 17 December 2017

How do you know when carbon filter needs to be replaced?

By latewood_ILGM on 18 December 2017

Oldpuffer. Carbon filters are generally replaced once a year minimum. Hope this helps. 🙂

By Chuck on 8 March 2018

How many square feet should I have per plant for growing in a shed built inside a farm shop. Was planning on a shed 8ft w x 16ftL x 10ft tall.

By latewood_ILGM on 9 March 2018

3 rows with 7 plants would fit nicely and have plenty of room in a 8'x16' shed. You want to be able to move around with disturbing the plants too much. Depending on grow method and genetics, a plant may require anywhere from 4 sf - 9 sf per plant. i.e. from seed untrained plants will take up a larger footprint than say, clones, which can be flowered smaller and so do not take as wide a foot print as a plant from seed.

Just do not cram too many plants in the space, and you will yield well.

Happy growing 🙂

By Sam Crow on 12 March 2018

Just starting my third indoor grow, currently using passive intake but plagued with high temps (30-35 degs) as my 4x4x7 tent with 6" exhaust and 600w HPS light is in a reasonably warm upstairs room quite a way from the window, with the blind only slightly open to allow air in due to privacy issues..

Do you think installing an intake fan would help bring the temps down sufficiently?

By latewood_ILGM on 18 April 2018


That all depends on whether you have cool air to draw from.

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