Understanding Cooling and Heating an Indoor Tent / Grow Space - Info Guide

Hi Grower,

I wanted to put together a good topic that can be a central point to guide people to answering some questions about heating and cooling an indoor tent or grow space from the viewpoint of an engineer.

I am an electrical engineer with a minor in bioengineering (human not plant). I will be covering some topics that most might not take into account when setting up a tent or if they begin to experience heat and cooling issues. While I have years of experience with my jobs and calculating thermal dynamic tests I would like to see others give their advice on what I will provide here. I use the methods I will cover in this myself and while I won’t go in-depth on the physics calculations I will at least discuss basic thermal dynamics and how they will affect your indoor grow space.

For starters it’s important to know the difference between cooler air and warmer air. At the most basics for creating a controlled environment you need to remember that cooler air is more dense and will be lower to the floor than warmer air. As well molecules are always trying to reach equilibrium with surrounding objects transferring hot to cold not cold to hot. These are important to remember when you experience temperature issues.

Difference between Absolute and Relative Humidity


This is important to know when understanding how your temperatures will affect the RH in the grow space. As your temperatures drop this can raise the RH as the temperatures rise they will lower your RH. The reason I bring this point up is that kit or cheap Hygrometers will have a common flaw amongst them which is that they are not as accurate as a calibrated meter. This is important to remember as if the meter is placed close to a moist spot or in a direct path of air that is combined with a humidifier this will drastically change the readings. Therefore it’s important to either have multiple measurements inside the grow space to determine a true average if you believe you have issues with humidity before changing temperatures.

With those understandings you can now take a look at your grow environment. Where you place your tent or grow space is important when you want to be able to control the temperatures with precision and less cost to your energy bill. It’s important to know that your tent or space needs to be as far from air vents, windows, electrical appliances, and other tents if you want them at different temperatures. A factor that most don’t think about is the radiating heat that devices put off even objects that are close to windows that have a high reflection material. This will also come into play with what you place in the grow space from the lights, fans, humidifiers, and sensors or controllers. Any electrical device is going to put off heat energy and without proper air exchange this can be a battle trying to balance as everything is always trying to reach equilibrium.

Knowing what is around your tent and inside it and how much heat they are generating can help you determine what you need to do to maintain your target temperature. While you don’t necessarily need to know formulas for calculating CFM air movement you should know how to minimize the energy needed to reach the target.

Moving air as it moves naturally

Simple solutions for this are to make the physics work for you. By keeping air intake as low to the ground as possible you can reduce the need of portable AC units. Or if you’re trying to warm the grow space having the intake as high up as you can to pull the warmer air in. You should also remember how long your exhaust lines are or your intake lines. If you’re bringing cold air past a hot object (LED Light) your air intake temperature will be different then the temperature at the end of the duct. The same goes for your exhaust, if your duct is super long the heat that it’s sending outside the tent is going to radiate to the surrounding air. If you have a fan near this it will have to work longer to reach the temperature that you want. This is why it’s important to be able to exhaust your space outside with as short of a duct as you can. If you believe that it is a problem due to higher temperature environments getting an insulated duct can help alleviate the issue.

Ideal Environment?

A crucial key to remember for your grow space is to keep the external temperature of the room 10-14 degrees warmer or colder from your target temperature. This allows you to use the room air to offset the equipment in the space. However if you know that your grow space is generating more heat having a 20 degree offset can help reduce the amount of time intake fans need to run as the space can be cooled faster. If you need to use a portable AC unit or a portable heater, its best to keep these relatively close to the intake but not close enough that any heat they are generating that you don’t want is able to change the tent’s exterior or grow space. Remember that Portable AC units can generate a lot of heat and always need to be vented to the outside. If you can’t accommodate that finding alternatives for your environment would be better as portable AC units dumping heat into a room will run non stop and waste more energy they they will help with. If this is the case I would recommend trying a Dry Ice cooling method with a fan. (I won’t go into how this process can be accomplished on this topic here as there are many factors that you need to take into account as this can generate large amounts of CO2)

If you can’t exhaust your space to the outside having your intake and exhaust on separate sides can help reduce recycled air problems.

Remembering to create a ventilation system in the direction of the change that you’re wanting to reach will reduce the energy costs for you in maintaining your target temperatures. Such as pulling cold air from the bottom of the space and venting at the highest point to remove the warmer air and vice versa.


While this isn’t something that you should be super concerned with if you’re having troubles with your grow space you might see if you have any leaks either releasing the cooler air or bringing in warmer air. To alleviate these the more sealed you can keep your space with clamped ducts, closed vents, or restricted flow channels can help maintain your target temperature longer. If the room you have to use is the only one available and you just can’t make many environment changes a simple solution is to insulate your grow space. This can be done relatively cheap using material like Reflectix®. You can order these from Amazon or most hardware stores. These are a great solution if you need two or more tents near each other that you want at different temperatures. As they will insulate the cooler air to stay inside while reflecting the heat energy away.

Fan’s Fan’s and More Fan’s

While this may seem like it’s working this is not the answer for changing and maintaining target temperatures. Imagine just putting a bunch of fans in a confined space each fan blowing a different direction, if you could see the air you would notice that its moving so much that the cool and warm air are basically becoming entangled with each other and not able to properly vent. This is just going to cost more energy and not reach your target temperature. Instead try to think about how your air is moving and ensure it’s moving the direction of the change you want to occur.

Now a good idea is you can add fans to your vents or low to the ground. These will bring in colder air and if your exhaust is at the top the heat is leaving and the cold air is taking it’s place until radiating energy changes it. If you’re using a tent you can create a vacuum in the tent which helps keeping the airflow moving in a constant direction with minimal energy expenditure.

Get a bigger fan?

Not always a good approach, again if you’re not looking at all possible reasons why your grow space is having temperature issue’s just getting bigger fans won’t solve your problem. More often temperature issues are a result of something either working harder than it should due to leaks or radiating energy or improper ventilation.

Engineer Approach

Now I am not going to say go out and buy calibrated equipment or fancy thermal cameras or IR sensors especially for a new grower. These can start racking up the costs and if you don’t know what you’re looking for you can end up getting wrong items or buying multiple ones. However, if you have tried all the solutions you can, good air exchange, changing the ambient environment, shortening up exhaust and intake ducts and you’re still not able to reach the temperatures you want getting a camera or an IR sensor can help. Being able to see areas such as exterior walls generating heat close to a tent, electrical equipment putting off more heat than you thought, or hot or cold spots in your tent/grow space can give you the answer you need to make adjustments to hit your target temperatures.

Other benefits can include being able to see your plants exact temperatures. This can be helpful in ensuring there are no hot spots being generated near or on the plants. But again you can get by without this, but if you’re planning to get one ask lots of questions don’t jump into a purchase!

True Temperature

True temperature vs the kit hygrometers. When you want to know what the temperature of a closed environment is that has air exchange the best thing to get is a liquid detached glycol hygrometer. The reason for this small spaces with moving air will change your readings. The glycol meters can help prevent this and give you a more precise reading as it takes longer for the liquid to respond to temperature changes in the environment. They can be bought for the same cost as a good hygrometer but will be more accurate in your min and max values for knowing what your plants are feeling. No reason to break the bank with these $30-40 is a good price point as you shouldn’t need a calibrated or certified meter, unless you really just want to be that precise. The benefit with these is that you can periodically move the probe around to get a good idea of how the temperature is in different spots in the grow space without running risks of air flow changing the results drastically.

Final words

As I mentioned this is something I have to deal with regularly as an engineer. While I am new to growing at the time of posting this, all of the advice I have provided I used when determining where to place my grow tent, length for my ducts, equipment placement, and energy offset. Now I do use a thermal imaging camera to monitor my tent along with a cheap hygrometer. However, I only use that meter as a reference point. I use handheld meters that are more accurate to take multiple readings at different places in my tent to know my true temperatures and relative humidity levels. Do you have to be as precise and data hungry as me? Absolutely not! This is meant to just get some gears turning for a problem you might have or help you plan a successful grow without having temperature and humidity issues right off the bat and spending extra money trying to find solutions.

I will always monitor this so if anyone has questions or want’s to know what equipment I use by all means tag me with a question. I won’t pretend I know everything especially with growing plants. But I have to think like an engineer and that means trying to perceive all variables and how to take them into account.


As I am new and do not know many people yet I wanted to tag some who have helped me and given me great advice. Hopefully this post will help others just as they’ve help sharpen my botany skills I can help others. @AfgVet @MadamCalamity @kaptain3d @MattyBear @patchman @latewood @imSICKkid @PurpNGold74


Useful info here

Thanks @PiratedSoldier


Wonderful information here! Thank you @PiratedSoldier for tagging me!


Lol, yep sorry I was typing to fast and my brain went opposite there when trying to keep points together. Unfortunately I didn’t catch it so not sure if someone else can fix that.


@MidwestGuy can someone edit this.

I’m not a moderator. @Myfriendis410 and @dbrn32 are usually around in the mornings.


Wasn’t trying to cause embarrassment I figured a mistake thought you would want to know. That’s a basic of understanding hvac and hate for a newbie to start out thinking 2x2 is -7


Say less :wink: nice topic.

Fixed. But again. Love this topic! Great info


I think I know where my brain went to for that. I’m currently running thermal shocking profiles on prototypes at work and we have to run them from cold to hot 70 cycles, and it’s very critical that the profile doesn’t change. So my brain is going Cold to Hot, cold to hot, don’t skip, don’t skip.

So it was yeah this looks right, good job :+1:t2:!

This is why engineers can’t be left unsupervised. We always need sanity checks from others before putting thoughts out into the world. Appreciate the catch.


Yeah I totally agree. Since joining the forum I’ve read through so many posts and I kept seeing people having issues with temperature and jumping straight to wanting to buy more equipment to fix it and I was like, wait think about your environment! I didn’t want to see people drop hundreds or thousands of dollars throwing equipment at a problem that might be as simple as moving a tent 3 feet to the left or moving a fan.

I tried to searched for a topic that looked like it was pretty complete for talking about indoor temperatures but they all seemed to be user problem-specific. So the hope is this topic can be a quick way to get a cumulative amount of information to help growers save money and make an awesome grow.


So my grow room is a section of a metal storage building.
Thru these last few months(hot,hot summer at least 90° days everyday) a 5000btu window unit has kept my approximately 8x12x7-8’,less than 700cf.
In the 70-80° range with my temp and humidity gauges placed all over the place…lol…I have them everywhere…and humidity 50 to 60% sometimes a lil over 60.
My worry is winter…because really it seems the more my aircon has to run…the lower my humidity stays(which has been a relief) .
But winter? With and electric heater…Im thinking I’m going to need a humidifier?
What comes to your mind as a solution for having to heat?
A combination you would try?
Am I correct that an electric heater of some kind is going to drive the humidity way down?
My mind went to the old oil heaters might not be as bad on humidity…but I don’t think a cheap one will keep it warm enuff.

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Look at doing infrared heating. These type of heaters will not dry the air as traditional ones. A couple options you could use are either a controller or there are heaters with humidifiers built in though I can’t speak to their efficiency at maintaining a certain level or just dump out till the tanks empty.


Lots of useful info! I’m giving this a try for the first time, and there’s so many factors to consider. Every time I have one under control I find something else I have to get into the proper range. So far the babies are growing, and they’re still green, so I’m getting it close enough I hope!