The two main being the RH (relative humidity) and the temperature. And one of the variables you may be facing to control these, is the actual climate or region you currently live in. Some places are more challenging than others to grow in due to a higher humidity and temperature range, so perhaps take this into account. These base fundamentals can be controlled in various different methods, but what I find to be the most important is the air exchange within the environment. One way to properly control air is by knowing the CFM of your grow room. CFM means, "Cubic feet per minute", and it measures the velocity in which air moves in and out of the environment. If you understand this, then you will understand what size of intake and exhaust fan you will need in order to exchange the proper amount of air. Simply take the height x the width x the length of your room to calculate out how many CFM (mass of air) you need to move.
Air exchange is responsible for everything from keeping CO2 levels optimal, preventing humidity from building up thus preventing mold issues like bud rot from developing, and even keeping the internal temperatures lowered and maintained.
So first let's talk about equipment. It's a given that the amount of watts your light puts out, will directly result in an emission of heat from it. But in some cases, the heat generated from a light source could be more or less coming from a driver affixed to it. Pro tip: Detach the driver (while still hooked up) and remove it from the tent entirely and mount it externally, thus drastically reducing the heat involved from lighting.
No matter what, you will need an exhaust fan to pump air out, and an intake fan to pump fresh air in. I live in Canada, where it is a colder climate compared to parts of the US. So a fundamental for me is a humidifier to combat the dry air, as well as a heater to help tackle the temperature fluctuations. I'm huge on automating most of this technology. Both my humidifiers and heaters have sensors on them that trip on or off to keep the environment within the range I have them set for. This is an absolute game changer, and I highly recommend getting the models with built in sensors. As for my exhaust fans to pump air out, they are also automated where I can set a desired temperature high/low, as well as a humidity high/low. You can also set the fan speed so it will dump out the old air. My intake fans, I have set up to a timer bar, and I manually set them as needed for whatever stage of life the plants are in.
What do I mean by that? Well, plants in different stages require different levels of these ranges. Here is how I always set mine for cannabis:
● During germination: and early seedling stage: Humidity @ 65-70% Temperature @ 20-26 °C
● During Vegetative: Humidity @ 40- 60 Temperature @ 22-28 °C
● During Bloom: Humidity @ 40-50 Temperature @ 20-26 °C
So back to that intake fan on the timer bar... if I'm in a stage that requires a higher humidity, I may set it to cycle on less frequently, like for 15 minutes for every two hours. If I’m at a stage where I require a lower humidity, I may bump the intake timer bar up to 15 minutes on every hour. The only time I set the intake and exhaust fans wide open 24/7 is during flower, because humidity is needed to be lower and maximum air exchange is of the utmost importance. In the case of wide open flow, I first set the intake and exhaust accordingly so that they can work together without over powering each other. To find this perfect flow, I make sure the tent has neither a positive or negative pressure within it by watching the sides of the tent and making sure it stays flush instead of concaving into itself (negative pressure), or puffing out (positive pressure). I find and maintain this sweet spot by controlling the CFM of the environment by altering the size or speed of the intake or exhaust fans.
Pro tip: I ended up giving years back to a gentleman in Texas. As mentioned prior, sometimes the climate you live in can be the problem. He asked me what he could do to get his RH and temperature down within his tent. But no matter what he did, because he lives in an area that both of those are normally higher than average, it seemed nothing helped the inside of his tent. So I told him this. I told him to stop worrying about what's going on inside his tent. But instead to focus on the room in which the tent was placed. Think about it. The intake fan on his tent is only sucking air into his grow tent from a surrounding environment that knowingly already isn't sustainable for cannabis. Start there. Treat the room the tent is in with a dehumidifier and some kind of air conditioning and it should, in turn, fix the problems within the tent at the same time. Sure enough, within a day after my suggestion, his issues were fixed and he was on his way to growing some nice dank nugs without a worry in the world. I like to think this story is where the term, "Sometimes it's better to think OUTSIDE the box", came from in the first place. Lol.
I hope you found this article helpful, and can now take this advice and properly calibrate your grow space to be worry free and have many bountiful future harvests. Don't forget to subscribe to my YouTube channel "Pilly The Stoner", for more helpful tips and tricks as well as follow me on Instagram.