The goal with an indoor grow room is to create a perfect growing season time and time again without the inconsistencies found in an outdoor growing environment. Much of this relies on man-made tools to improve on nature’s design. For example, we can make lights that mimic the sun’s spectrum and cover the walls with reflective materials to improve that light's coverage. We can feed nutrients to imitate fertile earth and watering systems that give the exact right amount of water down to the mL. But what we can’t create, is air. Fortunately, we don’t need to. There’s an abundance of fresh air outside; we just need to get it into our grow room. For that, we need ventilation.
What Is Ventilation?
Without trying to get too technical, ventilation is just simulated wind; with wind being a natural byproduct of higher pressure air moving into an area of lower pressure. So all we have to do is create an area of low pressure and an area of high pressure in our grow room and let physics take care of the rest. Simple, right? In a smaller room or in a grow tent, it really can be just that simple. The larger the space, the more complicated your ventilation setup will need to be.
Benefits of Good Ventilation:
- Provides Continuous Fresh Source of CO2
- Helps Remove Heat
- Helps Reduce Humidity
- Airflow Deters Growth of Mold & Mildew
- Strengthens Plant Stems
Ventilation Systems: Passive vs. Active Intakes
There are two basic types of ventilation setups but they serve the same purpose: to continuously bring fresh air into the room and expel warm, stagnant air. For this to happen, at minimum we need an intake vent and an exhaust fan.
Exhaust fan - This fan is placed near the top of the grow room and is attached to ducts to suck hot, humid air from the room and return it outside. It's recommended that the duct-work run through the light hoods to decrease temperatures in the room and inside the light reflectors which will help prolong the lifespan of your bulbs.
Oscillating fans - The goal of these fans is to produce a nice gentle breeze to cool the plants' leaves and keep air circulating within the room. This constant breeze will also help improve the strength of the plants' stems as they mature. The number you need will depend on the size of the space.
The two fans listed above make up what is known as a 'passive intake.' In a small, otherwise air-tight room, this system is preferred and works extremely well. As the exhaust fan removes air from the room, much like sucking on a straw, it creates a vacuum which pulls air in through the only other place it can enter, the intake vent.
The intake vent should be situated at the bottom of the room, preferably on the opposite side in relation to the exhaust fan, to create a cross flow of air. The size of the intake vent hole should be approximately twice that of the exhaust fan's hole to create the proper ratio of suction within the room.
In an active intake system, we're adding a fan to the intake vent to help pull outside air into our grow room. Large rooms and warehouse setups will often have an active intake. Also, since this system is less reliant on the size of the intake hole, rooms where larger intake holes aren't possible will also benefit from an active intake.
One secondary thing to consider when deciding what system is right for you is that an active intake system will be less efficient at cooling the space than a passive system. Again, without trying to get too deep into the physics of wind creation, just know that fan blades compress air which in turn creates heat. It may not be too much, but worth mentioning.
Air Exchange Rate
First, air exchange is very much what it sounds like: the replacing of the room's current air with all-new outside air. There is no clear-cut advice on what the optimal air exchange rate should be with ranges varying from once every 2-3 minutes up to once every 5 minutes. The reason for this is that every grow room is different, affected by external air temperatures, lights used, if you're running exhaust ducts through your light hoods, etc.
A good rule of thumb, however, is to let the temperature be your guide. If you find that you're unable to keep temperatures down in acceptable ranges, you will need to exchange air more frequently. To do this you may need a stronger fan or simply turn up the speed of your exhaust fan if it has multiple speeds. The likelihood is you'll want to exchange air closer to the 2-3 minute mark than the 5 minute mark. Also, if you're growing for a dispensary, you'll want to check that there aren't local regulations mandating air exchange rates.
To determine what your air exchange rate is, first determine the cubic feet of your space (Length x Width x Height) then compare that to the "CFM" (Cubic Feet per Minute) of your fan. For ease, if your space is 5' x 5' x 10' (250 cubic feet), and you use a 256 CFM fan, the air in your room will exchange at a rate of right around once per minute. In this example, the exchange is actually a little too fast, so you'll probably want a less powerful fan.
Ventilating Through Your Light Hood
Up until now, much of this guide has been about letting you know what your options are. As it relates to running your exhaust ventilation through the light hood, this is highly recommended as a best practice.
How this works: an exhaust vent is mounted next to your light with a duct that connects to the side of the light hood. On the other side of the hood, a duct runs outside where the exhaust fan is mounted. The idea is to suck the hot air both from the room and from the light simultaneously while creating your suction in the room to pull fresh air in through your intake vent. It's very important that the fan be on the opposite side of the light or else it won't dissipate as much heat. Remember, pulled air cools better than blown air.
And finally, as no system is complete without ducts to focus the direction of airflow, let's talk briefly about them with a couple do's and don'ts.
Flexible aluminum is generally the obvious choice for ducts as it's cheap and readily available. More expensive options are available but will depend both on your budget and your need. Ideally you'll want short, straight ducting to maximize your airflow. The further air has to travel the weaker the pressure will be. Avoid bends in your ducts as much as possible.
And last but not least, something to consider if you're building your room and ventilation system from the ground up: you might want to think about clean room technologies such as HEPA filters on your intake and exhaust. In tandem with oscillating fans inside the room, this can dramatically reduce the risk of powdery mildew and mold on your flowering plants.
And let's be honest, that's why we're here right? It's not because we love ventilation systems, or optimal humidity and temperature conditions. It's because our goal has always been creating a perfect environment to grow the highest quality buds time and time again.