Building A Grow Cabinet Part Two: Ventilation and Odor Control

by Alex Robles

When we decide to move our cannabis gardens indoors, ventilation and odor control should becomes very important parts of that plan.  In part one of this series I mentioned that when I build a cabinet I use the “Grow Cabinet Triangle” to make sure most of the big questions that come up are covered.  I’ve tried to put this basic information into bite size pieces that can be easily understood but that have to be considered before we put together a micro-grow or grow cabinet.  In this second part I’m going to go over the other two points of that triangle which are ventilation and odor control.  Some of you may think these are one in the same, but I like to split them up because I feel they’re equally important.


A well ventilated area is essential for overall plant health because high heat, high humidity, and stale depleted air will cause and attract mold, pests and stunt growth.  On the other hand a well ventilated space will promote strong, vigorous growth.  Since we are trying to recreate nature indoors a well planned system is going to do more than just exhaust (remove) the hot air.  It’s also going to refresh/exchange and circulate the air.  There’s also a little bit of math that needs to be done in order to get the proper ventilation.

The Cabinet

Before I design a ventilation system I make sure that the cabinet is well sealed so that no pests can get in.  Then I’ll make two holes in it that i’m going to use for exhaust and air exchange.  One of them will be made up high because the heat will rise and collect at the top of the cabinet and it’s going to be the best place to exhaust that air quickly from.  The other hole will be made down low because cool air is heavier the warm air and likes to hang out near the ground.  I also put these holes at opposite ends because I want to give that new refreshed air time to be pull across the grow area before it’s exhausted out.

Air Exchange/Fresh Air

If I don’t exchange or refresh the air that’s in a sealed area with plants in it, that air will quickly become stale, depleted of carbon dioxide and saturated with oxygen which can suffocate them.  Plants do need some oxygen but they need more CO2.

This air exchange will also help control the humidity that will be made by those plants.  The plants have pores called “stomata” and sweat during a process called “transpiration”.

The USGS defines transpiration as “the process by which moisture is carried through plants from roots to small pores on the underside of leaves, where it changes to vapor and is released to the atmosphere. Transpiration is essentially evaporation of water from plant leaves”.

Knowing this, I have to make sure that that moisture is removed from the grow space because it could attract fungus or pests.  Too much humidity will also slow down transpiration because it’s easier for evaporation to happen in drier, less moist air.  I also have to remember that  transpiration is part of photosynthesis, so it will also slow down and cause all other plant processes to slow down as well.  I need to have a plan to remove the excess humidity, I say excess because a certain level of humidity is needed for different stages of growth.  I like to have upto 60%-70% humidity for plants in vegetative growth and 40%-50% humidity for flowering but no lower than 20%.  This is why I make sure that the thermometer that I put in the grow will give me both temperature and humidity.  High humidity will also shorten the life of your carbon filter.

  • Growers Note:  Adding CO2 to increase growth will push up heat and humidity but most small and micro grows are too small to add a CO2 system.  There is usually enough CO2 in the ambient (surrounding) air of a small grow to help your plants thrive.  If I feel like it needs to be added I like to use a product call “Green Pads” that make CO2 and come in three different sizes.  Another easy way I add CO2 into a small space is to use Seltzer water in a spray bottle and spray the plants daily.  I do NOT use club soda or tonic water because they’ll have some kind of sodium (salt) in them and I’ll be spraying salt on the  plant.  This will harm and possible kill the plant, depending on what stage it’s in.  Read the lable on the bottle, seltzer only has water and CO2, that’s all it should have.

The two types of air exchange systems that I use are “Active” or “Passive” setups.  The main difference between these two is just the number of fans that are used.

  • Passive Air Exchange

With this type of system I’m going to use one fan to create a vacuum that’s going to pull the air in, through and out the cabinet.  This fan is going to be placed over or in front of the upper hole so that it’s blowing/pulling air out.  Since there is no air intake fan, I have to make sure that the intake hole (holes that let fresh air to be pulled in) are big enough so that the fresh air has time to circulate around the cabinet before being pulled out.  I also don’t want to overwork the fan by making it blow out air that can’t be replace.  I like to make the intake holes twice as big as the exhaust fan hole.  That’s either going to be one big hole or several smaller holes that add up to the size that’s needed.  All those holes should be covered with a mesh or screen to keep pests and particles from coming in.

  • Active Air Exchange

This type of system simply replaces the intake holes with an intake fan.  This other fan is going to go over the intake hole (lower hole) and help pull in air so the exhaust fan isn’t doing all the work.  With an active system the intake hole can be the same size as the exhaust hole and will be covered with a mesh or screen to keep things from being sucked in.  The thing I am going to have to pay attention to is fan speed.  I want the intake to move more air than the exhaust because I need to give the fresh air time to circulate around the cabinet before it’s pulled out.  I’m going to make this happen be either getting two different size fans or by putting a speed control on one of the fans.

  • Growers Note:  When I’m choosing an exhaust fan I’ll need to know how many CFM’s (cubic feet per minute) of air I’m going to need it to move.  So that means I’ll need to do some math and figure out how many cubic feet are in my grow space.  I do this by multiplying the length x width x height, that answer is the cubic feet of the cabinet and that’s how much air that fan will have to remove every minute.  If the fan has to pull or push air through a carbon filter I will add 25% to the CFMs that I need.

Heat Control

A well planned ventilation system is also going to be able to exhaust all the heat that’s going to be created by the lights, fans and anything else that runs on electricity that’s in that grow space.  Lights are going to make the most heat and it doesn’t matter what kind they are, they’re still going to make a certain amount of heat, the only question is how much.  We have to remember that more watts equals more heat and that heat is going to have to be removed.  Too much heat is bad cannabis plants and bad for equipment.  I don’t want my temps to get higher than the high 80s fahrenheit because it will start to stress the plant and cause it to stretch and could affect bud (flower) yields just to name a few things that high heat affects.

Air Circulation

In nature plants are exposed to slight breezes which make them sway back and forth, this swaying helps strengthen the stems and branches.  Think of it like lifting weights to help build muscle.  Those breezes also circulate the air through and around the plant which helps moisture evaporate off the plants surface.  This is important because if moisture is allowed to sit around to long it could attract pests like fungus gnats or spider mites and fungal diseases like powdery mildew or bud rot (a.k.a. gray mold).  Since we’re trying to mimic nature in a cabinet we need to find a way to recreate that breeze.  The best way to do this is to put a small fan inside the cabinet to blow across the top of the plant canopy.  If I have the room I’ll put another fans in below the canopy to help circulate the air around the pots as well.  If I’m using an “active air exchange” I’ll make the intake fan do double duty and circulate the air below the canopy.  Plants enjoy and thrive with a breeze, wind on the other hand can cause damage.  When a fan is turned up too high or to close to the plants it could also cause “windburn”.  I’ve seen plant “claw” (the leaves curve downward and look like a bird claw) when they get too much wind from a fan and will affects the leaves closest to the fan while the rest of the plant will look fine.

The first time this happened to me I thought I was over watering so I eased back on the watering schedule.  Then I thought it was nitrogen toxicity so I starved that girl for a couple more waterings.  By the time I figured out that it was windburn and that it can look like both of these conditions she was not happy as a matter of fact she looked pissed.  I loved that girl, she taught me a lot.

Odor Control

Odor control begins with a well sealed grow space, which means every unneeded whole has to plugged.  Those opening will let odors escape, will let light in and maybe even pests.

A female cannabis plants in vegetative growth will begin to smell and that smell will only get stronger as they mature.  When we’re growing indoors controlling those smells is an important part of our grow space and should be taken seriously.  A well planned odor control system will keep the peace in your house between you and your family or housemates because not everyone enjoys the smell of cannabis.  We also have to remember that when those odors escape they become a security risk because that could alert criminals, nosy neighbors or the law to what we’re doing.  It doesn’t matter if you’re in legal or prohibition land, it’s never a good time when uninvited people show up at your grow space.  A carbon filter is the best answer that I’ve found by far for this problem and I have total faith that it will scrub the air clean before it leaves the space, putting my mind at ease.  Another added benefit is that it’s going to remove any dust or airborn mold spores that might be floating around in the air.  Carbon filters and fans come in different sizes for almost all size of grow cabinet or closets.  If the space only has sprout, seedlings or clones I’m not going to worry too much about the smell, plants in these stages typically won’t give off much of an odor.  But remember that if the clones were cut off of a flowering plant, they will smell.

    • Growers Note:   If the cabinet size is too small for an off the shelf filter I’ll have to build one.

When we’re dealing with ventilation and odor control we need to make sure that we cover as many bases as we can because a well planned system will always repay you with peace of mind.  So I hope this information helps with the planning of you’re next grow cabinet.  And remember to always

Grow, Learn, Teach

Click here for “Green Pads” CO2 pads

Click here for small fan

2 thoughts on “Building A Grow Cabinet Part Two: Ventilation and Odor Control

  1. Taylor Bishop August 7, 2017 / 4:09 pm

    Thanks for explaining more about the ventilation and odor control for grow cabinets. I actually didn’t know that adding CO2 can increase growth, and that there is enough of it in the ambient area to help plants thrive. I’d be interested in learning if this is still the case for drier areas, or maybe even for areas that are too humid.


  2. inmygrow August 8, 2017 / 1:16 am

    I don’t know if CO2 levels are affected by humid or dry air. In a micro-grow or closet grow there is enough in the surrounding air when you have good air exchange. I don’t add CO2 if my space is a 4×8 tent or smaller. Once I start adding CO2 to the air I can’t exhaust that air anymore because I’ll be pushing out the gas I paid for. At that point I have to get a carbon scrubber to scrubber the odor out of the air, I would use this in a grow room with CO2. I hope this helps and didn’t make it too confusing. If you have any more questions, let me know.


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