Grow Room Ventilation: Calculating Your Active Incoming Air

Many indoor gardeners use ventilation to keep their indoor garden's environment within the optimal range for plant growth. Ventilation, or air-exchange, is all about getting that hot, carbon dioxide-depleted and humid air out and fresh, cool air in. Here we take a look at managing the incoming air.

Here's the bad news. There's no simple "one size fits all" formula for ventilating your indoor garden. A indoor grower in Montreal faces very different challenges to his green-fingered buddies in Florida. (A Montreal grower doesn't want to freeze his plants with -30F air in the depths of winter any more than a Florida grower wants to create an "indoor swamp" with 90F air and 95% relative humidity!) It's partly for these reasons that I always recommend drawing your indoor garden's incoming air from an adjacent room—i.e. from inside your house, rather than ouside. The air inside your house is far more stable in terms of temperature and humidity. You never know, there might even be a little more carbon dioxide in it too! ;-)

It could be argued that indoor gardeners who opt for the "sealed room" approach (i.e. split air-conditioner unit, carbon dioxide supplementation, scrubbers and dumps!) give themselves an easier life, albeit with a heftier electricity bill and markedly larger start-up costs. If they want to cool their grow room they just notch up the AC unit whereas a grower using ventilation to maintain his garden's environment is subject to the temperature and relative humidity of the air they happen to be pulling in. 

Grow Room Ventilation 101: Incoming Air Fan Size

Calculating your indoor garden's incoming air requirements.

Don't Get Bugged Out!

Growers using ventilation for their grow rooms need to take preventative steps to minimize the probability of bug infestations. Think about it. You're sucking air into your little botanical Shangri-La all the while. What's the possibility that, at some point, a heavily pregnant aphid is going to hop onto your little gulf stream and be delivered right into the heart of your garden? Once established she has access to endless juicy leaves and almost infinite possibilities when it comes to laying her eggs. In the absence of any natural predators, garden-destroying bugs can breed unhindered. It quickly becomes unmanageable and you basically have to tear everything down, buy a gallon of bleach, set off some pest foggers, and start over. 

Ideal Air duct collar and coupler to help create an air-tight seal

Active or Passive Grow Room Ventilation?

I don't really care. Okay, I prefer active (that is, using a slightly smaller inline fan to blow fresh air into your grow room rather than simply relying on atmospheric pressure) because it puts less strain on your garden's extraction fan and helps it to work more efficiently. However, it's certainly seems easier to filter your incoming air when using an inline fan—largely thanks to ... you guessed it ... incoming air filters!

Filtering all your garden's incoming air is essential to help you win the war against bugs in your grow room. Of course, no amount of air filtering is going to counteract the stupidity of walking into your grow room immediately after rolling around in the grass outside—it's just part of the solution—but it's really important. The question every indoor gardener asks is "How big a fan do I need?"

indoor garden / grow room ventilation duct accessories by Ideal Air

Calculating Your Fan Requirements—The Easy Way!

I've already harped on about this in an earlier article so I'll try not to repeat myself too much. There's also a handy infosheet on it. Basically, if you live somewhere like Vancouver, BC or London, England—you know, not too hot, not too cold, this is a pretty good rule of thumb:

1. Calculate your grow room's active garden volume.

I'm not talking about the volume of the entire room—just the portion dedicated to plants. Of course, you might have filled the whole room and, in that case, yes!

So—if, like me, you have two 1000W lights covering an area of 4.5 feet by 9 feet, and your ceiling is 8 feet high, that's an active growing volume of:

4.5 x 9 x 8 =  324 cubic feet.


Fitting Ideal Air duct collar and coupler for venting my indoor garden.

2. Aim to exchange your garden's air at least once per minute as an absolute minimum!

In North America, extraction fans are rated in CFMs - that's the amount of air (in cubic feet) per minute that they shift.

You may therefore be tempted into thinking you need a fan rated at 324 CFMS but remember these ratings are based on the fan running in an optimal environment. No ducting, no filters, no resistance. 

Add an intake filter. Reduce CFMs by 25%.

A 30 degree curve in your ducting. Reduce CFMs by 15%.

3. Master The Dark Art of Dialing In Your Garden's Ventilation System

It's recommended to overspec both your intake and your extraction fans—if they come with built in speed controllers, even better. Also, you never know, you may want to expand and add an extra grow light or two—it'd be nice if you didn't have to invest in brand new inline fans as a result. When ventilating your grow room, be mindful of your incoming air's attributes. If the air you're bringing into your garden is low in humidity (say 40% or less) then over-ventilating your garden will dry out your plants, stripping them of moisture and stopping them from photosynthesizing properly.