Why you need negative pressure in the grow room

Liz Filmer
30 Sep 2021

You may have heard the term "negative pressure" used regarding indoor growing. Do you know what it is or why it is necessary, though?

Negative pressure is when an enclosed area has a lower pressure than the area surrounding it. Think of a drinking straw. By sucking on the straw, you create a space of negative pressure or a vacuum by removing the air. To relieve that negative pressure, liquid rushes INTO the straw.

So how does this apply to your growroom? Well, your growroom is a perfect example of an enclosed environment, and your ventilation system is the air source that is entering and leaving it.

Negative pressure is created by the rate at which the air is entering and leaving your growroom. If more air leaves the grow room via your outtake fan than entering it via the intake fan, then you have achieved "negative pressure" and created a vacuum. You will know that this is the case by looking at the outside of your tent. The vertical sides should look concave (pulled in) in appearance.

Negative pressure is beneficial to your plants, making it easier to control and maintain humidity, temperature, and CO2 levels. Negative pressure helps to remove hot, humid air quickly and at a constant rate. This creates a stable environment and avoids any risk of mould or mildew.

The other benefit of negative pressure is that it controls unwanted odours created in your grow room. Negative pressure removes all the smells along with the unwanted heat, humidity and CO2. By fitting a carbon filter to your extraction ventilation, all odour leaving the grow room is "scrubbed" clean and is undetected! This stops the odour from sitting in the tent, growing stronger and seeping out into your home and surrounding area. Always match up the correct size carbon filter to your exhaust fan to achieve the best results.

Want to know how to create negative pressure? Adequate ventilation is the key! You need two fans, an intake fan that brings cool, clean air into the grow room and an extraction fan that excretes the warm odour filled air out. The size of your fans will depend on the size of your grow room and the amount of air you need to move. 

Your extraction fan should always be more powerful than your intake fan. This is so you can guarantee that more air is leaving the grow room than entering. It is always good to buy a bigger/more powerful extraction fan than you need anyway. This way, you can run your fan on a low, quiet setting which still has enough power to create negative pressure day today. However, you will also have that extra power boost should you need it on super hot or humid days. 

You should also ensure that your ducting is working for you too. I would recommend buying acoustic ducting as it keeps airflow noise levels to an absolute minimum. Keep it straight and short in length as your fans performance can be reduced by 2% for every meter of ducting you use. Changes in the direction of your ducting will slow airflow and cause condensation.

Liz Filmer