Ventilation and Environmental Control

Ventilation Basics

A five minute ventilation guide for indoor gardeners.

All indoor gardens require ventilation. 

Ventilation means removing old, warm air and replacing it with fresh, cool air.

Why do Grow Rooms Need Ventilation?

  1. Plants absorb CO2 during the day and levels can quickly become depleted. If your grow room CO2 levels drop too low, your plants will stop growing. Ventilation brings fresh air in and, along with it, more CO2.
  2. Plants emit water vapor which increases relative humidity levels. If relative humidity gets too high molds can attack buds and flowers. Ventilation removes this moist air and replaces it with drier air.
  3. Grow lights create heat as well as light. Most species of plants thrive at around 75 - 79 °F (24 - 26 °C). If temperatures rise above this, photosynthesis rates start to drop. If left unchecked, grow lights will heat up your garden too much. Ventilation is therefore necessary to prevent grow lights from overheating your indoor garden.
  4. Even so-called "sealed" growing environments supplemented with CO2 still need to have their air "dumped" at the end of each lights-on period and replaced with fresh air. This is to aid with nighttime plant processes where plants absorb oxygen instead of CO2.

More General Ventilation Tips

  • Smaller grow rooms heat up faster and become CO2 depleted quicker.
  • Larger grow rooms are more forgiving.
  • Low ceilings can be problematic for heat build-up.
  • Extract air from the top of your grow room (warm air rises!)
  • Too much ventilation is a bad thing. Always use a thermostatically-controlled fan speed controller. This prevents you from removing too much air and drying out your room too much. 
  • Air-cooled reflectors help you to remove the heat emited from your grow lights at source and give you more control over your garden's environment.
  • Always extract air to the outside. Draw fresh air from inside (e.g. an adjacent room) as it is less subject to temperature extremes.
  • Use air filters and bug screens to purify the air in your grow room to minimize the risk of contamination from bugs and molds.
  • Don't forget to move the air around in your grow room using oscillating fans. You should aim to simulate a light breeze. Do not point oscillating fans directly at your plants. Instead, move the air in between your plants' canopy and the grow lights.
  • You should aim to replace the air in your indoor garden once every minute. But an 800 cubic feet garden usually needs more than an 800 CFM fan due to ineffeciencies caused by carbon filters and ducting. As a general rule add 25% for a carbon filter and 10-20% for ducting. Keep ducting as straight and short as possible.