Indoor gardening is following the course of most emerging industries, in that solutions borrowed from mature industries are adapted to fit new needs. Over time, outdoor plant products, filters, fans, building supplies and lighting technologies have all been adapted to better fit the needs of the indoor gardener. As the marketplace continues to develop, information about product performance continually passes among growers, retailers and manufacturers. This shared information eventually leads to improved products that result in better, and more efficient, grow techniques.

Rapid changes might occur as an industry moves towards maturity, and gaps in knowledge can be common as users strive to keep up with the new tools, supplies and equipment, while manufacturers get caught up in developing and not take time to communicate with the end user. Thankfully, there are industry publications to provide a bridge.

Thanks to information published over the past several years, indoor growers have become aware that too much humidity can cause disease, rot, mold and mildew—and that this is especially true when plants get to be dense and full. However, what solutions work best and why, or even how much moisture needs to be removed from an indoor garden, are areas in which awareness isn’t as common. This article addresses humidity controls in growrooms.

The Difference Between Absolute Humidity and Relative Humidity

Rule #1: Relative humidity (RH) is not the same as absolute humidity

Absolute humidity is the quantity of moisture in the air—for example 100 pints of moisture in a grow room—whereas relative humidity refers to the amount of moisture in the air relative to the maximum amount of moisture that could be in the air (75% relative humidity = 75% of the maximum moisture content).

Rule #2: A warm environment can support more water vapor than a cool environment

An 80°F room can have twice the moisture in it as the same room at 60°F. In other words, a room that is 80°F and 50% RH during the lights-on cycle can reach 100% RH if the temperature drops to 60°F. When relative humidity reaches 100%, this is called the dew point—as the temperature drops, liquid water (or, dew) will quickly condense out of the vapor. Adding a small heater to raise the temperature by several degrees in an otherwise cool wet growroom is a simple way to reduce the percentage of relative humidity when the lights are off and can be a useful stopgap measure while working on a longer term solution.

Rule #3: Your key to mold control is moisture control

Mold growth requires fungal spores, organic material and moisture. Spores are ubiquitous and difficult to remove through filtration, so moisture is the only variable you can realistically control. The idea is to solve any moisture problems before they become mold problems. Experts consistently agree that 50% relative humidity or lower is your target for preventing mold.

Rule #4: Humidity equalizes rapidly

While temperature stratifies and requires air movement to achieve balance, humidity equalizes rapidly. This is useful because it means dehumidifiers don’t require ductwork in most growrooms. Rapid vapor equalization also poses a challenge, because water evaporates quickly, forcing growers to reduce evaporation by cutting unnecessary water exposure to open air.

Options to Control Humidity: Ventilation, Air Conditioning, Dehumidifiers


Ventilation is best for small growrooms and can work well for most hobbyists. This solution works by diluting the water vapor, which works well in dry climates with moderate temperatures year round but comes up short during humid seasons or in humid climates. An example of using ventilation to keep a growroom dry is when the conditions outside of the room are controlled by the use of a grow tent. Growers using this method should filter the incoming air with pleated air filters to help prevent the introduction of pests and other air-borne contaminants. One major issue with using ventilation is that it does not work well for growers using supplemental carbon dioxide (CO2). Another downside to ventilation is that it will have some effect on room temperature. Though outside air in New Mexico or Arizona is very dry, it can be extremely hot or cold at different times of the day, providing less than optimal results at those times.

Air conditioning

Air conditioning works well at removing moisture when lights are on, temperatures are near 80°F and there is a need for cooling. Using an air conditioner to dry a room when the lights are off is not recommended. Excessively cold temperatures can stress plants. While air conditioners use great amounts of energy to remove moisture and as temperatures go below 70°F, air conditioners tend to freeze up. Supplemental heat can be used to drive the need for cooling and to prevent the air from getting excessively cold and freezing up the air conditioner. The problem with this is that supplemental heat requires lots of energy and quickly becomes expensive to maintain. It isn’t wise to use your cooling system as a long-term moisture management solution.


Dehumidifiers are the only sure way of controlling humidity at all times. Although commonly available dehumidifiers are designed for light use in residential basements, there are two companies in the United States that build high-quality, high-capacity commercial grade dehumidifiers. These are extremely energy efficient, don’t create excessive heat and will last for many years. For smaller grow rooms, removing the condensate bucket and attaching a garden hose to drain the water, if needed, can modify basement-style dehumidifiers. Larger operations will want to consider the extended reliability and moisture removal capacity of commercial-grade dehumidifiers. Also, for best performance, install air conditioner vents and dehumidifiers as high in the room as possible to ensure that cold or hot air is not blown directly on plants. This will assist with water drainage from the equipment as well.

How Much Moisture Does My Air Conditioner Remove?

With so many variables in play, there is no general rule of thumb to estimate water removal based on air conditioner model, brand, air temperature, run time or room size. Fortunately, there is a relatively easy method any grower can use to find out how much moisture their air conditioner removes.

Collecting runoff from the air conditioner is usually a fairly simple task. Water from a 24-hour period is sufficient for finding your water removal rate, but the longer time period, the better. Yet, collecting water over a four-hour period—with air conditioning running, of course—is enough to estimate the amount of moisture your air conditioner would remove in 24 hours. Conduct this experiment during a time when moisture is most problematic, such as when plants are large and the external environment is humid. Don’t forget that air conditioners often don’t run continuously when the lights are off. In other words, if the air conditioner runs constantly when the lights are on, but only for, say, 30 minutes when the lights turn off, your air conditioner isn’t removing any water for a major portion of the cycle.

How Much Moisture Needs to be Removed to Protect My Plants?

Plants transpire about 99% of the water they receive, so the amount of water your plants receive is approximately the amount that needs to be removed each day. For example, 30 plants each receiving 0.5 gallons per day is 15 gallons total water per day. If you water every other day, divide by two in order to get an average per day.

So, 8 pints per gallon multiplied by 15 gallons is 120 pints per day of moisture into the room. Use the results of your moisture collection experiment to subtract whatever water your air conditioner removes from your total input water. If you have a remainder then that is the water that must be removed by a dehumidifier. It just so happens that dehumidifiers are sized by their capacity in pints per day. It is important to check these numbers at the peak of the plants’ growth or the final two weeks of flowering.

A little understanding and a little investigation coupled with a bit of math is all you need to ensure you have the right equipment to protect and grow your investment under all circumstances.