How to Control Humidity in Your Indoor Grow Room


If you intend to grow cannabis for either personal use or commercial purposes, then there are a few essential considerations to bear in mind. Firstly, you must invest in appropriate lighting, good air conditioning, and humidity control. By doing this, you will ensure excellent airflow and a level of control over your grow room’s temperature and humidity. This will also help to maximize your yield and minimize the threat from diseases and pests.

Most novice growers have a handle on controlling a room’s temperature. Many may even know a thing or two about air circulation. However, novices often tend to struggle when it comes to maintaining the ideal relative humidity (RH) level in their grow room.

For instance, did you know that humidity changes when you turn your grow room lights on and off? Sadly, many growers fail to pay attention to humidity until they encounter diseases such as bud rot or powdery mildew.

RH is the percentage of how much moisture is in the air versus the amount the air is capable of holding. RH and temperature have an inverse relationship. This means the RH in a room increases as temperature decreases, and vice versa.

As for how humidity affects plants, remember, your marijuana crop respires CO2 from the environment through their leaves. During this process, they lose some of the water retained in their foliage.

What Happens If the Air in the Grow Room Becomes Too Dry?

indoor grow room

Dry air results in your plants losing more water when respiring than in moist air. This reduces the overall moisture content in your weed. If the environment gets too dry (low humidity), your plants lose more moisture than they can regain through their roots.

If this happens, your plants close the pores in their leaves to minimize water loss. It is an effective tactic, but it means they receive less CO2 from the environment. The result is cell death within your cannabis plants.

It is tempting to think that you can just water your plants more often in dry conditions. However, you will end up over-watering the soil and reduce the amount of air in the growing medium. Your plants will suffocate and become more likely to develop root rot.

If you allow excessively high humidity levels, you increase the risk of diseases such as bud rot. The reason for this is that it allows moisture to build up in the plants’ thick foliage.

By setting your grow room to the right humidity level, your plants flourish. They open their pores, respire CO2, and grow quickly. Leading marijuana growers say beginners should look to achieve the following humidity levels in their grow room. Naturally, this is determined by the length of the marijuana plant’s life cycle.

These figures are a good starting point. However, you may wish to change the humidity ever so slightly depending on the strain:

  • Clones: 60-80% RH
  • Vegetative Stage: 50-60% RH
  • Early Flowering Stage: 40-55% RH
  • Late Flowering Stage: 40-50% RH (can be as low as 30-35% depending on the strain)

The RH should fall steadily throughout the growing cycle. Now, here are five tips to help you keep total control of the humidity in your grow room.

1 – Seal and Insulate Your Room

One of the fundamental tenets of successful indoor marijuana growth is to ensure your grow room is adequately sealed and insulated. Commercial builders use foam insulation and other materials to create a barrier between the grow room and the environment outside.

If you live in an area where outdoor humidity is low, you may think this tip doesn’t apply to you. However, it remains a necessity, regardless of whether you live in Colorado or New Mexico. It stops external factors such as humidity, wind, and sunlight from adversely impacting your weed.

These three factors, along with a few others, have a significant impact on temperature. As mentioned earlier, temperature and humidity levels are closely related. If you insulate your grow room properly, you don’t need to worry about outside factors damaging your crop.

2 – Control Grow Room Temperature

When you grow marijuana outdoors in a suitable climate, you don’t have to worry as much about the weather. However, the weather can still impact your crop, particularly if there is an unseasonable spell of weather.

For indoor growers, lighting is all-important. While it is tempting to use powerful lights, too much lighting causes the temperature in your grow room to soar.

A common mistake is to purchase an air conditioning system that’s far too large or small for the grow room. As a result, frequent fluctuations play havoc with the temperature. When it comes to AC and temperature change, short cycling and the deadband are the two key factors.

The deadband is a 3-5 ̊ Fahrenheit range around the temperature that you have set the room’s thermostat to. When the temperature reaches the upper end of the deadband, your AC unit will turn on. This helps to keep the grow room at the right temperature. When the temperature reaches the lower end, the AC switches off to stop the room from getting too cold.

The Importance of Using a Suitably-Sized AC Unit

If your AC system is too large, it will run in ‘short cycles.’ As a result, it consumes a ton of energy. It also creates an unstable growing environment where humidity and temperatures rise and fall rapidly several times a day.

If your AC unit is too small, the grow room temperature will increase to an uncomfortable level for your plants. Above all else, marijuana plants thrive in a consistent climate; therefore, short cycles can be disastrous for their growth.

Short cycles cause unwelcome spikes in temperature. As humidity levels are inverse to temperature, a boost in temperature leads to lower RH. On the other hand, a fall in temperature leads to a rise in RH. The resultant unstable growing environment becomes an ideal breeding ground for mold and mildew.

Therefore, the challenge for growers is to find a suitably sized AC unit for their grow room. In an ideal world, your grow room’s temperature will look like a lengthy and shallow wave on a graph. Once you find the right size AC unit, it will pull a sufficient amount of water from the air. Not only will it do this effectively, but it will also place a minimal strain on your dehumidifiers.

3 – The Importance of Air Movement

grow room control

Although humid air holds more water, it is lighter than the air that surrounds it. Therefore, it rises toward the ceiling of your grow room. Meanwhile, CO2, which is a critical component for plant growth, remains near the floor. As a consequence, your grow room must have excellent air circulation.

A lot of growers assume that oscillating wall fans are sufficient. In reality, they only reduce the temperature on the canopy and ultimately fail to provide air circulation. If you want a successful harvest, you need proper airflow throughout the room. It must be coming from the walls and the top and bottom of the room.

If you can afford them, we recommend purchasing floor fans. They pull air through the canopy of your marijuana plants and guarantee balanced humidity levels. They also ensure equal CO2 distribution and a stable grow room temperature.

4 – Good Drainage

In the natural world, moving water is healthy water because it flows in rivers, ponds, and rain. In municipal water systems, the air gets shaken in the water, which results in molecular oxygen getting trapped inside. However, in stagnant water, molecular oxygen is reduced to the point where anaerobic bacteria form and thrives.

A common issue in the grow rooms of novices is the formation of standing water. As well as becoming a breeding ground for bacteria, this water releases moisture into the air. This serves to increase the humidity level of your grow room. Therefore, the room must have proper drainage to prevent stagnant water from forming puddles on the floor. If you use a hydroponics setup, make sure you cover water reservoirs to ensure the liquid stays where it belongs.

It is common for growers to think that they have perfected their humidity systems erroneously. This is quickly proved not to be the case when they discover that the room’s RH is still too high. Rather than addressing the very basic issue of standing water, they meddle with a sound system and cause more problems.

5 – Use a Correctly Sized Dehumidifier

One would imagine that purchasing a high-quality dehumidifier is near the top of a grower’s list of new equipment. In reality, however, too many individuals bizarrely go cheap and invariably end up paying twice. Does this sound like you?

Residential dehumidifiers are ineffective because they’re not designed to handle the level of moisture in a marijuana grow room. Also, they use a lot of energy and are incredibly inefficient.

For many growers, the cost of a commercial dehumidifier initially seems to be too high. However, ultimately, they are forced to pay eventually because of problems with humidity in their grow rooms.

Bear in mind that plants transpire all but 3% of the water they absorb. Therefore, you need to correctly size a dehumidifier to pull the right amount of moisture from the air. If you have a large grow space, you’ll probably need several dehumidifiers. Make sure you design the system so that if one unit stops working, the rest continue to operate smoothly. Although it probably seems expensive at first, you will save a lot of money on electricity compared to residential units.

We hope that this guide has proved helpful. It is important to remember that plants with indica genetics tend to have dense buds. This makes them more susceptible to bud rot in humid conditions than their sativa counterparts.

We recommend checking your plants regularly, and you should also invest in a high-quality thermometer and hygrometer. There are all-in-one thermohygrometers available.

In summation, to keep your grow room humidity in check, you must:

  • Ensure it is properly sealed and insulated.
  • Monitor lighting and maintain a specific temperature range.
  • Increase the cool air supply.
  • Make sure stagnant water pools don’t form.
  • Use an appropriately sized dehumidifier.