How to Control Humidity in Your Indoor Grow Room

Our top tips on this essential aspect of growing


If you intend to grow cannabis either for either personal use or commercial purposes, it is imperative that you invest in adequate lighting, good air conditioning, and humidity control. By doing this, you will ensure excellent air flow and a level of control over your grow room’s temperature and humidity. When this happens, you can expect to maximize your yield and minimize the threat from diseases and pests.

Although most novice growers have a handle on controlling a room’s temperature, and may even know a thing or two about air circulation, they tend to struggle when it comes to maintaining the ideal grow room relative humidity (RH) level. For instance, humidity changes when you turn your grow room lights on and off. However, many growers pay no 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, bear in mind that your marijuana crop respires CO2 from the environment through their leaves. During this process, they lose some of the water retained in their foliage. Dry air results in your plants losing more water when respiring than in moist air, which reduces the overall moisture content in your weed. If the environment gets too dry (low humidity), your plants will lose more moisture than they are able to regain through their roots.

If this happens, your plants will 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.

In a room set to the right humidity level, your plants will flourish because they open their pores, respire CO2 and grow quickly. If you allow humidity levels to get too high, you increase the risk of diseases such as bud rot because moisture builds up in the thick foliage of your plants.

According to leading marijuana growers, beginners should look to achieve the following humidity levels in their grow room, according to the marijuana plant’s life cycle. These figures are a good starting point, although 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).

As you can see, 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

If you’re trying to grow marijuana in a dilapidated building, or else you live in an old, relatively uninsulated building, you’re doing it wrong! One of the key tenets to successful indoor marijuana growth is to ensure your grow room is properly sealed and insulated. In the modern era, commercial builders use foam insulation and other materials to create a thick barrier between the marijuana grow room and the environment outside.

If you live in an area where outdoor humidity is relatively 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 because 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 we 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 how the weather impacts your crop, unless of course there is an unseasonable spell of weather. For indoor growers, lighting is all-important. While it is tempting to blast your room full of powerful lights, you need to be wary because 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.

In case you’re not aware, 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 to keep the room at the right temperature. When the temperature reaches the lower end of the deadband, the AC switches off to stop the room getting too cool.

If your AC system is too large, it will run in ‘short cycles’, which means it consumes a ton of energy and creates an unreliable 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, so 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, and a fall in temperature leads to a reduction 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 properly 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 enough water from the air to ensure there is minimal strain on your dehumidifiers.

3 – The Importance of Air Movement

Although humid air holds more water, it is actually 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 completely fail to provide air circulation. If you want a successful harvest,you need good air flow 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, 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, air gets shaken in the water, which results in molecular oxygen getting trapped inside. However, stagnant water is bad news because the molecular oxygen is reduced to the stage where anaerobic bacteria form and subsequently thrive.

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 and increases the humidity level of your grow room. Therefore, it is essential that the room has proper drainage to prevent stagnant water from forming puddles on the floor. If you are using a hydroponics setup, make sure you cover water reservoirs to ensure the liquid stays where it belongs.

It is all too common for growers to perfect their humidity systems, only to find that the RH in the room is still too high. Rather than addressing the very basic issue of standing water, they meddle with a good system and cause even 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?

As it happens, residential dehumidifiers are a waste of money because they are not designed to handle the level of moisture in a marijuana grow room. Also, they use a lot of energy and are extremely inefficient. For many growers, the cost of a commercial dehumidifier seems too high, but 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 and are 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 a properly sized dehumidifier.