How To Control Temperature and Humidity in a Grow Room

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Controlling temperature and humidity in your grow room is an indispensable part of proper cannabis cultivation. Failure to attend to these factors will still allow you to grow cannabis, but with diminished results.

However, attending to these factors will boost productivity of your crop and they can be employed with just a little effort. Here, we will break down the important facts and steps you may take to optimize your cannabis crop for peak productivity.

What Are The Optimal Growth Conditions?

Before launching into a discussion of controlling environmental conditions in your grow room, it is germane to review the approximate growth conditions for most cannabis cultivars. Below are the target ranges for temperature and humidity.

Stage 1 (seedlings): 24°-27°C (75°-80°F), 70% RH (relative humidity)

Stage 2 (vegetative growth): 21°-29°C (70°-85°F), 40-60% RH

Stage 3 (flowering): 18°-27°C (65°-80°F), 40-50% RH

These are the optimal ranges for vegetative growth and bud production. However, if you are into trichome farming, the following light stress parameters should be applied for 2-3 weeks prior to flowering: 21°-27°C (70°-80°F), 30% RH

Tools For Measurement

If you are going to aim for optimizing growth conditions, you need a few tools in your bag for monitoring the situation. These include an outdoor thermometer and a means of measuring humidity (hydrometer or sling psychrometer).

An optional tool is an infrared (IR) thermometer. They can be somewhat expensive and are not an absolute must for monitoring temperature, however, if you are going to get serious, we recommended investing in one.

There are several reasons for this. You can monitor leaf surface temperature with an IR thermometer.

A second, very important reason is that you can use this tool in a strategy to find that balance point between humidity and temperature. Typically, as you drop humidity, leaf temperature increases due to a loss of evaporative cooling.

Changing one parameter, then the other, then having to go back and change the first one again can quickly become an exercise in frustration. Keep in mind, those are for the conditions at the moment, which can change later in the season (more on this subject below).

Environmental Equipment

Equipment you use will depend on the season and your location. The same equipment will not be used in Arizona as in Michigan.

These pieces are provided as part of a set of basic guidelines. Which ones you use will vary based on your particular situation.


Before any discussion of temperature control, we need to talk about lights. Grow lights are the single greatest heat contributor to any grow room, regardless of climate.

Your choice of grow lamps will factor heavily into how much effort goes into heating or cooling systems for the grow room. Electricity consumption should also be considered, as different types of lamps use different amounts of electricity.

Lamps that run hot include metal halide lamps, high-intensity discharge lamps, and compact fluorescent lamps. LED lamps are cooler and use less electricity, yet they are more expensive.

Remember that lamp placement is also very important. The closer the lamps are to the canopy, the more radiant heat will be focused on leaf surfaces, so a bit more distance can make a noticeable difference in canopy temperature.

Air flow:

Control of air flow in the grow room has a big effect on temperature. Simply removing hot air from the room makes a substantial contribution to temperature control.

Simple box fans set up in an exit can produce modest effects, but for more efficient air flow we recommend using a 6-inch ducted inline fan. Many types of grow lights have 6-inch duct ports and can be easily integrated to a fan and ductwork setup to create a more efficient air exchange and temperature regulation system.

Climate Control Supplements:

We finally get into the meat and potatoes of the humidity and temperature regulation game. Humidifiers and air conditioners.

Room air conditioners can quickly cool a room, as well as dehumidify the air through their condenser coils. Although many locations will not need an air conditioner, you may find that you are unable to rely on ambient temperatures when adjusting humidity.

Having an A/C on station which comes on even occasionally is a great help. Many models of air conditioners are also heat pumps and can switch on auto mode between cooling and heating.

Heating up a grow room is usually not an issue for most locations; however, this statement is an exception for those in northern climates who wish to maintain their grow room year-round. Most of the time, just running the lights is enough to heat the room.

If you live in a northern climate and find that your grow room is just not hot enough in the winter months, try adjusting your light:dark cycle to run the grow lights at night instead of during the day. This simple change will make for a warmer grow room in the evening hours.

Low humidity can be remedied by using a room humidifier, which is typically a type of vaporizer that sends out a mist of water vapor. Make sure to read the specs regarding the amount of humidity change the unit will produce for a given room area and choose one that is appropriate for your space.

Choose units with at least a 12-hour runtime (24-hour runtime is better) with a simple design, few moving parts, and a reservoir volume of at least 4 liters. For spot humidification, simply misting your plants does a lot as well.

For high humidity conditions, a room dehumidifier will do the job. Make sure to review the specs on any unit you buy so that the machine can accommodate the volume of your grow room. We recommend getting a machine that can handle up to 25 liters of water per day that has a drain port so that you can connect a drain hose.

Balance (Omm…)

Finding a balance between humidity and temperature can be a real game and finding that balance point a real challenge. Often, changing one parameter causes a shift in the one you just finished setting.

You could engage in chase-your-tail optimization, if that is your thing, dropping the temperature only to find you need to increase humidity, then need to drop the temperature some more. Quite frustrating.

Just cut to the chase and employ a little science and all will be right and well in the world. Remember that IR thermometer we discussed above?

This is where that little gem comes into play. An IR thermometer for measuring leaf surface temperature (or at least taking temperature measurements in the canopy with a standard thermometer) can be combined with your humidity measurements to calculate the optimal humidity and temperature balance for your situation.

That can be accomplished by calculating the vapor pressure deficit (VPD), which is determined in the following equations.

Saturation vapor pressure (SVP) = 610.78 x e^[T / (T + 238.3) x 17.2694]

Where T is temperature in Celsius

e is Euler’s Number (2.71828)

SVP is expressed in Pascals (divide by 1000 to get kiloPascals or kPa)

Vapor pressure deficit (VPD) = SVP x (1 – RH / 100)

For most cannabis cultivars, the target VPDs are as follows:

Stage 1 (propagation and early vegetative growth): 0.5-0.8 kPa

Stage 2 (late vegetative growth and early flowering): 0.8-1.2 kPa

Stage 3 (mid-late flowering): 1.2-1.6 kPa

If you are not a math geek, the equation above doesn’t get you call excited, and you just want the results, there are several calculators out on the internet that will assist you in your decision making. Here is an easy to use version by gRobot.

Happy growing!

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