Getting Hot & Heavy: Dealing With Heat in Growrooms

By David Kessler
Published: June 1, 2016 | Last updated: April 21, 2021 10:14:30
Key Takeaways

Knowing how to deal with the heat effectively can mean the difference between success and failure when growing indoors. In part one of this two-part series, David Kessler examines what equipment makes heat, and how to design a space that is capable of handling heat effectively.

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The first law of thermodynamics states that the amount of energy in the universe is constant. This means that energy is never destroyed; it only changes form. From the electricity that goes into your ballast, to the light emitted from your lamp, every bit of energy that is not turned into light is eventually turned into heat. Therefore, the more efficient your equipment is, the less heat you will introduce into your garden.


The heat in growrooms is primarily the undesirable by-product of four pieces of equipment: HID ballasts, HID bulbs, CO2 generators and dehumidifiers. Although there are other culprits that heat up growrooms, these four products are responsible for the vast majority of our heat problems, so a well-designed growroom capable of handling heat effectively is a must, considering all of the heat created by our equipment.

How to Design Your Grow Room to Beat the Heat

Much like real estate, the first consideration when designing a growroom should be location, location, location. For example, a basement will naturally be cooler and better insulated than an attic. If your room has no windows, it might be necessary to cut holes in the ceiling or walls for ventilation. If you are not comfortable with this type of construction, then using a grow tent is a better option.


When designing a growroom, consider your access to electricity. Check your panel, plan where your electricity will come from and consider how much electricity you need versus how much is available. Safety is paramount. It is not worth burning your house down for an extra 400 W. Does the room have air conditioning? Where is the thermostat located? You don’t want the whole house to be 60°F so your garden can be 78°F—that would be both expensive and inefficient.

Once you have assessed your options and chosen a room to grow in, there are a few more things to keep in mind before beginning to cut holes willy-nilly. First, are you going to build a closed growing environment or a room with intake and exhaust? A closed room will necessitate a larger air conditioner and also a CO2 source, which could increase your initial expenses. However, a closed room can be dialed-in more precisely. On the other hand, a ventilated room can be less expensive to construct, but will require cutting more ventilation holes.

Ventilation Tips

A standard rule when venting a growroom is to provide at least one complete air exchange every five minutes. This is a minimum and, depending on the heat load, location and size of the room, it may be wise to exchange 100% of the air as often as once each minute. To determine how big your exhaust fan should be, divide the cubic footage by the number of minutes you want all of the air exchanged.


To determine cubic footage, calculate the total square footage by multiplying the length of your room by the width of your room, then multiply the total square footage by the height of the room to find the total cubic volume. For example, a 10- by 14-ft. room with an 8-ft. ceiling has a total volume of 1,120 cu. ft. of air:

10 ft. x 14 ft. = 140 sq. ft.


140 sq. ft. x 8 ft. = 1,120 cu. ft.

To figure out how big your exhaust fan needs to be, divide the cubic footage by the number of minutes you want all of the air exchanged. For this example, let’s say five minutes:

1,120 ÷ 5 = 224

This means your exhaust fan will need to provide a minimum of 224 CFM (cubic feet per minute). Remember that 224 CFM is with no resistance. Adding a length of duct will increase the size of the fan needed.

A 25-ft. length of flexible ducting will decrease air flow by about 7% and each bend will further reduce air flow by an additional 1-4%, so make your runs of ducting as straight and short as possible to maximize air flow and minimize resistance.

Now that you know the size of your exhaust fan or fans, cut the holes for your ventilation. Remember that you want to vent hot air out and since heat rises, exhaust holes should be cut high and intake holes should be cut low. Intake air can be passively brought in by the negative pressure created by your exhaust fan or actively with a fan with less CFM than your exhaust fan; this will maintain negative pressure.

Try to place the exhaust and intake openings on opposite sides of the room to create cross-ventilation. Two quick tips before you begin: measure twice and cut once, and be aware of where your electrical lines and plumbing pipes are before you cut into a wall or you may be in for a wet shock!

A well-insulated room keeps hot air out and cold air in. When you build your growroom, line the interior walls with extruded polystyrene foam insulation board, commonly called pink or blue foam board, to resist heat flow.

Another lesser-used but useful type of insulation is a radiant barrier. Radiant barrier insulation is a reflective insulation designed to reflect heat, not absorb it. It is applied to the exterior of the walls to prevent heat from the house from entering the growroom.

High-airflow Rooms

Another type of room design is called a high-air-exchange room, which does not include CO2 enrichment or air conditioning, just lots of air movement. By exchanging the air in the room faster than the heat can be added, you can essentially maintain the temperature of your growroom at the temperature of your intake air.

For this type of growroom design, you need to exhaust 100% of the air in your room at least every minute. Fans use less electricity than air conditioners, so this can save you money on your electric bill. Remember that a high-air-flow growroom will only work for those growing in areas with a moderate climate.

Ballast Placement

Finally, when picking your grow area, make sure you can place the ballasts outside of the growroom. Ballasts are the second greatest heat-producing piece of growroom equipment and putting them in the growroom due to a lack of planning is a cardinal sin. If you currently have your ballasts in your room, this is an easy problem to overcome. If you are not lucky enough to have any external space for your ballasts, then consider making a vented ballast chamber.

Place your ballasts in a 12-in.-diameter section of hard metal ducting and blow air across them using an inline fan, which exhausts out of the growroom. This will remove the heat from the ballasts before it can enter your room.

Now that you’ve dialed in your growroom design, check out part two of this series, which outlines accessories that can effectively reduce heat, and methods for dealing with the heat that can’t be avoided.


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Written by David Kessler

Profile Picture of David Kessler
David Kessler heads research and development at Atlantis Hydroponics and writes for their popular blog. David has more than two decades of experience and multiple degrees from the State University of New York. An accredited judge for the American Orchid Society, he travels the world judging events. Follow his blog at

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