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Let them breathe! Cannabis and correct ventilation

Smokin Moose

Fallen Cannabis Warrior
Proper ventilation and air circulation are essential to growing healthy, happy plants.

An important aspect to consider when growing plants indoors is proper ventilation, air circulation and temperature control. This practice becomes especially necessary when working with lights over 400 watts, in very small spaces, any time the temperature exceeds 32°C (90°F), or if the humidity gets too high.

The Perfect Temperature

Though sense of feel is adequate to gauge the "perfect climate" for a given plant, there is no real substitute for a thermometer and humidity gauge.

Thermometers are cheap and accurate enough for our purposes. I usually employ several thermometers in different areas in and around the grow room. Somewhere between 32-35°C (90-95°F) is the absolute highest room temperature your plants would care to tolerate. The perfect temperature would be somewhere between 24-29°C (75-85°F). Peaks of 38°C (100°F) are allowable for most strains, but not for any longer than a half hour or so. And only above the root level.

Roots and Aeration

The main area of concern involving temperature are the roots of the plants. Ideally, the roots should be kept at as constant a temperature below 21°C and above 10°C (70-50°F) as possible. The fact that warm air rises and cool air sinks works to our advantage in this case. Also, the plants end up under the larger lights by the flowering cycle, and so they're usually large enough to help shade and cool their root areas.

Still, some rooms build up sufficient heat to require a separate circulating fan, or fans, focused specifically on the root systems. A soil thermometer may be a wise investment.

Proper aeration of organic based soils is crucial in high temp/humidity areas. Perlite and vermiculite, are the tips here — add more to the soil if need be. In hydroponic systems make sure that the nutrient water temp is below 21°C (70°F). If necessary, store the reservoir outside of or below the grow room.

Squirrel Cage and House Fans

There are many different types of fans and air movers available on the market. Most fans can be purchased at the average home improvement store. Proper research and smart shopping will net the best purchases. Careful planning will help avoid costly mistakes. Using the "hot air rises, cool air drops" rule, one can figure out the right solution.

The two most common types of fan are the squirrel-cage and what I call the "common house fan" (box or oscillating fans). Both come in a seeming endless variety of shapes and sizes. Generally speaking, squirrel-cage fans move air either in and/or out of the room, while common house fans move the air up, down and around the room. There are also neat little "muffin" fans that can be used for many things such as light-hood venting and passive ventilation systems.

A passive ventilation system is one that moves air either in out of room (not both). The room is not sealed and so air exchange is allowed free movement from inside and outside the room.

Squirrel Cages

The squirrel-cage fan is the most popular fan for moving large volumes of air into or out of a room or rooms. A common placement for this fan is inside of the room, up high, blowing out. This will help move the hot air out. This method is what is used to stimulate the passive intake of cool air with vent holes cut in the floor or lower walls to access the cooler areas outside of the room.

Other hardware such as dryer-vent tubing or muffin fans may be used to best access the cool, dry air outside of the grow room. It is a simple step further in this type of system to add an oscillating fan or two on the floor, pointing at any angle up, to help circulate the cooler air up and around the plants. This is the simplest of vent systems and works quite well. Choosing the correct squirrel-cage fan is part of the trick to success.

Measuring Air Movement

Squrrel cage fans are rated by their volume of air movement in cubic feet per minute or CFM. A fan with a rating of 100 CFM is able to move 100 cubic feet of air per minute. A room that is eight by ten feet and eight feet tall holds 8 X 10 X 8, or 640 cubic feet of air. Therefore, it would take an optimally running 100 CFM fan 6.4 minutes to fully circulate the air in that room.

Generally speaking, most fans move a little less than their rated CFM due to intake resistance or a dirty fan cage. Bigger fans usually will work more efficiently. Potentiometers, or a "volume control", could be installed in the power line of the larger fans to adjust the fan speed. This would give further aid in the specific control of air volume and ventilation.


The ideal ventilation system utilizes automation in the form of thermostats and regulators. A thermostat, as with the common household thermostat, would cause the fans to turn on at a certain temp, and turn off at another. That is, a sensor would turn on the fans on at around 30°C (86°F), and turn them off if the temperature dropped below 21°C (70°F). A well-stocked, high-tech grow shop will have several types of thermostats available in a variety of systems.

Box and Oscillating

Common summer house fans also come in a wide array of types and sizes. The most common being the box and the oscillating. Box fans are self explanatory. They can be used in a variety of ways, depending on the innovation and imagination of the user. Experimentation will yield the most efficient uses for these devices.

Oscillating fans are perhaps the most efficient devices for circulating air in a room. The gentle back and forth sway of the fan is very beneficial for the developing plants. These fans tend to keep anaerobic molds down by constantly freshening any potentially stagnant air. There are wall-mounted styles available as well. Home improvement centres carry a large array of various types and styles of air-moving fans, some relatively inexpensive.

A warning needs to be expressed concerning the cheaper, discount-store, oscillating fans (or any cheap fan for that matter) that have a tendency to burn out after a period of time. Some of these products are potentially dangerous if left plugged in and turned on after they burn out.

Therefore, it is a wise idea to check one's fans (and all electric devices and equipment for that matter) on a regular basis as often as possible.

Noise Concerns

Another fan consideration is noise. Some fans, especially the squirrel cage, may be a bit too noisy for a given situation. There are higher quality fans available that do run more quietly — expect to pay more, of course. It also helps to mount the fan directly to a main stud or support, by at least two of its support holes, and preferably more.

Rubber dampers and gaskets can be easily made and used on the support holes or around the overall mounting surface. Keep the fan's bearings sufficiently lubricated as well.

High Humidity

Humidity is another factor that influences the overall quality and quantity of a crop. Generally speaking, high humidity (over 80 or 90%) is bad. It inhibits plant transpiration and ultimately stunts growth. Mold and fungus love high humidity as well. Note that warmer air holds more moisture than cooler air.

There are a few simple practices to help reduce humidity. First and foremost, keep the room as dry as possible. When watering, use just as much as the plants need. Pump, siphon or mop up any remaining water and remove it from the room.

Keeping the room clean also helps. Moisture likes to hide and store itself in material such as dead leaves, spilled dirt or any garbage. Therefore, keeping the room clean and free of debris will help keep moisture and organisms such as mold, fungus and bacteria down.

Temperature and moisture levels directly affect the plant's ability to metabolize nutrients and supplements such as fertilizer and carbon dioxide.

If these practices fail to lower humidity enough, the only solution may be a de-humidifier. However, de-humidifiers are expensive, consume a large amount of electricity and produce heat. These factors will need to be considered in choosing whether or not to employ one.

Ventilation and Circulation are Essential

Proper ventilation and air circulation are essential to maintaining a healthy indoor grow environment. The basic rule of thumb is to move the warm, moist air out and to move the cool, dry air in and around the plants and their roots. Many various types of fans and devices are available to achieve this goal.

Careful planning, basic research and smart shopping will acquire all that one needs to keep it cool and dry, and experimentation will fine tune the system to provide the most perfect indoor environment possible.

Thanks DJ Short for another informative read
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