The ventilation system is the most important aspect of environmental control in any greenhouse or indoor garden. Plants can be given perfect lighting conditions and the most complete diet of nutrients, but without a properly functioning ventilation system they will inevitably suffer and fall short of their potential.
Ventilation systems for both greenhouses and indoor gardens serve the same four very important purposes: temperature control, humidity control, air circulation, and—if it is not being supplemented—CO2 replenishment.
Probably the most obvious reason for ventilation systems is to control temperature. Ventilation systems are commonly used to remove excess heat—either created by trapped radiant energy from the sun or by high intensity discharge lighting—from the growing environment. An exhaust fan, like its name suggests, exhausts unwanted heat and stale air from the garden and is usually the first piece of equipment purchased for any ventilation system.
The humidity level within an enclosed garden is affected by multiple variables. Temperature differences between the inside and outside of an enclosure can cause condensation, which directly affects humidity levels. Plants naturally transpire water, which will also raise the humidity level within an enclosed space. Exhaust fans used to remove unwanted heat can also serve the secondary purpose of removing excess humidity, which would otherwise be trapped in the garden space.
Air circulation refers to the air movement within the physical garden space. Greenhouses and indoor gardens alike can benefit greatly from oscillating fans, which create consistent air movement. Not only does consistent air movement result in uniformity of temperature and humidity within the growing environment, it also helps strengthen the structural integrity of your plants.
Plants consume CO2 for photosynthesis and they expel oxygen. If the CO2 in an enclosed environment gets used up by the plants and is not replenished, the plants’ ability to photosynthesize will be compromised—ventilation systems replace stale, CO2-depleted air with fresh air containing new CO2 for plants to process.
Ventilation set-up for indoor gardens
Heat naturally rises, so it is most efficient to have your exhaust fan positioned high in the grow room. The fresh air intake point should be placed low in the grow room, preferably on the opposite side of the room from the exhaust—this will ensure fresh air movement across the garden.
Air follows the path of least resistance, so it is important when setting up a ventilation system to determine where the air will flow. Imagine a string from the fresh air intake point to the point of exhaust—assuming there aren’t any large physical obstacles impeding airflow, this will be the ventilation path.
Fan Sizing for Indoor Gardens
Many factors will influence what size of fan you choose. Additional equipment such as air conditioners, dehumidifiers and CO2 burners will all affect your choice of ventilation fans. For a starting point, let’s assume there will be no additional equipment and that the climate of your garden is about average.
A good rule of thumb is to have 265 cubic feet per minute (CFM) of air movement per 1,000 watt light—for example, a room with 4,000 watts should have an exhaust system with a CFM rating of 1,060 or higher. For growers who decide to use a passive intake—with no fan—a slightly higher CFM rating for their exhaust might be necessary to create enough negative pressure to allow sufficient airflow.
The opening for a passive air intake should be a minimum of twice the size of the exhaust. Gardeners who utilize an intake fan should choose one with a slightly lower CFM rating than the exhaust fan, which will ensure a slight negative pressure in the grow room.
An air conditioner can dramatically decrease the CFMs needed to exhaust heat and this should be taken into consideration when setting up any ventilation system. Generally speaking, every 10,000 BTUs of air conditioning will replace 265 CFMs of exhaust and should be able to efficiently cool a 1,000 watt light. On the other hand, dehumidifiers and CO2 burners add heat to a grow room and might require you to install a larger exhaust fan.
New cooling technologies and ventilation systems
There have been many technological advances in indoor garden heat management in recent years, all of which can affect the sizing and set-up of ventilation systems. Air or water cooled reflectors will drastically reduce heat in the room, minimizing the required size of your exhaust fan.
Some cutting-edge growers are now combining air or water cooled reflectors, dehumidifiers and super-efficient mini-split ventless air conditioning units. Used in conjunction, these technologies reduce heat to a minimum and—as long as CO2 is being supplemented—make exhaust fans unnecessary.
Excess heat or stale air within a greenhouse will result in slow growth and poor overall crop performance. Ventilation systems can often prove the downfall of the novice greenhouse grower; many will never associate the mold, insects or diseases they have managed to acquire with having poor ventilation. There are two common ways to set up a ventilation system in a greenhouse: naturally or mechanically.
The keys to natural ventilation are wind and thermal buoyancy. ‘Thermal buoyancy’ refers to the rising of warm air within the greenhouse, a process which actually contributes to efficient ventilation. A greenhouse utilizing natural ventilation will have either retractable or removable sides or roof panels, or a series of vents.
Hoop houses with retractable sidewalls are great examples of the use of natural ventilation—with the sides raised, the wind can flow through the greenhouse and replace warm, stale air with fresh air from outside.
Greenhouses with a series of roof and sidewall vents remove heat via wind and natural thermal buoyancy. As wind passes over a roof vent it creates a vacuum within the greenhouse, which draws air through the sidewall vents and out the roof vents. Thermal buoyancy is most effective when there is a large temperature difference between the outside air and the air in the greenhouse.
Cooler air enters the greenhouse through the sidewall vents and as the air heats up it rises and exits the greenhouse through the roof vents. On warm days—where the temperature difference is minimal—the buoyancy effect is not as powerful.
Mechanical ventilation in greenhouses is very similar to that found in indoor gardens. Fan systems create air movement, which brings fresh air into the greenhouse and exhausts unwanted heat and humidity.
Greenhouse Fan Sizing
Fan systems operating during summer months should be sized to provide one volume of air exchange per minute, to a height of 10 feet. We use 10 feet as a constant when determining fan capacity for year-round greenhouses. The general rule of thumb for sizing fans is to multiply the greenhouse’s length in feet by its height in feet and then multiply by the constant—10 feet.
That number will give us the cubic foot per minute capacity needed to sufficiently cool the greenhouse—for example, calculating the fan capacity required for a 15 by 60 foot greenhouse would look like this: 15 feet x 60 feet x 10 feet = 9,000 CFM capacity required for year-round cooling.
Ventilation needs will vary with the seasons and cooler months will require a less aggressive amount of air exchange. In winter months most ventilation systems can be reduced to one third of full fan capacity, so it is advantageous for growers who plan on using their greenhouses year-round to invest in variable speed fans or fans that can be controlled by a dimmer or thermostat. Larger greenhouses might require multiple large fans for cooling during the summer months, but a reduced number of fans will be required during the winter.
Exhaust fans should be positioned high on the wall opposite the intake vents so that air flows over the plant canopy on its way through the greenhouse. If possible, set up your fans to work in conjunction with the prevailing winds—this can help ventilation systems work up to 20 per cent more efficiently.
Passive intakes (with no fan) should be 1.5 times the size of the exhaust fan in order to ensure sufficient intake and to make sure exhaust fans aren’t overworked. If strong negative pressure is apparent—for example, if doors are hard to open because they are literally being sucked shut or the greenhouse plastic is being pulled tightly against the frame—the size of the passive intake opening needs to be increased.
One highly efficient way of taking in and distributing fresh air in a greenhouse is by using a perforated polyethylene tube that inflates and extends among the plants. The intake fan fills the tube with fresh air and pushes it through the holes in the plastic, which allows fresh air to reach the plants in a direct and uniform manner.
This technique is especially effective in larger greenhouses that would otherwise only have fresh air entering one end of the building. The use of perforated polyethylene tubes does require intake fans and—as with an indoor garden—these fans should have a slightly lower CFM rating than the exhaust fans so they’ll produce a gentle negative pressure within the greenhouse.
The ventilation systems we install in our greenhouses and indoor gardens are a vital component in our attempt to recreate nature indoors. Just as the wind strengthens, revitalizes and nourishes plant life outdoors, the ventilation systems in artificial environments directly influence plant health and production.
Sufficient air movement—combined with proper fan sizing and placement—will reduce the potential for problems and can help create ideal conditions for efficient horticultural production.