Ventilation in Green House Environment

Greenhouse Ventilation

The greenhouse ventilation system plays a pivotal role in plant health and crop production. 

Many novice horticulturalists do not attribute slow growth, poor yields, or problems with pathogens to an inadequate greenhouse ventilation system. 

Most gardeners understand the importance of ventilation systems for temperature control but benefits derived from ventilation systems are multifaceted. 

Not only are ventilation systems vital to maintaining environmental conditions but they also directly influence a plant’s ability to perform photosynthesis, uptake essential elements, and complete reproductive cycles (pollination). 

Greenhouse ventilation systems serve four imperative functions

• temperature control, 
• humidity control, 
• CO2/oxygen replacement, and 
• air circulation.

Temperature Control

Greenhouses trap solar radiation thereby raising the temperature in the environment (the greenhouse effect). If this heat is not removed from the environment it can cause undesirable conditions for plant growth or worse. Ventilation systems, whether natural (passive) or mechanical (powered), serve the basic function of the removal of excess heat.

Humidity Control

Like heat, high concentrations of humidity can become trapped in a greenhouse’s environment. Although some varieties of plants thrive in high humidity conditions, most plants are hindered by the pathogens associated with high humidity levels. Just about every mold, fungus, and mildew thrive in high humidity conditions. Plants transpire moisture that, if not removed, can build up in a greenhouse, raising the humidity levels. It is also very common for the environment within a greenhouse to be affected by the dew point which causes condensation and excess humidity.

CO2 / Oxygen Replacement

Just as humans would run out of oxygen in a poorly ventilated and enclosed area, plants “breath” CO2 which needs to be replenished in any greenhouse that is not supplementing it artificially. Fresh air drawn into a greenhouse carries with it two important gases needed by plants to carry out some of their most basic functions. During photosynthesis, plants use CO2 which they acquire directly from the air. When CO2 levels decrease the rate of photosynthesis and all other plant functions associated with photosynthesis decrease accordingly. Maintaining an adequate level of fresh air intake guarantees sufficient CO2 levels for plant growth.

Air Circulation

Air circulation, or air movement, within a greenhouse serves many purposes. Proper air circulation creates uniformity in temperature, humidity, CO2, and oxygen within the environment. Plants respond better to environmental consistency and proper air circulation ensures each plant within the greenhouse receives the same atmospheric conditions. Air movement is also our way to simulate the wind in an enclosed environment. 

Natural (Passive) or Mechanical (Powered) Ventilation Systems

When building a greenhouse or setting up a ventilation system one of the first decisions a gardener must make is whether to use a natural or mechanical ventilation system. 

A natural ventilation system is a system that has no powered fans but instead relies on wind and thermal buoyancy for air movement

Mechanical ventilation systems use fans or other mechanical devices to produce adequate airflow and circulation. The main advantage of mechanical systems is a higher level of control. 

Sizing for Natural Ventilation Systems

For some greenhouses, like a hoop house, simply rolling up a portion of the wall to achieve airflow is all that is needed for natural ventilation. However, greenhouses that are built from more rigid materials need to follow a few guidelines to ensure proper ventilation. The most important aspect of natural ventilation for these greenhouses is the size of the ridge (roof) vents and sidewall vents. It is recommended that the total combined ridge vent area should be equivalent to the total combined area of the sidewall vents and each should equal approximately 20% of the floor area. For example: a 10 ft x 30 ft greenhouse would have a floor area of 300 square feet; 20% of 300 is 60, so a 10 ft x 30 ft greenhouse should have a total area of 60 square feet in ridge vents and 60 square feet in sidewall vents to be properly ventilated naturally.

Sizing and Fan Placement for Mechanical Ventilation Systems

Before sizing mechanical equipment for a greenhouse it must first be decided if the greenhouse is for seasonal or year-round use. There is a great difference in temperature and humidity from summer to winter and this affects the internal environment of a greenhouse. If you plan on using a greenhouse year-round it would be best to size your equipment for the hotter summer months that require more aggressive ventilation. Just make sure to purchase equipment that is either thermostatically controlled or includes variable speed fans so it can be used throughout the year when less intensive ventilation is needed.

Fan Placement

Heat rises so it is generally advantageous to place exhaust fans toward the top of a greenhouse. If possible, place intake vents or fans lower to the ground, preferably on the opposite side of the greenhouse than the exhaust. This will create a path of airflow across the entire greenhouse which makes a ventilation system operate more efficiently.