The glass and plastic materials of a greenhouse trap solar energy, creating an effect similar to Earth’s atmosphere. This rapidly causes internal temperatures to spike far above the outside air. These temperature increases can quickly cause heat stress, desiccation, or premature flowering (bolting) in plants, making active temperature management necessary for successful cultivation. Cooling a greenhouse requires a multi-faceted approach: preventing heat from entering, removing existing hot air, and using water to lower the ambient temperature. Controlling this intense solar load is the primary challenge in maintaining a stable, healthy growing environment.
Managing Air Exchange Through Ventilation
Ventilation is the most immediate method for heat removal, replacing hot, stagnant air inside the structure with cooler exterior air. This air exchange can be achieved through passive or active means. Passive ventilation utilizes the natural buoyancy of heated air: warm air rises and escapes through high-level roof vents, drawing cooler air in through low-level side vents. This movement, known as the chimney effect, is an energy-efficient way to move large volumes of air.
To automate passive ventilation without electricity, some systems use temperature-sensitive wax cylinders that expand when heated, opening the vent. Active ventilation systems use exhaust fans paired with intake shutters to force a complete air change for more intense cooling. Commercial standards recommend sizing a greenhouse for at least one Air Change Per Minute (ACPM) to manage solar heat gain. This requires moving approximately 7 to 10 cubic feet of air per minute (CFM) for every square foot of floor area.
Smaller circulation fans, often called Horizontal Air Flow (HAF) fans, are used internally to prevent microclimates and hot pockets. These fans ensure the cooled air is evenly distributed throughout the growing space. Proper sizing of exhaust fans and intake openings is important, as restricted airflow reduces cooling efficiency.
Reducing Solar Load with Shading Techniques
Reducing the solar radiation that penetrates the glazing decreases the heat load. External shading is more effective than internal methods because it blocks the sun’s energy before it converts into heat inside the space. External shade cloths (e.g., 30% to 50% density) are positioned above the roof to reflect light and radiant heat away.
Internal shading, using curtains or screens, is easier to deploy but traps heat inside the greenhouse, making it less efficient. Solar energy passes through the glazing, hits the internal shade material, and radiates warmth. Shading compounds, such as temporary whitewash or specialized paint, are applied directly to the outside of the glazing in the spring and washed off in the fall. This compound reflects a portion of the solar spectrum, providing seasonal heat reduction.
The orientation of the greenhouse also plays a role; structures aligned east-west often have less intense solar gain on the long walls. Combining these methods manages light intensity to prevent plant scorching while lowering the solar energy the ventilation system must remove. Shade density must be matched to plant requirements to ensure adequate light for photosynthesis.
Utilizing Water for Temperature Drop (Evaporative Cooling)
Evaporative cooling uses the principle that heat energy (the heat of vaporization) is absorbed when water changes into vapor. This process draws heat directly from the surrounding air, resulting in a temperature drop. This method is effective, particularly in hot, arid climates where low relative humidity allows the air to absorb large amounts of water.
Fan-and-pad systems draw outside air through a thick, water-saturated porous pad. As air passes through, the water evaporates, cooling the air before it is pulled into the greenhouse by an exhaust fan on the opposite wall. These systems achieve substantial temperature reduction but often create a temperature gradient, with the coolest air near the pad and the warmest air near the exhaust fan.
Misting and fogging systems use high-pressure nozzles to inject tiny water droplets (10 to 20 microns) directly into the air. These droplets evaporate instantly, cooling the air with minimal wetting of the plant canopy or floor. While powerful, evaporative cooling increases relative humidity, which can be detrimental in humid regions or for plants susceptible to fungal diseases.