A greenhouse is a structure designed to create a controlled environment, extending the growing season and allowing for the cultivation of plants that might not otherwise survive in a local climate. While often associated with protecting crops from cold weather, successful summer operation requires managing extreme heat. The enclosed space that provides warmth in winter can rapidly become an oven when solar intensity is high. Effective year-round gardening relies on specific techniques to dissipate or block the intense solar energy that leads to overheating.
The Mechanism of Excessive Heat Gain
The primary reason a greenhouse overheats in summer is the conversion and trapping of solar energy. Shortwave solar radiation, including visible light, easily passes through the transparent glazing material, such as glass or polycarbonate. Once inside, this energy strikes surfaces like soil and plants, which absorb the radiation. These heated surfaces then re-emit the absorbed energy as longwave thermal radiation, or infrared heat. Unlike the incoming shortwave radiation, these longer wavelengths are much less able to pass back out through the glazing. This difference in transparency traps the thermal energy inside, causing the internal air and surface temperatures to climb rapidly. Furthermore, the enclosed nature of the structure prevents the natural heat dissipation mechanisms that occur outdoors, such as convection and wind. This limitation on air exchange allows the air temperature to quickly rise far above the ambient outdoor temperature.
Cooling Through Active Ventilation
The most effective method for controlling summer heat is active ventilation, which physically removes the hot, stagnant internal air and replaces it with cooler outside air. This process is governed by the air exchange rate, which measures how frequently the entire volume of air is replaced. For proper cooling during hot, sunny conditions, a greenhouse should aim for a high air exchange rate, ideally replacing the entire air volume once every minute, or 60 air changes per hour.
This high rate requires a powerful mechanical system, typically consisting of motorized exhaust fans on one end wall and louvered intake vents on the opposite wall. The fans create a negative pressure inside the structure, drawing in fresh air through the intake vents and ensuring a consistent flow of air across the crop area before the heated air is expelled. Fan systems are frequently sized based on the greenhouse’s floor area, with a common design parameter being 7 to 10 cubic feet per minute (CFM) of air movement for every square foot of floor space.
The strategic placement of these components is important for efficiency. Intake vents should be positioned low, near the floor, to introduce the freshest, coolest air where it is most needed. The exhaust fans are then placed high, near the roof, to pull the hottest air from the structure’s peak. Relying solely on natural ventilation, such as open roof vents and side walls, is usually insufficient to remove the substantial heat load generated on a summer day.
Reducing Solar Load with Shading
Preventing solar energy from entering the greenhouse is a highly efficient way to manage summer heat and reduce the heat load. Shading systems achieve this by intercepting the shortwave radiation before it can be converted into trapped longwave heat inside the structure. External shading methods are significantly more effective at cooling because they block the sunlight before it passes through the glazing.
Materials like shade cloth (polypropylene or polyethylene mesh) or white reflective paint applied directly to the exterior reflect the sun’s energy away. External shade cloth can prevent up to 90% of solar heat gain, substantially reducing the load on subsequent cooling systems.
In contrast, internal shading systems, such as curtains placed inside the structure, are less effective for cooling. While they shade the plants, the solar radiation has already passed through the glazing and generated heat before being intercepted. This absorbed heat then radiates into the greenhouse air, meaning the internal temperature will remain higher than with an external system. Shade materials are categorized by their density, expressed as a percentage of light blocked. The appropriate percentage of shading, typically 30% to 60% in summer, must be balanced to reduce heat stress without limiting the light required for photosynthesis and healthy plant growth.
The Role of Humidity and Evaporative Cooling
Evaporative cooling is an active method that can lower the internal greenhouse temperature below the ambient outdoor temperature, a feat simple ventilation cannot achieve. This process relies on the physical principle of the latent heat of vaporization, where energy is absorbed from the air as water changes from a liquid to a gas. When water evaporates, it draws heat energy from the surrounding air, converting sensible heat (temperature) into latent heat (stored in the water vapor), thereby cooling the air mass. Evaporative cooling systems are particularly efficient in hot, dry climates, where the air has a low relative humidity and a greater capacity to hold additional moisture.
The most common application is the fan-and-pad system, where exhaust fans draw air across a wall of water-soaked, cellulose cooling pads. As the warm outside air passes through the wet pads, the water evaporates, and the air entering the greenhouse is cooled by as much as 10 to 20 degrees Fahrenheit in arid regions.
Other systems, such as misters or high-pressure foggers, introduce very fine water droplets directly into the air. These tiny droplets quickly evaporate while suspended, cooling the air and simultaneously increasing the humidity. Managing the resulting humidity is a necessary consideration, as excessively high levels can inhibit plant transpiration and increase the risk of fungal diseases, requiring a balance between cooling and maintaining plant health.