The glass or plastic shell of a greenhouse traps solar energy, which extends the growing season but creates a serious challenge in summer. This phenomenon, known as the greenhouse effect, causes internal temperatures to rise rapidly, often exceeding the ambient outdoor temperature by 15 to 30 degrees Fahrenheit. Uncontrolled heat quickly leads to plant stress, causing wilting, scorching, and the denaturation of necessary proteins and enzymes. Effective temperature regulation is mandatory to protect crops and ensure productive summer growth.
Maximizing Air Exchange and Ventilation
Removing the superheated air trapped inside the structure is the most direct method of cooling. Mechanical ventilation relies on exhaust fans and motorized intake shutters to achieve a specified air exchange rate, measured in Cubic Feet per Minute (CFM). The industry standard for summer cooling requires exchanging the entire volume of air inside the greenhouse at least once per minute. To calculate the necessary fan capacity, determine the greenhouse volume (length x width x average height); this figure is the minimum CFM rating required. Exhaust fans should be placed high on one end wall to draw the hottest air out, while fresh air is pulled in through intake shutters on the opposite end wall.
A complementary strategy is passive ventilation, which relies on the principle of thermal buoyancy, often called the chimney effect. Hot air naturally rises and escapes through vents positioned at the highest point of the structure, such as the ridge or roof. This upward movement draws cooler, fresh air in through side vents located lower to the ground.
For effective passive cooling, the total open vent area should ideally be between 25% and 40% of the greenhouse floor area. Roof vents are significantly more effective at releasing heat than side vents alone because they capture the highest concentration of hot air. Combining high-level roof vents with low-level side vents maximizes the convective flow.
Managing Solar Load with Shading
Reducing the amount of solar radiation that enters the greenhouse is an effective preventive cooling measure. External shading is the most effective solution because it intercepts sunlight and heat before they penetrate the glazing material. The heat absorbed by the shade cloth is then dissipated into the atmosphere outside the structure.
Shade cloth is available in densities ranging from 30% to 60%, depending on the crop’s light requirements. Placing the cloth several inches above the glazing allows for a small air gap, which helps vent trapped heat away. Internal shade screens trap absorbed heat between the screen and the glazing, offering only a marginal cooling benefit compared to exterior applications.
For a temporary, low-cost solution, specialized whitewash or shade paint can be applied directly to the exterior of panels. These materials contain reflective pigments that scatter incoming solar rays and are designed to wear off or be easily removed at the end of the season. While inexpensive, this method does not allow for dynamic control over light levels.
Utilizing Evaporative Cooling Systems
Evaporative cooling is an active process that uses the latent heat of vaporization, where water absorbs heat energy as it changes from a liquid to a gas. This heat absorption lowers the temperature of the air within the structure. The method works best in dry climates where ambient relative humidity levels are below 60%.
The most common system is the fan-and-pad setup, where exhaust fans draw air across water-saturated cellulose or aspen pads. As the air passes through the wet pads, water evaporates, cooling the air by as much as 10 to 20 degrees Fahrenheit in arid conditions. This cool, humidified air is then pulled through the entire length of the greenhouse before being exhausted.
High-pressure fogging or misting systems inject ultra-fine droplets of water directly into the air. These droplets evaporate almost instantly, cooling the air immediately around the plants. Proper installation is necessary to prevent surfaces from becoming wet, which could lead to fungal growth and disease if airflow is inadequate.
These cooling systems naturally increase relative humidity. Growers must monitor the environment to ensure humidity remains within a target range (60% to 75%) to prevent plant diseases. Maintenance involves regularly cleaning the pads or nozzles to prevent mineral buildup, which reduces system efficiency.
Passive and Structural Adjustments
Structural choices and passive environmental controls can supplement active cooling systems. Incorporating thermal mass into the floor or foundation helps modulate temperature swings by absorbing excess heat during the day. Large containers of water or a concrete floor slab slowly store this thermal energy, acting as a heat sink that stabilizes the interior temperature.
The stored heat is released gradually during the cooler evening hours. Additionally, wetting down a gravel or porous concrete floor can provide a small degree of evaporative cooling at ground level. This action contributes to a slightly cooler microclimate around the base of the plants without requiring a complex mechanical system.
The surrounding environment can also be managed to reduce solar gain and air temperature. Planting deciduous trees on the east, west, and south sides provides excellent shade during the summer. Once the leaves drop in the fall, the sun’s angle allows maximum light penetration during the cooler months.