A greenhouse is designed to capture solar energy, creating a warmer environment than the outside air. While the transparent structure allows sunlight to enter and heat the interior during the day, this effect is often insufficient to maintain warmth during cold weather or at night. The structure is not inherently insulated against the subsequent heat loss once the sun sets. For a greenhouse to support plant life through a cold winter, natural solar gain must be supplemented by strategic design and active intervention. This requires understanding how solar energy is converted into heat and how that heat escapes the structure.
Understanding Solar Gain and Trapping
The warming phenomenon relies on converting solar energy into thermal energy, known as the greenhouse effect. Sunlight reaches the structure as shortwave radiation, easily passing through transparent glazing materials like glass or polyethylene. Inside, this energy is absorbed by the soil, plants, and structural components, causing their temperature to rise.
These warmed objects re-radiate the absorbed energy as longwave infrared radiation, which is perceived as heat. Unlike the incoming shortwave radiation, this outgoing longwave radiation is largely blocked or absorbed by the glazing material. The transparent covering acts as a selective filter, trapping thermal energy inside the enclosure. This cycle allows the structure to achieve temperatures significantly higher than the ambient exterior air on a sunny day.
Mechanisms of Heat Loss in Cold Weather
Despite daytime solar gain, a greenhouse is highly susceptible to heat loss once the sun disappears because it is a lightweight structure with a large surface area. The primary method of thermal energy escape is conduction, which is the transfer of heat directly through the glazing material. Different coverings resist this heat flow differently, often measured by an R-value. Single-pane glass and thin polyethylene offer very little resistance, while a double-layer system with an insulating air gap transmits heat much less readily.
Another significant path for heat loss is convection and air infiltration, often considered the most substantial source of heat escape. Warm air inside the structure rises and escapes through small gaps, cracks, and poorly sealed vents. This pulls cold exterior air in to replace the lost volume, dramatically cooling the interior environment. Poorly sealed structures can experience rapid air exchange, quickly diminishing stored warmth, especially when exterior winds are strong.
The third mechanism of cooling is radiative heat loss, occurring when internal surfaces emit longwave infrared radiation toward the cold night sky. While glass blocks most of this outgoing radiation, many common plastic glazing materials, such as standard polyethylene film, allow a substantial portion of this energy to pass through. This energy loss is particularly noticeable on clear nights, causing the internal temperature to drop quickly.
Passive Solutions for Winter Temperature Stabilization
To counteract heat loss without relying on external power, growers utilize passive strategies focused on maximizing retention. A highly effective method involves incorporating thermal mass, which is any material that absorbs heat during the day and slowly releases it at night. Water is an excellent choice due to its high specific heat capacity, making black-painted water barrels popular for stabilizing nighttime temperatures. Materials like concrete, stone, or brick flooring also contribute to thermal mass, helping to reduce temperature swings.
Reducing conductive heat loss is accomplished by improving the insulation value of the structure’s envelope. Adding a second layer of glazing, such as greenhouse-grade bubble wrap or a second film of polyethylene, creates a dead air space. This significantly decreases the rate of heat transfer. For the northern-facing wall, which receives minimal winter sunlight, applying a reflective, opaque insulation material prevents heat from escaping.
Controlling air infiltration requires sealing all unintended openings. Vents must be tightly closed, and any gaps around doors, fans, or joints need to be sealed with weather stripping or specialized tape. Strategically siting the structure to block prevailing northern winds or using a windbreak hedge reduces the convective cooling effect. Utilizing an insulated thermal curtain pulled over the interior glazing at night dramatically reduces both conductive and radiative heat loss.
Implementing Active Heating Systems
When passive methods are insufficient to maintain the minimum temperature for plant survival, supplemental active heating systems are necessary. For smaller structures, electric heaters provide a clean, reliable, and easily controlled heat source. Larger greenhouses require more powerful options, such as natural gas or propane-fired unit heaters. These forced-air units quickly generate and distribute warmth throughout the space.
A primary safety consideration for combustion heaters is the need for proper venting to exhaust byproducts like carbon monoxide and ethylene outside the growing area. Unvented heaters release gases toxic to humans and sensitive plants, potentially causing leaf damage. Radiant infrared heaters offer an alternative by directly warming the plants and surfaces rather than the surrounding air. This can be a more energy-efficient approach because less heat is lost to the structure’s envelope.
Regardless of the heat source, the system’s efficiency relies on maintaining a minimum set point using a reliable thermostat appropriate for the specific plants. Small circulation fans are employed to move air horizontally throughout the greenhouse. This prevents the formation of cold pockets and ensures the heated air is evenly distributed. Constant air movement maximizes the efficiency of the heating system by uniformly maintaining the target temperature.