A greenhouse is a specialized structure designed to create a controlled environment for the protection and cultivation of plants. This sealed space primarily functions by trapping the sun’s energy, which permits year-round gardening and the growth of crops that would not otherwise survive the local climate. Understanding how much warmer the interior gets compared to the outside air is important for anyone utilizing these structures. The actual temperature difference is not a fixed number, but depends entirely on the physics of heat retention and a range of environmental factors.
The Science Behind the Heat Retention
The warming mechanism within a greenhouse relies on trapping solar energy. Solar energy, predominantly shortwave radiation, easily passes through the transparent glazing material, such as glass or polycarbonate. Once inside, this radiant energy is absorbed by internal surfaces like the soil, plants, and structural elements. These objects warm up and re-emit the heat as longwave infrared radiation. Unlike the incoming shortwave radiation, the longwave radiation has difficulty passing back out through the glazing material. This trapping of re-radiated heat causes the interior temperature to climb significantly above the exterior temperature.
Typical Temperature Differences
The practical temperature difference between the interior and exterior of an unheated greenhouse can vary dramatically under typical sunny conditions. On average, an unheated, unventilated structure can easily be 5°C to 15°C (9°F to 27°F) warmer than the outside air during the day. This differential can soar to much higher extremes if the sun is intense and no cooling measures are in place. Under clear, sunny winter conditions, the temperature inside can be 20°C to 30°C (36°F to 54°F) warmer than the freezing outside temperature during peak solar hours. Conversely, on a heavily overcast day, the difference shrinks considerably, perhaps only maintaining a margin of 3°C to 5°C (5°F to 9°F) warmer than the outside.
Factors Influencing the Temperature Swing
External Factors
External elements determine the magnitude of the temperature difference achieved within the structure. Higher wind speeds increase the rate of heat loss through the glazing material, reducing the overall internal temperature. The angle of the sun and the season also affect the intensity and duration of the solar radiation entering the structure.
Structural Composition
The structural composition is a significant factor, particularly the type of glazing material used. Double-layered materials like twin-wall polycarbonate or insulated glass provide better insulation than single-pane glass. This leads to greater heat retention, especially at night.
Size and Orientation
The size and orientation of the greenhouse also matter. A smaller structure with a large surface-area-to-volume ratio loses heat more quickly than a larger structure.
Managing Internal Temperature
Because the temperature inside a greenhouse can quickly exceed 38°C (100°F) on a sunny day without intervention, active management is necessary to prevent plants from dying. There are three primary methods used to control internal temperatures:
- Ventilation: This is the most common technique to prevent overheating, involving opening roof vents or side louvers to allow hot air to escape and draw in cooler outside air. Many systems use automatic vent openers based on an internal thermostat to ensure consistent air exchange.
- Shading: This control method uses shade cloth or a temporary whitewash applied to the exterior glazing. Shading physically reduces the amount of solar radiation that enters the structure, thereby lowering the solar heat gain.
- Supplemental Heating: During cold periods, systems such as electric heaters or propane burners are employed to maintain a minimum temperature and prevent frost damage.