The winter temperature inside a greenhouse is a variable result of physics, construction, and active management. A greenhouse functions by allowing short-wave solar radiation to pass through its transparent covering, where it is absorbed by internal surfaces, soil, and plants. This absorbed energy is then re-radiated as long-wave infrared heat, which the glazing material traps inside the structure. The actual temperature achieved depends entirely on the interaction of external weather conditions with the structure’s ability to capture and hold heat.
Understanding Baseline Temperature Gains
The greenhouse effect creates a noticeable temperature difference between the interior and the outside environment. On a sunny winter day, an unheated greenhouse can experience significant solar gain, often becoming 30 to 40 degrees Fahrenheit warmer than the ambient outside temperature. This daytime increase relies entirely on direct sunlight.
The challenge occurs during cloudy days and at night, when the greenhouse loses accumulated heat. Without supplemental heating or insulation, a single-layer structure may only maintain temperatures 5 to 10 degrees Fahrenheit above the outdoor minimum at night. For most cold-tolerant plants, natural solar gain is enough to keep the interior just above freezing, typically around 37°F (3°C), even when outside temperatures drop below freezing. This baseline protection prevents frost damage for cold-hardy species, but it is insufficient for tropical or tender plants.
Structural Factors Influencing Heat Retention
The materials of a greenhouse dictate its ability to resist heat loss, measured by the R-value (resistance to heat flow). Single-pane glass offers minimal insulation, yielding an R-value of less than 1.0, making it a poor choice for cold climates without supplemental heating. Multi-walled options, such as 6mm twin-wall polycarbonate, significantly improve performance, often reaching an R-value between 1.5 and 1.7 due to insulating air pockets. High-performance materials like 16mm five-wall polycarbonate can push the R-value up to 3.2, drastically reducing the energy needed to maintain warmth.
The size and shape of the structure also influence heat retention through the surface-area-to-volume ratio. A larger greenhouse volume relative to its exterior surface area loses heat more slowly than a smaller structure. Heat loss through air infiltration can account for a substantial percentage of total energy consumption. Sealing air leaks around vents, doors, and the base with weatherstripping or silicone caulk is necessary to prevent warm air from escaping and cold drafts from entering.
Passive Techniques for Maintaining Warmth
Growers can employ passive strategies that use the sun’s energy without consuming electricity or fuel.
Using Thermal Mass
The most effective passive technique involves introducing thermal mass, materials that absorb solar heat during the day and release it slowly at night. Water is the most efficient thermal mass material, possessing a heat capacity superior to concrete or stone. Placing large, dark-colored containers of water, such as 55-gallon drums painted black, along the sunniest walls maximizes solar energy absorption. This stored heat is then radiated into the greenhouse air as the interior temperature drops after sunset, moderating the nighttime temperature swing.
Temporary Insulation
Installing temporary insulation, such as horticultural bubble wrap with large bubbles, can further enhance heat retention by creating an insulating air layer against the glazing. This application can reduce heating costs by up to 35%. It is particularly effective when applied to the north wall, where it blocks radiant heat loss without sacrificing much-needed winter light.
Managing Air Exchange
Proper management of air exchange is necessary to conserve both heat and plant health. Since cooler air holds less water vapor, high humidity and condensation can quickly lead to fungal diseases. Short bursts of venting during the warmest hours of a sunny day allow humid air to be exchanged with drier outside air without causing a significant temperature drop. Using internal circulation fans to move air consistently also prevents pockets of cold, moist air from settling on plant surfaces.
Active Heating Solutions for Winter Protection
When the goal is to maintain a temperature higher than the natural baseline, active heating systems are required.
Types of Heaters
Electric heaters, such as forced-air or radiant units, are a clean option for smaller greenhouses, offering precise thermostatic control. While easy to install, the cost of electricity often makes them expensive to operate continuously for large spaces. Gas heaters, fueled by propane or natural gas, provide higher heat output and are typically more cost-effective for commercial operations. These units require a dedicated vent system to exhaust combustion byproducts like carbon dioxide and water vapor, which can be harmful to plants. Kerosene or paraffin heaters are an alternative without an electrical connection, but they also produce moisture and require manual monitoring.
Thermostat Placement and Strategy
A reliable thermostat is necessary to regulate the system and prevent unnecessary energy consumption. For accurate temperature sensing, the sensor must be placed at plant height, away from direct heater airflow and sunlight, often near the center of the growing area. The most cost-effective strategy is to maintain the lowest temperature that still protects the plants, such as a frost-free minimum of 37 degrees Fahrenheit.