The temperature difference inside an unheated greenhouse compared to the outside air during winter is highly variable. This dynamic range is influenced by solar intensity, structural design, and external weather conditions. Understanding this variability requires looking at the physics of how solar energy is converted and retained within the structure. This article explores the physical mechanisms and practical results of passive heating to determine the temperature gains a grower can expect.
How the Greenhouse Effect Traps Solar Energy
The warming mechanism within a greenhouse mimics a natural atmospheric process, driven by the difference in energy wavelengths. Solar radiation, which is primarily short-wave energy, easily passes through the transparent glazing material. Once inside, this energy strikes interior surfaces, such as the ground and plants, where it is absorbed.
Upon absorption, the energy is converted into heat and re-radiated as long-wave infrared radiation. Glazing materials, like glass or polycarbonate, are designed to be largely opaque to this long-wave radiation. This trapped infrared energy cannot easily escape, leading to a rapid increase in the internal air temperature. The physical enclosure also prevents the warm air from rising and mixing with the colder exterior air, significantly reducing heat loss via convection.
Typical Temperature Gains in an Unheated Winter Greenhouse
The most significant temperature increase in a passive greenhouse occurs during the day under clear, sunny conditions. An unheated greenhouse can experience a temperature spike of 15°F to 30°F above the outside ambient temperature. For example, if the outdoor air is 30°F, the interior temperature may quickly reach 45°F to 60°F or higher. This substantial daytime warmth is beneficial for photosynthesis and plant growth, effectively extending the growing season.
This temperature gain is directly dependent on the sun, and the situation changes dramatically after sunset. Without a heat source, the interior temperature rapidly drops due to conductive heat loss through the glazing and infiltration through gaps. The minimum temperature reached overnight is often the most limiting factor for plant viability. While some stored heat may keep the interior slightly warmer than the outside air, the temperature commonly falls back to near-ambient levels. In poorly insulated structures, the temperature can even drop a few degrees below the outside air due to radiative cooling toward the cold night sky.
Structural and Environmental Factors that Influence Warmth
The actual temperature difference achieved is highly dependent on the greenhouse’s inherent structural properties. The glazing material’s R-value, a measure of its resistance to heat flow, is a major factor. Single-pane glass or single-layer polyethylene film offers minimal insulation. In contrast, multi-wall polycarbonate or double-layer polyethylene film creates an insulating air pocket, significantly increasing the R-value and reducing conductive heat loss.
Air tightness is equally important, as infiltration through small cracks or unsealed vents allows warm air to escape and cold air to rush in. This infiltration can account for a large percentage of total heat loss. The orientation of the structure also dictates how much solar energy is captured. A greenhouse oriented with its longest side facing south maximizes exposure to the low-angle winter sun, optimizing solar gain.
Environmental conditions outside the structure introduce further variability. Wind speed dramatically increases heat loss, potentially doubling the heating requirement if the wind speed increases from zero to 15 miles per hour. Furthermore, the presence of clouds or a heavy overcast layer immediately reduces solar gain, preventing the daytime temperature spike.
Maximizing Passive Heat Retention
Growers can employ several practical, non-mechanical methods to mitigate the nighttime temperature drop and stabilize the internal climate. The most effective passive technique involves incorporating thermal mass: dense materials that absorb and store heat during the day, releasing it slowly at night. Large, dark-colored water barrels, concrete floors, or stone walls are excellent examples of thermal mass. These materials absorb long-wave radiation and radiate the stored heat back into the space after sunset.
Temporary insulation measures are also highly effective at reducing heat loss through the glazing itself. Lining the interior walls with horticultural bubble wrap creates an additional layer of still air, acting as supplemental insulation. After sunset, deploying thermal blankets or reflective insulating curtains can be drawn across the ceiling and walls to trap the accumulated heat. These interventions primarily function to slow the rate of heat loss, ensuring the minimum overnight temperature remains high enough to prevent cold damage.