Greenhouses provide a warm, protected environment for growing plants year-round. But how much warmer does a greenhouse stay compared to outside temperatures? There are several factors that affect how much heat is retained in a greenhouse. The main considerations are the glazing material, the size and shape of the structure, environmental conditions like sunlight and wind, and any supplemental heating systems. With the right design and materials, greenhouses can stay significantly warmer than exterior temperatures.
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How Do Greenhouses Retain Heat?
Greenhouses create a warmer microclimate by trapping heat from the sun during the day and reducing heat loss at night. The transparent glazing materials like glass or plastic allow sunlight to pass through and heat up objects inside the greenhouse. This heat gets absorbed by the plants, soil, and other contents rather than escaping back out.
At night, the glazing helps hold in the accumulated heat. Greenhouses have a high proportion of surface area compared to their volume, so they can lose heat quickly without glazing. The glazing acts as insulation to reduce conductive heat loss through the walls and roof. It also reduces convective heat loss by preventing warmer interior air from mixing with cooler exterior air.
Greenhouse designs that are more enclosed or have double polycarbonate panel glazing can further reduce heat loss. Greenhouses may also incorporate energy-saving features like thermal curtains, foam board insulation, or high-performance glazing materials. With good heat retention, greenhouses can stay up to 20-30°F warmer than the outside air.
Factors That Affect Greenhouse Heating
Several variables impact the temperature difference between a greenhouse environment and the external conditions:
Glazing Material
The type of glazing is one of the biggest factors. Traditional glass has high clarity to let in sunlight but also conducts heat readily. Plastic polyethylene film has better insulation properties but reduced light transmission. Polycarbonate panels offer a good compromise, with decent insulation, transparency, and durability.
Insulation
Added insulation around the greenhouse further reduces conductive and convective heat loss. Foam board, rigid plastic sheets, insulation blankets, or bubble wrap add protective layers to keep heat in. Airtight seals around doors, vents, and fans also minimize air leaks.
Size and Shape
The geometry of the greenhouse affects the surface area to volume ratio. More compact shapes like domes or square structures retain heat better than long, lean-to greenhouses. Larger greenhouse spaces also hold heat more effectively.
Growing Season
The temperature difference is most pronounced in the winter when sunlight hours are shorter and outdoor temperatures are substantially colder. In the summer, greenhouses may not stay much warmer since heat builds up quickly on sunny days. Greenhouse cooling and ventilation are needed in hotter months.
Supplemental Heat
Heating systems like heaters, hot water pipes, or heat exchangers can actively add warmth to the greenhouse. This allows more consistent control over temperatures year-round.
Environmental Conditions
Wind, precipitation, cloud cover, and humidity affect the greenhouse’s ability to retain heat. Calm, dry days allow for more solar gain. Cloudy or wet weather reduces heat accumulation and increases losses.
Typical Temperature Differences
Despite many variables, well-designed greenhouses often maintain temperatures around 20-30°F higher than ambient air when sunlight is present. On a sunny 50°F winter day, the interior may reach 70-80°F. At night and on cloudy days, the greenhouse temperature drops closer to the outside temperature but still remains warmer. Adding insulation and heating can help moderate this temperature swing.
Here are some typical temperature differences for greenhouses:
Winter Daytime
– Outside: 25-45°F
– Greenhouse: 50-75°F
– Difference: +20 to +30°F
Winter Nighttime
– Outside: 20-40°F
– Greenhouse: 30-55°F
– Difference: +5 to +15°F
Summer Daytime
– Outside: 70-90°F
– Greenhouse: 80-100°F
– Difference: +5 to +15°F
Summer Nighttime
– Outside: 60-75°F
– Greenhouse: 65-85°F
– Difference: +5 to +10°F
These ranges demonstrate how greenhouses maintain warmer growing environments, especially during colder months. With greenhouse heating systems, the temperature difference can be increased further.
Example Temperature Data
Here is sample data showing greenhouse versus outdoor temperatures over a one week period in March:
| Date | Outdoor Low Temp (°F) | Greenhouse Low Temp (°F) | Outdoor High Temp (°F) | Greenhouse High Temp (°F) |
|---|---|---|---|---|
| March 1 | 32 | 41 | 46 | 68 |
| March 2 | 33 | 42 | 48 | 72 |
| March 3 | 34 | 44 | 51 | 76 |
| March 4 | 36 | 46 | 53 | 78 |
| March 5 | 37 | 48 | 55 | 80 |
| March 6 | 35 | 45 | 52 | 74 |
| March 7 | 34 | 43 | 48 | 70 |
This data shows the greenhouse maintained interior temperatures about 10-15°F warmer than outdoor lows and 20-25°F warmer than outdoor highs. These ranges are typical for a well-insulated greenhouse in springtime conditions.
Estimating Temperature Difference
Greenhouse operators can estimate the expected temperature difference by considering these factors:
– Desired minimum interior temperature
– Average low/high exterior temperatures for the location and time of year
– Glazing material (glass vs. polycarbonate vs. plastic sheeting)
– Insulation type and rating
– Greenhouse size
– Presence of energy curtains, thermal screens, etc.
– Type of supplemental heating system
With this information, you can predict about how much warmer the greenhouse will stay. If the structure is unable to maintain the minimum desired temperature, changes like adding insulation or installing heaters can help raise the interior temperature difference. Ongoing monitoring of interior and exterior temperatures will verify if the greenhouse is operating as expected.
Conclusion
Greenhouses provide growers with more climatic control compared to open-field agriculture. Through greenhouse designs that maximize light transmission while minimizing conductive and convective heat losses, interior temperatures can remain 20-30°F warmer than external conditions. However, many variables affect the efficiency of heat retention. The glazing material, insulation, size, environmental factors, and supplemental heating all impact the temperature difference. With strategic choices and active heating, greenhouses allow year-round plant growth despite cold exterior conditions. Tracking temperature data provides insight into how much extra warmth a particular greenhouse maintains.