A greenhouse is typically somewhere between 5 and 30 °C warmer than the outside air during the day, though the actual number swings dramatically depending on the time of year, the type of structure, and whether any cooling systems are running. One experimental solar greenhouse recorded interior temperatures ranging from 3 to 35 °C above ambient across different conditions. That enormous spread hints at the real answer: there is no single number. A small, sealed cold frame on a sunny July afternoon can easily climb 30 °C above outside air, while a well-ventilated commercial greenhouse with shade cloth might hover only a few degrees warmer, or even dip below outdoor temperature under certain conditions.
Where the Heat Comes From
Sunlight passes through the transparent covering of a greenhouse and is absorbed by the soil, plants, and interior surfaces, which then re-emit that energy as heat. In an open field, warm air simply rises and drifts away. Inside a greenhouse, the walls and roof block that convective mixing, trapping the heated air. This suppression of air movement is the primary reason a greenhouse warms up, not (as is commonly assumed) infrared trapping by the glass or plastic. The covering does absorb some outgoing infrared radiation, but convection suppression does most of the heavy lifting.
The intensity of the heating depends on how much sunlight gets in, how tightly sealed the structure is, and how much mass is available inside to absorb and store heat. On a cloudless summer day with no ventilation, temperatures inside even a modest backyard greenhouse can climb past 50 °C. During winter, with low sun angles and shorter days, the same greenhouse might only manage a few degrees of advantage over outside air.
The Nighttime Surprise
Most people assume a greenhouse stays warmer than the outside air around the clock. That is true for heated structures, but unheated greenhouses can actually become cooler than the outside air after sunset. Research on unheated greenhouses found that on clear nights, the air inside dropped about 2.5 °C below outside air temperature, and the roof surface was as much as 4.4 °C cooler than the ambient air.1Acta Horticulturae. COMPUTATIONAL FLUID DYNAMIC MODELLING OF NIGHT-TIME ENERGY FLUXES IN UNHEATED GREENHOUSES This phenomenon, called thermal inversion, happens because the thin covering material radiates heat into the clear sky faster than the enclosed air mass can replace it. The covering essentially becomes a cold surface, chilling the air near it.
Radiation losses through the outer surface of the cover are the main driver of nighttime energy loss in an unheated greenhouse.2Acta Horticulturae. THERMAL PERFORMANCE OF AN UNHEATED GREENHOUSE UNDER SEMI-ARID CONDITIONS DURING THE NIGHT Cloud cover helps, because clouds reflect some of that outgoing radiation back down. But on a still, clear night, an unheated greenhouse can offer little frost protection and may even put plants at greater risk than if they were outdoors, where turbulent air mixing moderates temperature drops.
One practical fix is an external shading screen, which, somewhat counterintuitively, helps at night too. Research showed that external screens reduce radiative heat loss to the sky, cutting the risk of thermal inversion and keeping nighttime temperatures closer to or slightly above outside air.3Spanish Journal of Agricultural Research. Shading screens for the improvement of the night time climate of unheated greenhouses The effect is strongest under clear skies, which is exactly when the problem is worst.
How Covering Material Changes the Equation
Not all greenhouse coverings perform the same. Glass, polyethylene film, PVC, and polycarbonate panels all transmit different amounts of visible light and infrared radiation, and those differences change how much heat builds up inside. Research comparing enclosures covered in silica glass, PVC, and low-density polyethylene found measurable differences in interior temperature, with the semi-transparent plastic films and the more opaque glass trapping heat differently.4Solar Energy. Theoretical and experimental study of solar thermal performance of different greenhouse cladding materials
Glass tends to hold heat well because it is largely opaque to long-wave infrared, meaning it absorbs outgoing heat radiation and re-emits some of it back into the greenhouse. Single-pane glass also conducts heat readily, though, so it loses warmth faster than double-wall polycarbonate. Polyethylene film is cheap and lets in plenty of light, but it is relatively transparent to infrared, which means more heat escapes at night. Double-layer polyethylene with an air gap between the layers narrows that gap somewhat. Polycarbonate panels, especially twin-wall or triple-wall varieties, insulate better than single-layer anything, reducing both daytime overheating on mild days and nighttime heat loss.
For most hobby gardeners, the practical takeaway is that the covering you choose matters as much for nighttime heat retention as for daytime warmth. A greenhouse that heats up beautifully during the day can still freeze plants overnight if the covering lets infrared radiation escape too freely.
Ventilation and How Quickly the Advantage Disappears
The moment you open a vent or a door, the temperature gap between inside and outside shrinks fast. Ventilation works because it allows the trapped warm air to escape and cooler outside air to flow in, restoring the convective mixing that the structure was designed to block. Research on three-span arched greenhouses showed that opening side vents significantly affected interior temperature: with side vents open to 0.8 meters and no wind, the interior was roughly 3 °C cooler than with only roof vents open. Even a light breeze of 1 meter per second shrank the interior-to-exterior difference further, down to about 1.5 °C.5Computers and Electronics in Agriculture. Effects of vent opening, wind speed, and crop height on microenvironment in three-span arched greenhouse under natural ventilation
This is why commercial greenhouses in warm climates often run with vents wide open for much of the growing season. The goal is not to maximize the temperature difference but to keep it in a productive range, usually around 20 to 30 °C for most crops. An unventilated greenhouse on a hot day is not a growing environment; it is an oven. Growers spend more energy and engineering on cooling greenhouses than heating them across large parts of the world.
When a Greenhouse Can Be Cooler Than Outside
It sounds paradoxical, but a well-managed greenhouse full of actively transpiring plants can actually have air temperatures below outside air, even at peak solar radiation. Plants release water vapor through their leaves, and the evaporation of that water absorbs heat from the surrounding air, the same principle behind sweating. Research on a ventilated greenhouse growing roses found that with about 30 air volume changes per hour and outside humidity below 50 percent, plant transpiration cooled the air inside the greenhouse below the outdoor temperature even when solar radiation was at its maximum.6Agricultural and Forest Meteorology. Evaporative cooling of a ventilated greenhouse rose crop
Under non-water-limiting conditions, rising temperatures drive more transpiration, which cools the leaves and surrounding air, creating a natural negative feedback loop that moderates temperature spikes.7Agricultural and Forest Meteorology. Transpirational cooling of a greenhouse crop with partial ground cover The denser the canopy and the more water the plants have access to, the stronger this cooling effect. An empty greenhouse will always be hotter than the same greenhouse filled with well-watered tomato plants.
This is one of those findings that surprises people outside of horticulture. A greenhouse is not just a passive heat trap; it is a managed ecosystem where plants, ventilation, and humidity all interact to determine interior climate. Experienced growers use this to their advantage, timing irrigation and vent openings to keep temperatures in a target range.
Shading and Evaporative Cooling in Hot Climates
In hot, sunny regions like the Mediterranean basin, the Middle East, or the American Southwest, the challenge is not warming the greenhouse but preventing it from overheating. Shade cloth, reflective screens, and evaporative cooling systems are standard equipment. Research on a greenhouse with external shade cloth and a water film flowing over it found that the shading alone reduced interior air temperature by about 2 °C compared to an unshaded greenhouse, while adding the water film brought the reduction to about 6 °C.8Energy and Buildings. Modeling and experimental validation of a greenhouse with evaporative cooling by moving water film over external shade cloth
A broader review of shading in sunny regions found that combining shading with a cooling method could maintain greenhouse air temperature 5 to 10 °C below outside air, while also increasing relative humidity by about 15 to 20 percent and cutting incoming solar radiation by 30 to 50 percent.9Scientia Horticulturae. Shading greenhouses to improve the microclimate, energy and water saving in hot regions: A review That figure, 5 to 10 °C cooler than outside, is the flip side of the wintertime warming advantage. In summer, a well-designed greenhouse in a hot climate is a cooling structure, not a heating one.
This completely reframes the question in the title. For a significant part of the year in warm climates, the answer to “how much warmer is a greenhouse” is actually “it isn’t, and it shouldn’t be.” The same structure that provides warmth in winter provides shelter and cooling in summer, depending on how it is managed.
The Role of Soil and Thermal Mass
The ground inside a greenhouse acts as a thermal battery. During the day, the soil absorbs solar energy that passes through the covering. At night, it slowly releases that stored heat back into the air. This buffering effect is significant enough that mathematical models of greenhouse energy balance that ignore soil heat flux produce inaccurate predictions of heating needs. Research found that accounting for the soil’s heat storage led to a considerable reduction in calculated heating demand, particularly when heating set points were low.10Acta Horticulturae. INFLUENCE OF THERMAL STORAGE EFFECTS OF THE SOIL ON GREENHOUSE HEAT CONSUMPTION
Some greenhouse designs amplify this effect deliberately. Water-filled barrels, rock beds, and buried pipe systems all serve as thermal mass, soaking up excess daytime heat and releasing it after dark. These passive solar strategies have been studied and deployed worldwide, with five main categories of heat storage: water, phase-change materials, rock beds, buried pipes, and hybrid systems. The choice depends on climate, crop type, and budget. A few hundred liters of water in dark containers along the north wall of a small greenhouse can raise nighttime temperatures by several degrees at essentially no operating cost.
For the home gardener asking how much warmer their greenhouse will be at 2 a.m. in March, the answer depends heavily on how much thermal mass is inside. An empty greenhouse with bare concrete will cool faster than one with raised beds full of moist soil and a row of water barrels. The insulating value of the covering matters too, but thermal mass is the variable most hobbyists overlook.
Shape, Size, and Vertical Stratification
The geometry of a greenhouse affects how much heat it retains. A low-profile greenhouse with a shallow roof pitch loses less heat through natural convection than a tall, steep-roofed one. A study of even-span gable-roof greenhouses found that decreasing the aspect ratio and roof pitch reduced convective heat transfer by up to 25 percent for aspect ratio changes and about 15 percent for roof pitch changes.11Thermal Science and Engineering Progress. Characterising the effects of geometry on the natural convection heat transfer in closed even-span gable-roof greenhouses In practical terms, a squat, wide greenhouse holds heat better than a tall, narrow one.
Size matters too, because the ratio of surface area to volume decreases as a greenhouse gets larger. A small 2-by-3-meter hobby house has a lot of exterior surface for its modest interior volume, so it heats up fast during the day but also cools fast at night. A large commercial range covering thousands of square meters has proportionally less surface area, giving it more thermal inertia. Commercial growers benefit from this scaling effect even before they add any heating or insulation.
Inside a greenhouse, the temperature is not uniform. Warm air rises, creating a vertical gradient that can be surprisingly steep. Research in semi-closed greenhouses found that the temperature difference between the top and bottom of the crop canopy exceeded 5 °C when outside solar radiation was high.12Acta Horticulturae. Vertical Temperature Gradients in the Semi-Closed Greenhouses: Occurrence and Effects The humidity gradient tracked the temperature gradient, with drier air at the top and more humid air near the soil. For tall crops like tomatoes or cucumbers, this means the fruit at shoulder height may be experiencing a meaningfully different microclimate than the roots just centimeters above the ground.
Humidity as the Hidden Variable
Temperature gets all the attention, but humidity inside a greenhouse is arguably just as important and just as different from outdoor conditions. The enclosed space traps moisture released by plant transpiration and soil evaporation, pushing relative humidity higher than outside air. In a well-stocked, poorly ventilated greenhouse, humidity can approach saturation, creating ideal conditions for fungal diseases.
The relationship between temperature and humidity inside a greenhouse is inverse at any given moisture content: as temperature climbs, relative humidity drops, and vice versa.13Computers and Electronics in Agriculture. Process-based humidity control regime for greenhouse crops This means that the daytime warming effect actually lowers relative humidity, which can be beneficial for disease prevention but stressful for plants if it goes too far. Growers monitor vapor pressure deficit rather than just temperature or humidity alone, because VPD captures the drying power of the air and drives transpiration rate more directly than either number in isolation.
For home greenhouse owners, the practical lesson is that heating or cooling the air automatically changes the humidity situation, and those changes can matter more for plant health than the temperature itself. A greenhouse that is 15 °C warmer than outside on a winter morning might also have dramatically lower relative humidity than the foggy air just outside the door, and that bone-dry interior air can stress seedlings and slow growth.
Why a Greenhouse Does Not Work Like the “Greenhouse Effect”
The phrase “greenhouse effect” is one of the most successful and most misleading metaphors in science communication. An actual greenhouse warms up primarily because its walls and roof physically block the movement of warm air. Hot air cannot rise and drift away as it would in an open field. The covering does absorb some outgoing infrared radiation, but experiments going back over a century have shown that even a covering transparent to infrared still produces most of the warming, because convection suppression is the dominant mechanism.
The atmospheric greenhouse effect, by contrast, works entirely through radiation. Gases like carbon dioxide and water vapor absorb outgoing infrared radiation from Earth’s surface and re-emit it in all directions, including back toward the ground. There is no physical barrier preventing convection in the atmosphere; in fact, convection (thermals, weather systems) is one of the main ways heat moves from the surface to the upper atmosphere. The two phenomena share a name but rely on different physics. A paper in the International Journal of Modern Physics B highlighted that there are no common physical laws between the warming of glass houses and the atmospheric process often called the greenhouse effect.14International Journal of Modern Physics B. FALSIFICATION OF THE ATMOSPHERIC CO2 GREENHOUSE EFFECTS WITHIN THE FRAME OF PHYSICS
Understanding this distinction is useful because it clarifies what you can and cannot change about your greenhouse. Adding a second layer of glazing improves performance mainly by creating a dead-air insulation layer that further suppresses convective heat loss, not by doubling the infrared absorption. Choosing a covering that blocks more infrared helps a little, but adding mass, reducing air leakage, and managing ventilation all matter more.
Seasonal and Regional Expectations
The temperature advantage of a greenhouse follows the sun. In mid-winter at high latitudes, when the sun barely clears the horizon and the days are short, an unheated greenhouse might gain only 2 to 5 °C during the brief sunny hours and lose most of that overnight. In the shoulder seasons (early spring, late fall), the same greenhouse might gain 10 to 20 °C during a sunny afternoon. At the height of summer, the challenge flips entirely, and the grower’s job becomes preventing the interior from overheating.
Climate matters just as much as season. A greenhouse in the cloudy Pacific Northwest of the United States will rarely see the dramatic daytime temperature spikes common in the sunny Intermountain West. A desert greenhouse in Arizona can easily reach 50 °C inside by mid-morning without active cooling, while one in coastal Scotland might struggle to hit 25 °C on a good day. The one experimental solar greenhouse that recorded a 3 to 35 °C range above ambient was demonstrating exactly this variability: the temperature advantage is not a fixed number but a function of sun angle, cloud cover, wind, and the design choices built into the structure.15IOP Conference Series: Earth and Environmental Science. Evaluation of the effectiveness of a helio-greenhouse with soil heating
For anyone planning a greenhouse, the honest answer to the title question is: it depends on everything. The covering, the thermal mass, the ventilation, the crop load, the time of day, the season, and the local climate all interact. A sealed, empty, glass-covered greenhouse on a sunny day might be 30 °C warmer than outside. That same greenhouse at 3 a.m. on a clear winter night, unheated and poorly insulated, might be cooler than the outdoor air. The skill in greenhouse growing lies in narrowing that wild swing into a range your plants can thrive in.