The climate inside the glass

A greenhouse is not simply “warmer than outside.” It is a small climate system that can change completely between a bright afternoon and a cool British night.

Sunlight can raise greenhouse air temperature very quickly, even when the outdoor day feels mild. Plants add moisture through transpiration. After sunset, glass, aluminium and leaves cool; relative humidity rises and cold surfaces may reach the dew point. This cycle is especially important in the UK, where cool nights, cloudy spells and humid maritime air can follow sunny conditions within the same week.

Good greenhouse management therefore means controlling relationships rather than one thermostat setting. Temperature affects photosynthesis, respiration, flowering and fruit set. Humidity affects transpiration, nutrient movement, pollination, disease pressure and condensation. Air movement changes the leaf boundary layer; watering changes humidity; shading changes leaf and air temperature.

For a home grower, the practical goal is a stable, responsive microclimate that suits the crop. Lettuce and young plants need different conditions from tomatoes, cucumbers, peppers and basil. The greenhouse strategy in Cornwall or a sheltered London garden will also differ from an exposed site in Scotland or northern England.

If you are choosing the structure itself, climate control begins with the greenhouse. Browse the Bloomcabin UK greenhouse collection with usable vents, doors, air volume, height and glazing in mind.

There is no single ideal temperature. A useful home-greenhouse approach is to separate cool-season crops from warm-season crops. Lettuce, spinach and many seedlings perform at substantially lower temperatures than tomatoes, cucumbers, aubergines, basil, peppers and chillies. Growth stage and light level also shift the optimum.

For many warm-season vegetables, daytime conditions around 20–27°C are a useful starting zone, with somewhat cooler nights. That is not a universal setpoint: fruiting crops can lose flower quality during sustained heat, while cool crops may bolt long before a tomato appears stressed.

The practical rule is to define the house by its crop mix. A greenhouse used for salads can run cooler. One dominated by tomatoes and peppers should run warmer. Mixed houses need zoning, good spacing and ventilation rather than trying to force every crop to the same number.

During daylight, photosynthesis is active and leaf temperature may exceed air temperature under strong sun. At night, photosynthesis stops while respiration continues. A moderate day–night difference is normal; problems arise when nights become cold enough to check growth or warm enough to keep respiration unusually high.

On clear nights, leaves and glazing can cool below the measured air temperature through radiation. Condensation depends on this surface temperature. A leaf may reach the dew point even when the air sensor still reads a few degrees warmer.

In much of Britain, buffering cool nights is a major greenhouse advantage. During summer heatwaves, however, the job reverses and the house must release stored heat. Larger air volume changes more slowly, which is one reason a greenhouse such as the Bloomcabin Classic behaves differently from a very small enclosure.

Use a maximum/minimum thermometer or logger. A single noon reading can hide the two measurements that often explain crop performance best: yesterday’s maximum and last night’s minimum.

03 • Humidity

What is the ideal greenhouse humidity?

Humidity is where simplistic greenhouse advice becomes misleading. Relative humidity is temperature-dependent: warm air can hold more water vapour than cool air. If the water content stays similar while temperature falls, RH rises. That is why an apparently comfortable afternoon can become near-saturated air after sunset.

The wettest microclimate is often inside the crop canopy rather than by the door. Transpiring leaves release moisture while dense foliage restricts air exchange. A hygrometer near the entrance may therefore look acceptable even when the tomato canopy is far more humid.

For a mixed greenhouse, think in terms of a moderate working humidity rather than a magic percentage. Very dry air increases evaporative demand and wilting risk. Very humid air slows drying, encourages condensation and can interfere with pollination or increase fungal disease risk.

One practical rule is especially useful in Britain: do not let the house stay near saturation for long periods overnight. Keeping leaves dry and air moving is a better disease-prevention objective than forcing one exact RH value every hour.

Vapour-pressure deficit, or VPD, expresses the drying demand of the air more directly than relative humidity alone because it combines temperature and moisture. It helps explain why the same RH percentage can feel very different to a plant on a cool spring morning and during a hot July afternoon.

At low VPD the air is moist relative to its capacity, so transpiration slows and surfaces dry more slowly. At high VPD the air has strong drying power; leaves lose water rapidly and roots must replace it. Neither extreme is automatically desirable, and the preferred zone depends on crop and stage.

Home growers do not need commercial climate computers. A temperature/RH sensor that calculates dew point or VPD can already reveal useful patterns. The key lesson is simply that 70% RH at 12°C is not the same growing condition as 70% RH at 28°C.

Think of it this way

RH describes the air. VPD better describes the drying demand on the plant.

That is why serious greenhouse climate management uses more than one number.

The dew point is the temperature at which air can no longer keep all of its water vapour in vapour form. If glass, aluminium, pipes or leaves cool below it, water condenses. The coldest glazing and framework usually become wet first; with continued cooling, the crop itself can become wet.

That moisture is not merely untidy. Dripping condensation can splash soil and debris, while wet foliage creates favourable conditions for Botrytis and other diseases. Closing every vent tightly at dusk to preserve heat can therefore trap the day’s moisture and create a disease-friendly night.

The solution is not to leave a greenhouse wide open all night. It is to combine sensible heating, short moisture-purging ventilation, air circulation, morning watering, clean floors and suitable spacing so the canopy does not remain wet from dusk to dawn.

A useful misconception to break

“It is raining outside, so opening the greenhouse cannot make it drier.”

It often can. Cool saturated outside air may contain less total water than warm humid greenhouse air. Once that incoming air is warmed, its relative humidity falls and it can absorb additional moisture.

Ventilation removes heat and moisture, refreshes carbon dioxide and reduces temperature differences. Passive ventilation uses buoyancy and wind: hot air leaves through high vents while cooler replacement air enters through lower openings and doors. Fans can assist exchange or circulate internal air.

RHS advice stresses the importance of adequate roof ventilation, with doors and side openings helping replacement airflow. For the home grower, several useful openings plus air circulation are far stronger than one tiny vent. Automatic vent openers are particularly helpful when a bright morning begins while nobody is at home.

If easy climate management is a priority, compare structures with usable openings and generous air volume such as the Bloomcabin Prestige or a Lean-To Greenhouse.

Roof vents, side openings or doors?

Roof vents: excellent for releasing rising hot air.

Doors / lower openings: supply replacement air and create useful cross-flow.

Circulation fans: do not replace outside-air exchange, but reduce hot, cold and humid pockets.

High tunnels: rely heavily on passive ventilation, so large openings matter. See the Bloomcabin High Tunnel Greenhouse for another protected-growing format.

Ventilation comes first. Shading reduces incoming solar radiation rather than removing heat already stored inside, so the two methods work best together. During a British heatwave, shading can be particularly useful for seedlings, lettuce and fruit exposed to intense sun.

Manage the root zone as well. Dark containers on paving can become significantly hotter than the surrounding air. Larger pots, mulch, lighter surfaces and stable watering reduce root stress. Avoid heavy defoliation immediately before hot weather because exposed fruit can scorch.

Air volume matters. A more spacious structure such as the Bloomcabin Orangery changes temperature more slowly than a tiny enclosure, although good venting is still essential.

Cooling order of operations

1. Open high vents and doors early.

2. Increase internal air movement.

3. Add shading if solar load remains excessive.

4. Stabilise root-zone moisture and temperature.

5. Reconsider crop placement if one zone repeatedly overheats.

Water early. Morning watering gives foliage, benches and floors time to dry as the greenhouse warms.

Avoid standing water. Every puddle becomes another moisture source.

Space crops properly. Dense canopies create humid pockets exactly where disease begins.

Move the air. Continuous circulation reduces temperature differences and helps surfaces dry.

Vent moisture in cool weather. Short exchanges of humid greenhouse air for cooler outside air can reduce total moisture once the incoming air warms.

Keep surfaces above dew point where practical. Insulation, controlled heat and air mixing reduce cold surfaces.

Reduce late-day watering. Do not send crops into the coolest part of the night with wet foliage and saturated air.

Remove unnecessary plant mass. Dead leaves, weeds and overgrown canopies add moisture and obstruct airflow.

11 • Measurement

Where should you place temperature & humidity sensors?

Place the primary sensor near crop-canopy height, where leaves actually experience the climate. Keep it out of direct sun unless it has a radiation shield. Avoid positioning it immediately next to a heater, door, roof vent or fan outlet unless you specifically want to measure that local zone.

One sensor is adequate for a very small greenhouse, but larger houses often contain distinct climates. The sunny end, shaded end, centre aisle and dense canopy may all differ. If one crop repeatedly struggles in a particular corner, put a second logger there for a week instead of guessing.

In a spacious structure such as the Bloomcabin Orangery, multiple measurement points can reveal differences between the sunny side, shaded side, central aisle and crop canopy.

A practical sensor set records current temperature and RH, daily minimum/maximum and ideally dew point or VPD. Continuous logging is even more valuable because it shows what happens after sunrise, watering and vent opening.

How hot is too hot for a greenhouse?

It depends on the crop, but sustained conditions around 30°C and above begin to stress many mixed greenhouse crops, with flowering often affected before mature leaves show dramatic symptoms.

Should I ventilate when it is raining?

Often yes, especially if moisture is trapped. Cool outside air can contain less total water than warmer greenhouse air. Once admitted and warmed, its RH falls and it can help carry moisture away.

What humidity is too high?

Do not judge by RH alone. High humidity becomes most risky when temperature falls towards the dew point and leaves remain wet. A brief daytime spike is different from a saturated night.

Do I need a fan?

Not every small greenhouse requires mechanical circulation, but it becomes increasingly useful as the canopy closes. Fans reduce stagnant pockets; they do not replace genuine ventilation.

Is a larger greenhouse easier to control?

Greater air volume generally changes temperature more slowly, but design, glazing, vent area, orientation, wind and crop density still matter. A large greenhouse can overheat if its ventilation is poor.

What greenhouse design should I consider for serious climate control?

Prioritise ventilation, working height, sufficient air volume and room for crop spacing. Compare the Classic, Prestige and larger Orangery formats according to your garden and crop plan.

A practical weekly routine

Five minutes of climate observation can prevent weeks of problems

1. Record last night’s minimum and yesterday’s maximum.

2. Compare aisle RH with canopy RH if you have two sensors.

3. Look for morning condensation on glazing and leaves.

4. Check whether automatic vents opened and closed at useful times.

5. Feel the root zone before changing irrigation.

6. Watch plant responses: flower drop, soft growth, leaf curl, wilting, disease spots or slow ripening.

Use the Bloomcabin UK greenhouse collection and the Classic Greenhouse page when planning ventilation and working volume.