The environment beneath glass

Glazing does more than warm a garden space. It creates a small, fast-changing environment with its own daily pattern.

Sunshine can quickly push indoor air above outdoor temperature as crops release vapour through transpiration. By evening, foliage, aluminium and glazing cool, RH rises, and cold surfaces may reach dew point. Ireland’s Atlantic climate varies by location: coasts are often windier and more maritime, while inland air frost can occur even in May. Guard against cold nights in spring and vent stored solar heat promptly in summer.

Good management therefore follows relationships, not one thermostat reading. Temperature affects photosynthesis, respiration, flowering and fruit set; humidity shapes transpiration, nutrient transport, pollination, disease pressure and condensation. Air movement alters the boundary layer at leaves, irrigation adds moisture, and shading changes leaf and air temperature.

For home growers, the practical aim is a stable but responsive microclimate suited to the crop. Lettuce and seedlings need different treatment from tomatoes, cucumbers and peppers. Warm sheltered plots, urban gardens, inland sites and exposed coastal locations also call for different routines.

Choose with climate control in mind from the outset. Use the Irish Bloomcabin greenhouse range to compare vent area, height, air volume and glazing, as well as appearance.

There is no ideal setting for every crop. In a mixed domestic house, distinguish plants that prefer cool conditions from warmth-loving ones. Lettuce, spinach and many seedlings thrive at lower readings than tomatoes, cucumbers, aubergines, basil, peppers and chillies. The best range also moves with light levels and growth stage.

Many warmth-loving vegetables are comfortable at roughly 20–27 °C by day, with somewhat cooler nights. This is a flexible starting point, not a fixed setpoint. Long-lasting heat can reduce fruit set in tomatoes; peppers may drop flowers after unsuitable days or nights; lettuce bolts more readily when it is too warm.

Base the operating routine on what is planted. Leafy crops can use a cooler area, whereas tomatoes, peppers and basil favour greater warmth. With mixed planting, divide the space sensibly, leave suitable gaps and ensure that ventilation reaches every zone.

Daylight powers photosynthesis, and direct sun may warm leaves beyond the surrounding air. Photosynthesis pauses at night, but respiration continues. A moderate difference between day and night is natural. Problems arise if cold nights slow metabolism too far, or if warm nights deny plants a cooler interval in which to recover.

On a clear night, radiative heat loss may cool glazing and foliage below the measured air temperature. Condensation depends on those surfaces, so a leaf can reach dew point while the air sensor still reads several degrees higher.

Greenhouses buffer falling night temperatures in cooler districts, but during warm spells they must also shed heat after dusk. A larger body of air changes more slowly, so designs such as the Bloomcabin Classic therefore develop climate differently from a very small sealed unit.

A min–max thermometer or data logger is especially useful. One midday observation can miss the two readings most likely to explain a plant’s response: yesterday’s maximum and the minimum recorded during the night.

03 • Humidity

How humid should the growing space be?

Simple humidity targets can mislead because RH depends on temperature. Warm air accommodates more water vapour than cool air. When the actual amount of moisture stays constant but temperature falls, RH increases. A house that seemed unremarkable in late afternoon may consequently be almost saturated a few hours later.

The most humid air is often among the plants, not near the doorway. Leaves continually release vapour, and dense foliage restricts movement. A hygrometer by the entrance can therefore report acceptable conditions while the air enclosed by tomato leaves is substantially damper.

Mixed crops are better served by a moderate operating band than by a supposedly perfect percentage. Overly dry air increases evaporative pressure and may promote wilting. Excess humidity slows drying, can hinder pollination in some crops, and raises the likelihood of condensation and fungal disease.

The main rule is to prevent hours near saturation, especially overnight. Dry leaf surfaces and softly moving air usually do more to discourage disease than forcing humidity to match a precise figure throughout the day.

Vapour pressure deficit, or VPD, measures how far actual atmospheric moisture lies below the maximum possible at a given temperature. Because the calculation combines temperature and humidity, it describes the drying demand experienced by foliage more usefully than relative humidity considered alone.

A very low VPD means the atmosphere is moist for its temperature. Plants transpire more slowly and their leaves take longer to become dry. A very high VPD creates powerful drying demand, so water leaves the plant rapidly and roots must replace it. The useful range varies between crops and development stages.

Home growers need no commercial control system to understand the pattern. A combined temperature and RH sensor that also works out VPD or dew point is enough to show useful relationships. Crucially, 70 % RH at 12 °C produces different plant conditions from 70 % RH at 30 °C.

In plain language

RH tells you how moist the air is relative to capacity; VPD reflects its drying pull on foliage.

Effective control therefore cannot rest on one reading alone.

Dew point is the temperature at which the air can no longer keep all its moisture as vapour. If glass, metal, pipework or leaves cool below it, water condenses. The coolest material becomes wet first; further cooling may put droplets onto the crop itself.

That water causes more than visual inconvenience. Drops from the glazing may splash compost and plant debris, while sustained leaf wetness creates favourable conditions for Botrytis and other pathogens. Sealing the house each evening in an effort to retain every bit of warmth can therefore intensify dampness.

Leaving the greenhouse wide open all night is not the answer. Combine suitable heating with short, controlled ventilation, circulating air, morning irrigation, clean ground and generous plant spacing.

One myth worth reconsidering

“Rain outdoors means ventilation cannot make the greenhouse any drier.”

That conclusion is not always correct. Cold saturated outside air often contains less total vapour than warm indoor air. Once admitted and warmed, its relative humidity falls and it gains room to absorb moisture from the greenhouse.

Ventilation takes out surplus heat and vapour, renews CO₂ and reduces uneven temperatures. With natural flow, warm air rises towards roof vents and cooler outside air enters at doors or lower apertures. Fans can either help replace indoor air or keep it circulating around the crop.

In a domestic greenhouse, roof vents, doors or low-level inlets work best when paired with circulation among the plants. This is generally more effective than one small window. Automatic vent openers are valuable when sunshine arrives and nobody is present to respond.

For easy daily adjustment, compare designs offering practical vents and enough internal air volume, such as the Prestige range or a lean-to greenhouse from Bloomcabin.

How each opening contributes

Roof vents: release hot air rising overhead.

Doors and lower openings: admit replacement air and promote cross-ventilation.

Circulation fans: move indoor air but do not replace ventilation to outdoors.

High tunnels: rely heavily on passive airflow, making generous openings especially important. Another protected option is the Bloomcabin high tunnel greenhouse.

Ventilate first. Shading reduces the solar load still arriving, whereas it cannot remove heat already stored, so the two measures work best together. Through an extended hot spell, shade netting or solar-control glazing may benefit seedlings, lettuce and fruit prone to scorching.

Remember the roots. Black pots standing on dark slabs may heat far beyond the surrounding air. Larger containers, mulch, lighter surfaces and even watering help moderate this stress. Do not strip away substantial foliage immediately ahead of very hot weather.

Conditions swing quickly in a compact structure, so air volume matters alongside the particular model. For greater thermal stability and more room to work, compare the Bloomcabin Orangery range.

A practical cooling sequence

1. Vent early through roof openings and doors.

2. Circulate air within the crop.

3. Add shading while solar gain remains excessive.

4. Stabilise moisture and warmth around roots.

5. Move crops away from repeatedly overheated spots.

Irrigate earlier. Morning watering allows foliage, paths and surfaces time to dry.

Eliminate pooled water. Every puddle continues adding moisture to the air.

Give foliage breathing room. Crowded leaves create humid pockets where disease can start.

Maintain gentle circulation. Steady circulation evens temperature and speeds drying.

Exchange damp air during chilly weather as well. Short ventilation can exchange humid warmth for drier outside air.

Limit surfaces that fall below dew point. Root warmth, insulation and circulation limit cold surfaces.

Avoid unnecessary late irrigation. Keep leaves dry as the night approaches its coolest period.

Remove needless plant growth. Old leaves, weeds and crowding release moisture and impede airflow.

11 • Measurement

Choosing positions that represent the crop’s conditions

Place the main sensor near canopy height, where leaves experience the climate. Keep direct sunlight off it unless suitable radiation shielding is fitted. Readings beside a heater, door, fan outlet or roof vent are unlikely to represent the whole house, unless that exact area is under investigation.

A single sensor often serves a tiny greenhouse, whereas a larger structure can contain distinct zones at its sunny side, shaded side, centre, edges and inside dense foliage. If trouble appears in just one corner, put a second logger there and compare it with the main reading.

For a roomy structure such as the Bloomcabin Orangery several sensors can reveal differences among the sunny end, shade, middle aisle and canopy.

A useful kit reports current temperature and RH, records each day’s minimum and maximum, and ideally supplies dew point or VPD. A logger also shows how rapidly conditions respond to sunrise, watering and vent operation.

How hot is too hot under glass?

It varies by crop. In a greenhouse containing several vegetables, sustained readings approaching 30–35 °C or higher bring a marked increase in heat stress. Flowers, pollination and developing fruit often suffer before established leaves reveal obvious damage.

Is ventilation worthwhile during rain?

Frequently, especially when moisture is trapped in a heated house. Cool saturated air outside may contain less vapour in total than warm air indoors. Bringing it inside and heating it lowers its RH, enabling it to take up additional water.

When is greenhouse humidity too high?

The percentage alone is not decisive. Concern increases when cooling brings leaves near dew point and keeps them wet for hours. A brief humidity peak generally presents less danger than maintaining almost saturated conditions throughout the night.

Do I need a fan?

Not necessarily in every small greenhouse. It becomes more useful as the canopy thickens, because it disperses still pockets. However, circulating indoor air cannot substitute for roof vents, doors or another route for exchanging air with outdoors.

Is a larger greenhouse easier to regulate?

More enclosed air usually warms and cools more slowly than the air in a very small structure. Nevertheless, vent capacity, glazing, orientation, exposure to wind and crop density still matter. A large greenhouse can also overheat if its openings are inadequate.

Which design details assist closer climate management?

Look for usable vents, sufficient working height, useful air volume and sensible planting space. Compare the Classic, Prestige and larger Orangeryoptions against your location and crop plan.

A weekly climate routine

Five minutes of observation can prevent weeks of crop trouble

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

2. With two sensors, compare aisle humidity with the canopy.

3. Check morning leaves and glazing for condensation.

4. Confirm that automatic vents operated when conditions required it.

5. Test root-zone moisture before changing the watering routine.

6. Watch for flower drop, weak growth, curled leaves, wilting, marks or slow ripening.

For structural planning, see Bloomcabin’s local greenhouse category and Classic greenhouse.