Why Is Your Multi Span Greenhouse Too Hot? Causes and Solutions

High temperatures can limit yield, increase water needs, and make climate control harder. If a multi span greenhouse is too hot, ventilation, shading, airflow, equipment setup, or even structural design may be the culprit.

The first step is quantitative. Measure the temperature rise above ambient and compare it with what the installed systems are rated to deliver. In a well-designed multi span house, ventilation alone at 40–60 air changes per hour should hold the internal temperature within a few degrees of outside air; only what remains after that is the job of shading and evaporative cooling. If the observed rise is far larger, the problem is a capacity or distribution fault, not the weather.

Identify The Main Causes Of Excessive Greenhouse Heat

If additional cooling is needed, determine the source of unwanted heat and the reason why it is not moving outside. The right diagnosis will solve the real issue, not just add to energy and operating expenses without making things better for the plants.

Insufficient Roof Ventilation

There is natural rising air, and it could gather under the roof of the multi span greenhouse. This heat can build up in the roof when roof vents are too small, in poor location, blocked, or not open adequately. The situation is worsened in times of higher sunshine. Hot air removal may not be sufficient in large areas that are side-ventilated.

The governing ratio is vent area against floor area. For natural ventilation to carry the peak heat load, combined roof and side opening area should be at least 15%–20% of the floor area, measured as effective free opening. Scaling that ratio to a 5,000 m² multi span house gives roughly 750–1,000 m² of genuinely open vent area, and the ridge outlets have to be paired with low-level inlets. Testing of opening combinations shows that a side inlet combined with windward roof vents achieves the highest natural flow — around 12.3 air changes per hour at a wind speed of just 0.5 m/s — while roof-only or leeward-only configurations fall well short.

Inspect vent openings for adequate opening without obstructions and surrounding buildings, screens, and equipment blocking adequate ventilation. Automated vent controls should also be checked for incorrect temperature settings or mechanical problems.

Limited Air Movement

Ventilation openings are openings that allow ventilation, but they do not necessarily provide uniform ventilation. If the airflow is not distributed to the center or to the lower part of the crop area, there are hot zones in the growing area. Fans can help circulate air inside and minimize still areas.

Internal circulation should be sized against the floor area, not guessed. The established horizontal air flow rule of thumb is about 2 CFM of fan capacity per square foot of floor area, roughly 0.6 m³/min per square metre, arranged so air travels as a continuous loop through the canopy. This is a separate calculation from the wall-mounted exhaust fans that create the exchange rate, and under-sizing it is the most common reason a house with visibly working ventilation still has hot zones in the middle spans.

Their location should be part of a continuous air pathway; do not just push air around to one part. Insufficient air exchange can also raise humidity levels in close cropping. This is important for air circulation not only as a temperature control measure, but also for maintaining overall environmental stability.

Excessive Solar Heat Gain

Strong solar radiation can raise greenhouse temperature quickly, especially during peak sunlight hours. The amount of heat entering the greenhouse depends on the covering, greenhouse orientation, shading, and local solar conditions.

Shading is a load-reduction measure, so the screen specification should be chosen against measured radiation. Screen selection is normally expressed as a shading percentage and, for internal screens, also as a reflection characteristic: reflective aluminium screens hold the greenhouse 4–8 °C cooler than a black screen of the same nominal shading percentage, because they reflect radiation back out instead of absorbing it and re-radiating it into the crop. The same screen family, installed as an energy screen, also saves 30%–50% of heating energy in cold periods, which is why a movable reflective screen often earns its cost twice.

Movable shading can reduce solar heat gain when needed, but excessive shading can also reduce useful light. Shading should therefore be used together with ventilation and other cooling methods. Note that a glass Venlo roof already transmits only 75%–85% of incident light as a complete assembly, before any screen is closed — so the screen strategy must be built around the structure's baseline transmission rather than around raw solar radiation.

Cooling Capacity Does Not Match The Load

The cooling equipment should be in line with the size of the multi span greenhouse and the local environment. An undersized system might be operating around the clock and yet not be able to maintain a desired temperature. A professional multi span greenhouse manufacturer can estimate the heat load before determining the cooling equipment to be used.

Two common errors appear here. The first is sizing to average rather than peak heat load, which guarantees shortfall on the design day. The second is ignoring that evaporative efficiency is finite: fan-and-pad systems are designed for 70%–80% wetted efficiency of the wet-bulb depression, so at roughly 80% overall system efficiency the achievable temperature drop is about 9 °F (5 °C), and supplied air typically arrives 10–20 °F below ambient. On a humid day, the wet-bulb depression shrinks, so the same system delivers less — the design must be checked at the local design wet-bulb, not at the dry-bulb peak.

This is particularly important for large structures, when multiple growing areas might be subject to varying conditions. Cooling performance should also be assessed in the hottest operating period. This equipment will run well in a cooler climate but not during peak heat.

Check Where Heat Is Entering And Building Up

Before adding cooling equipment, check where the heat is coming from and where it is collecting. In a large multi-span greenhouse, different spans may not have the same temperature because of roof orientation, crop density, equipment, and airflow.

Check Vent Open Area And Screens

The actual open area of a vent matters more than the vent size on a drawing. Insect screens can also restrict airflow, especially when they become dirty or clogged.

Quantify the screen penalty rather than assuming it away. Wind-tunnel testing of insect-proof screens covering 25%–53% porosity shows ventilation rate falling as porosity falls, because the screen lowers the discharge coefficient of the opening. In practice, a standard screen reduces airflow through an opening by 10%–20%; a 75-mesh screen reduces natural ventilation by roughly 25% compared with a 40-mesh screen; and a 50-mesh screen with a 55% open-area ratio on a 100 m² opening passes air equivalent to only about 55–60 m² of free opening.

Check that roof and side vents open fully and that screens are clean. If screens are part of the ventilation openings, their airflow resistance should be considered when sizing the ventilation system.

Check Internal Heat Sources

Some equipment can add heat to the greenhouse. Pumps, motors, lighting, heating pipes, and electrical equipment may increase the heat load, particularly in enclosed areas.

Equipment locations should be reviewed when investigating a hot zone. Moving heat-producing equipment outside the growing area, where practical, can reduce the internal heat load.

Compare Temperatures Between Spans

A multi-span greenhouse should not always be treated as one temperature zone. Compare readings from different spans and crop areas to identify where heat is highest.

If one area is consistently hotter, check its vent openings, shading, crop density, equipment, and airflow before increasing cooling capacity for the entire facility. A useful diagnostic is to log simultaneous readings at gutter height, at canopy height and near the floor in three or more spans; a warm pocket that persists at canopy level while the gutter reads normal points to a circulation problem, whereas a whole-span elevation points to a venting or load problem.

Incorrect Climate Control Settings

However, if the controls are not set properly, a good system can never achieve poor performance. Sensors can be placed in the wrong place, thresholds set too high, or equipment may turn on too late. Examine how sensors have been placed and compare sensor readings for different parts of the growing area. Conditions in a large multi span greenhouse may not be well represented by a single sensor.

Also take into account crop requirements for control settings. Setting temperatures similar to those used in spring and summer during the fall and winter can cause unnecessary stress and waste energy. Ventilation and screen stages should be staged rather than switched: a stepped programme that opens vents progressively, then closes the screen, then starts the pad system, avoids the overshoot that a single setpoint inevitably produces in a large air volume.

Apply Practical Solutions To Reduce Internal Heat

Cooling should be planned as a system once the cause is determined and not as a single equipment replacement. The optimal choice is to use a combination of ventilation, shading, airflow, greenhouse equipment size, and control settings based on the operating conditions of the greenhouse.

Improve Natural Ventilation

If the heat is caused by air that has become trapped, the first action should be to boost effective ventilation. Roof and side openings must allow hot air to be vented and cool outside air in. Ensure good ventilation openings are clear, and mechanical components are checked on a regular basis.

If vents are automatic, test to see if temperature sensors are operating at the right times. The structure is also an important factor. Ventilation and air quality should be considered at the design phase and not as an afterthought by a multi span greenhouse manufacturer.

Add Controlled Shading

During periods of extreme heat, shading systems can restrict direct sun during the day. They are very handy and can be used during square sun when it is especially hot outside. The system should be movable and not permanently obstruct light. Growers can provide more shade during hot periods of the day and less shade during periods of better light availability.

Use adjustable shading: Change coverage according to sunlight and crop requirements.

Avoid permanent heavy shading: Excessive coverage can reduce useful light.

Coordinate with ventilation: Shading works better when hot air can also escape.

Monitor crop response: Adjust settings when plants show signs of heat or light stress.

Increase Internal Air Circulation

Circulation fans can be used to move air around an area that is warmer than the rest of the greenhouse. They may also even lessen the area between the upper and lower growing areas. The positioning of the fans should be dependent on the layout. Don't establish isolated air circulation patterns, which allow stagnant air to build up in the center. Size the loop at approximately 2 CFM per square foot of floor area, equivalent to about 0.6 m³/min per square metre, and check that the air actually arrives at the far side of each span rather than dissipating within a few metres of the fan.

Regular checks are also required. For instance, dirty parts or components with damaged fan blades or clogged airflow may lead to decreased performance that won't immediately be apparent.

Review Cooling Equipment

If ventilation and shading are ineffective, check cooling system capacity and condition. Cooling equipment should be designed to cool the peak heat load of the greenhouse, not the average heat load. The number of growing zones, the density of the crop, covering, outside temperature, and anticipated operating time should be taken into account in a commercial project.

When reviewing an existing pad system, verify four things in order: is the pad fully wetted across its whole face, is the approach velocity within the pad manufacturer's limit, is the pad achieving 70%–80% of the available wet-bulb depression, and is the design wet-bulb figure realistic for the site. A system that passes all four and still cannot hold setpoint genuinely needs more capacity.

Businesses should also look for equipment that will be easy to maintain and will have room to expand upon as a multi span greenhouse manufacturer. Difficult-to-service systems can result in extended downtime during important growing time.

Upgrade Monitoring And Automation

Early detection of temperature issues can help to manage them. Growing zones can be discerned, and the time at which temperatures start to rise can be indicated by sensors positioned at appropriate sites.

The automation system can then be programmed to open or shut the ventilation, shade, and fan, as well as activate the cooling system, under specific conditions. This would minimize the need for manual adjustments in changing weather conditions.

Monitoring should not be used as a substitute for physical inspections. The condition and use of the equipment and environment should also be regularly checked because sensors can give false readings, drift, or fail.

Frequently Asked Questions

Open vents may not provide enough airflow for the greenhouse size. Poor vent placement, weak air movement, strong sunlight, or insufficient cooling can still cause excessive temperatures. Check the effective vent-to-floor area ratio against the 15%–20% design baseline, confirm that windward inlets exist to pair with the ridge outlets, and deduct 10%–20% (or more for fine mesh) for any insect screen before concluding that the vents are adequate.

Peak summer cooling is normally designed around 40–60 air changes per hour, with a controllable minimum of about 1 air change per hour for winter humidity control. If a measured house is running far below the lower figure on a hot day, ventilation capacity or distribution is the limiting factor, not the cooling equipment.

Shading can reduce solar heat gain, but it should usually work alongside ventilation and airflow management. Excessive shading can reduce the light available to crops. A movable reflective screen is the better option where both cooling and winter heat retention are needed, because the same screen that holds the greenhouse 4–8 °C cooler than an equivalent black screen also returns 30%–50% of heating energy when closed at night.

Wet pad systems are designed for 70%–80% of the wet-bulb depression. At roughly 80% overall system efficiency, the practical temperature drop is about 9 °F (5 °C), and supply air typically reaches the crop 10–20 °F below ambient. A pad that produces materially less is usually under-wetted, running above its rated face velocity, or allowing air to bypass the pad face.

Use about 2 CFM of fan capacity per square foot of floor area, which is roughly 0.6 m³/min per square metre, arranged so that air moves in a continuous loop through the canopy rather than in isolated pockets. This is separate from, and additional to, the exhaust fans that generate the air exchange rate.

Log temperature and humidity simultaneously at gutter height, canopy height and floor level in several spans, with all vents open. A whole-span elevation that tracks outdoor conditions indicates a ventilation or load problem. A persistent warm pocket at canopy level while the gutter reads normal indicates a circulation and distribution problem.

An overheating greenhouse is usually the result of several factors working together rather than one isolated fault. Businesses should evaluate ventilation, solar heat gain, airflow, cooling capacity, and control settings before investing in major changes.

Checking against published design values turns that evaluation into something measurable: 15%–20% effective vent-to-floor area, 40–60 air changes per hour at peak, a 10%–20% screen penalty, about 2 CFM per square foot of circulation capacity, and 70%–80% pad efficiency delivering roughly a 9 °F drop. Whichever of those numbers the greenhouse currently fails is the one worth fixing first.

Choosing the right design with a reliable multi span greenhouse manufacturer can prevent many heat-management problems from the beginning. For existing structures, systematic inspection and targeted upgrades can create a more stable environment while avoiding unnecessary equipment and operating costs.