Winter greenhouse climate control means coordinating heating, ventilation, air circulation and moisture management around the crop, rather than maintaining a thermostat reading alone. Interpret air and leaf temperature, relative humidity, vapour pressure deficit (VPD) and outdoor conditions together.
- Select temperature targets for the crop, growth stage and available light.
- Assess condensation using surface temperature and dew point.
- Coordinate heating and ventilation through shared control logic.
- Plan for sensor faults, power loss and loss of internet access.
This is the cold-season climate guide within our smart greenhouse and automation series. Final equipment capacities and operating setpoints require project-specific calculations and review by the crop specialist.
Which variables does greenhouse climate control connect?
The greenhouse climate results from energy and mass exchanges between the air, crop, covering and outdoors. Opening a vent affects moisture, CO₂ and heating demand as well as temperature. Independent equipment controls can therefore issue conflicting commands unless their objectives and priorities are coordinated.
| Variable | Operating question | Relevant measurement |
|---|---|---|
| Air and leaf temperature | Does the display represent the crop zone? | Canopy-level air temperature and surface measurements at vulnerable locations |
| Humidity, VPD and dew point | Is transpiration restricted or a surface approaching condensation? | Simultaneous air temperature and RH, plus leaf temperature where needed |
| Light | Are temperature and irrigation programmes aligned with radiation? | Radiation or PAR; DLI for accumulated photosynthetic light |
| CO₂ | Are supply, uptake and ventilation losses coordinated? | Appropriate calibrated sensor and ventilation status |
| Outdoor weather | What will ventilation do, and can vents safely open? | Temperature, humidity, wind and rainfall |
What is VPD, and why is RH alone insufficient?
VPD is the difference between saturation and actual water-vapour pressure, expressed in kilopascals. The same RH produces different air VPD values at different temperatures. Leaf temperature also matters for transpiration: air VPD and the leaf-to-air vapour pressure difference should not automatically be treated as identical.
A simple air-VPD calculation has three steps. At 20°C, saturation vapour pressure is approximately 2.34 kPa. At 70% RH, actual vapour pressure is 2.34 × 0.70 ≈ 1.64 kPa. Subtracting gives 2.34 − 1.64 ≈ 0.70 kPa. A controller or validated calculator can perform the calculation; temperature and RH inputs should describe the same place and time.
VPDair = eₛ(Tair) × (1 − RH / 100)
Enter RH as 70 for 70%, not as 0.70 in this formula.
Full equations and symbol definitionsTair and Tleaf are air and leaf temperature in degrees Celsius. eₛ is saturation vapour pressure, while eₐ is actual vapour pressure; both are in kPa. The exp function is the exponential function, available as EXP in spreadsheets.
eₛ(T) = 0.6108 × exp[17.27 × T / (T + 237.3)]
eₐ = eₛ(Tair) × RH / 100
VPDair = eₛ(Tair) − eₐ
VPDleaf-air = eₛ(Tleaf) − eₐ
The last equation uses leaf temperature for saturation pressure and the surrounding air's actual vapour pressure. A colder leaf can therefore have a different leaf-to-air vapour pressure difference from the air VPD displayed by the controller.
The saturation-pressure relation follows FAO's meteorological-data guidance. The Ohio State University VPD fact sheet also discusses using canopy temperature.
| Calculated example | Air temperature | RH | Air VPD |
|---|---|---|---|
| Case 1 | 20°C | 70% | About 0.70 kPa |
| Case 2 | 30°C | 70% | About 1.27 kPa |
These are calculated examples, not field observations or recommended setpoints. Select a VPD operating range for the crop, cultivar, growth stage and root conditions; increasing VPD is not itself an operating objective.
When can condensation form on leaves?
Condensation can occur when a leaf surface reaches or falls below the dew point of the surrounding air. A central RH reading therefore cannot establish the condition of every leaf. At night, leaves beside a sidewall or below cold glazing can be colder than the measured air.
Illustrative calculation: air at 20°C and 85% RH has a dew point of approximately 17.4°C. A leaf at 16°C is below that dew point even though the air-temperature display still reads 20°C. This demonstrates the value of surface measurements; 17.4°C is not a universal control threshold.
Heating can lower RH without removing water from the greenhouse. Moisture is removed only through a removal process, such as air exchange or dehumidification with condensate collection. Check surface temperature, leaf-wetness duration and the actual moisture-removal path together.
Request a review of humidity and cold spots
Where should sensors be installed, and what should be logged?
Sensors should represent the crop-zone air without direct influence from heaters, sunlight or foggers. A single point may not represent a long or multi-span greenhouse. Measure spatial differences and distinguish installation error from a genuine climate problem before buying additional control equipment.
- Review measurement height as the canopy grows.
- Select shielding and aspiration according to the sensor manufacturer's instructions.
- Check cold perimeter areas and poorly mixed zones separately.
- Compare readings with a valid reference and record calibration dates.
- Log heater, pump, fan, vent, screen and alarm states alongside temperature and RH.
- Detect missing data, stuck readings and implausible differences between sensors.
A useful log records time, sensor location, measured value, target, actuator state and reason for the command. Sampling intervals should match the process and control dynamics; a five-minute interval is not a universal requirement.
How should a cold-night control sequence work?
Respect crop and safety limits first, then coordinate heating with moisture removal. The sequence below is a review framework. Vent openings, operating times and setpoints must be selected and tested for the actual greenhouse.
- Before sunset: verify heaters, water circulation, fans, fuel and backup power.
- Heat distribution: record crop-zone temperatures at the beginning, middle and end of the house.
- Energy screen: coordinate closure with radiation and moisture management. Neither complete closure nor a permanent gap is a universal rule.
- Internal circulation: inspect the airflow path and stagnant zones. Circulation does not replace ventilation.
- Moisture removal: where required, use controlled ventilation with heat compensation or an appropriate dehumidification system. Assess outdoor drying potential using vapour pressure or humidity ratio, not outdoor RH alone.
- Interlocks and logging: limit conflicting commands. Simultaneous heating and ventilation can be intentional during dehumidification and is not always a fault.
- Alarms: provide local response arrangements and a named person for low temperature, sensor faults and power failure.
See greenhouse heating and ventilation systems for equipment implementation. Operating logic must match the installed capacities.
How should heating demand and heat loss be assessed?
Heating demand depends on envelope area, heat transfer, indoor-outdoor temperature difference and air exchange. Floor area alone cannot establish heater capacity. Review covering, leakage, distribution and design conditions before selecting equipment.
Qcover ≈ U × Acover × (Tin − Tout)
In plain language: heat loss through the covering is approximately the heat-transfer coefficient multiplied by covering area and indoor-outdoor temperature difference.
- U: the covering's heat-transfer coefficient in W/(m²·K), selected for the actual covering.
- Acover: roof and wall covering area in m², not greenhouse floor area.
- Tin − Tout: the design temperature difference. Inside 20°C and outside 0°C give a difference of 20 degrees; temperature differences have the same numerical value in Celsius and kelvin.
- Qcover: the resulting heat loss in watts. Divide by 1,000 for kilowatts.
Worked example: U = 5 and covering area = 1,500 m² give 5 × 1,500 = 7,500 W per degree. Multiplying by a 20-degree difference gives 150,000 W. Dividing by 1,000 gives 150 kW.
This is only the covering-loss component, not a heater or boiler specification. Assess leakage, ventilation, other heat exchanges and design conditions separately. Heat capacity, fuel consumption and equipment price are different quantities; the 150 kW example does not establish a purchase price or fuel bill.
UMass energy guidance cites potential savings of 20–50% for thermal blankets. This is not a guaranteed reduction in a particular greenhouse's bill. Interpret before-and-after records alongside outdoor weather, operating hours and crop conditions.
The greenhouse covering guide complements the discussion of heat loss and surface temperature.
How do ventilation, circulation and evaporative cooling differ?
Circulation moves internal air; ventilation exchanges it with outdoor air; evaporative cooling changes temperature and humidity through water evaporation. These processes interact but are not interchangeable.
| Method | Main function | Important limitation |
|---|---|---|
| Circulation fans | Reduce stagnant zones and improve uniformity | Do not directly exhaust water vapour |
| Vents or exhaust fans | Exchange air, heat and moisture with outdoors | Consider heating demand and wind conditions |
| Fan-and-pad or fogging | Evaporative cooling under suitable conditions | Depends on outdoor air, water quality and evaporation capacity |
| Dehumidifier | Remove moisture from air | Check capacity, energy, condensate drainage and thermal effects |
Warm-season fan-and-pad selection needs outdoor dry- and wet-bulb conditions and airflow resistance. On a cold night, moisture removal without an unacceptable crop-temperature drop may be the priority. See smart greenhouse irrigation for the link with water management.
What do greenhouse heating and cooling systems cost in Iran?
The following preliminary equipment allowances come from Sadra's Iranian greenhouse cost guide. They are in Iranian tomans, not rials, and are not international price benchmarks or a complete climate-control quotation.
| Equipment | Allowance per m² of greenhouse floor area | Main variables |
|---|---|---|
| Heating | 360,000–3,600,000 tomans/m² Price basis: 13 October 2026 | System type, capacity, furnace and burner specification, and heat distribution. This wide range covers different designs, not one comparable system. |
| Fan-and-pad cooling | Approximately 480,000 tomans/m² Price basis: 13 October 2026 | Fan capacity, brand, motor and shutter, pad height and quality, and water-system specification. |
For a hypothetical 3,000 m² house, simple multiplication gives 1.08–10.8 billion tomans for heating equipment and 1.44 billion tomans for fan-and-pad equipment. These are budget calculations, not capacity selections or project quotations.
Confirm whether installation, transport, tax, electrical work, piping, controls, commissioning and warranty are included or separate. Fuel, electricity, water, service and consumables are operating costs and need their own estimate. Obtain a current, project-specific quotation before purchasing.
Where do rule-based control, PID and AI fit?
Judge a system by coordinated actions, reliable data and its response to faults, rather than an app or an AI label. Advanced methods still require valid sensors, an appropriate model and clearly defined operating limits.
| Approach | Role | Assessment requirement |
|---|---|---|
| On/off and rules | Specified thresholds and sequences | Hysteresis, minimum run times and actuator coordination |
| PID and adaptive control | Continuous adjustment in a suitable control loop | Tuning, actuator limits and changing conditions |
| MPC | Model-based prediction with constraints | Model validity, forecasts and data quality |
| Machine learning and RL | Data-driven prediction or decisions | Validation, safety limits and comparison with baseline control |
The GreenLight research model examines greenhouse climate and supplemental lighting. Differences in heat transfer under LED and HPS lamps illustrate why changing lighting can affect heating demand. This source does not establish fixed savings or imply that Sadra has implemented that model.
Request a review of sensors and control logic
What should a climate-control quotation specify?
A controller price is not the cost of a complete system. Specify zones, sensors, panels, wiring, actuators, communications, commissioning and support separately. A connection capability does not necessarily mean the relevant sensor or equipment is supplied.
- Input/output schedule, zones and supplied equipment
- Sensor model, manufacturer-stated accuracy, calibration and placement
- Heating, ventilation, screen and CO₂ logic and alarm priorities
- Local operation without internet and recovery after power failure
- Data logging, export access, training and maintenance responsibilities
- Installation, any software licence or subscription, warranty and service
For the broader budget, use the greenhouse construction cost guide, checking each price's date and scope. See greenhouse automation and control for integration.
Troubleshooting and commissioning checklist
Commissioning should demonstrate the relationship between sensor data, control commands and actual equipment response. A dashboard screenshot or a manually running fan alone does not verify performance in operating conditions.
| Symptom | Check | Record |
|---|---|---|
| Wet leaves despite acceptable air temperature | Leaf temperature, dew point and local circulation | Simultaneous surface temperature, air temperature and RH |
| Long burner operation and cold zones | Envelope losses and heat distribution | Multi-point temperature trends and heater states |
| Rapid switching | Hysteresis, minimum timings and sensor noise | Commands and actual state changes |
| Poor control after communication loss | Local logic and fault mode | Controlled test supervised by technical personnel |
Electrical, burner, flue, motor-protection and equipment-failure tests require appropriately qualified personnel. Coordinate climate reviews with crop and disease management; controller settings do not replace plant-disease diagnosis or nutrition planning.
Frequently asked questions
What is the best greenhouse temperature and humidity?
There is no universal pair of settings. Assess crop, cultivar, growth stage, light, roots and disease risk alongside leaf temperature, VPD and outdoor conditions.
Does falling RH after heating mean moisture has been removed?
No. Heating can reduce RH without exhausting water vapour. Removal requires an air-exchange or dehumidification process.
Can circulation fans replace ventilation?
No. Circulation mixes internal air; ventilation exchanges air with outdoors. Better mixing does not by itself guarantee net moisture removal.
Is air-temperature VPD sufficient to assess condensation?
Not always. If leaves are colder than the air, surface temperature and dew point matter. Air VPD does not capture every local canopy condition.
Does AI reduce energy use by a fixed percentage?
No. Results depend on the baseline, equipment, climate, data and control quality. Savings claims need a defined comparison period and measurement conditions.
What happens to remote control without internet access?
It depends on the architecture. Specify and test local control, alarm routing and communication recovery at commissioning.
From a display reading to a traceable decision
Check measurement quality and heat losses first, then coordinate heating, circulation and moisture removal. Reliable data, explicit logic and commissioning tests are the foundation for advanced control. Prepare the location, crop, dimensions, covering, equipment and climate logs before discussing a project.
Discuss your greenhouse requirements with Sadra
Sources and further reading
- Prenger & Ling (2001), Greenhouse Condensation Control, AEX-804, Ohio State University Extension; copy hosted by the University of São Paulo.
- FAO, Irrigation and Drainage Paper 56, Chapter 3.
- Pawlowski et al. (2009), Sensors 9, 232–252, DOI: 10.3390/s90100232. Simulation study.
- Chen et al. (2025), Sensors 25, 1388, DOI: 10.3390/s25051388. Review.
- Katzin et al. (2020), GreenLight, Wageningen University & Research record, DOI: 10.1016/j.biosystemseng.2020.03.010.
- UMass Extension, Greenhouse energy guidance. Interpret savings ranges in their installation context.
Smart greenhouse and automation
Specialist guide: cold-season climate management
This guide connects heating, humidity, and ventilation management in cold weather to monitoring and automation.
Cold-weather operating check
This is a decision checklist, not a substitute for crop-specific setpoints or project calculations.
- 1Check the cover, openings, and sealing before the cold period.
- 2Confirm the measurement point represents the crop zone, not the heater or an exterior opening.
- 3Define who receives and responds to an alert.
- 4Review heating, air movement, ventilation, and irrigation together after each meaningful setting change.
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