Greenhouse Dehumidification: Where It Fits, What It Costs, and How to Size It

Almost every litre of irrigation water that does not leave as drain leaves as water vapour. In a greenhouse, that vapour is the humidity problem — and it means the moisture load is not a weather condition to be endured but a quantity that can be measured from records the grower already keeps.

That reframing changes the discussion from "our humidity is too high" to "we generate this many litres per day and currently remove them this way, at this cost." Once the load is a number, equipment selection becomes an engineering comparison rather than a preference.

Greenhouse Humidity Is a Load Problem

Building HVAC treats humidity as something that arrives with outdoor air and with occupants. A greenhouse is different: the dominant moisture source is inside, it is continuous during daylight, and it is proportional to the crop's productivity.

A healthy crop transpires. Water moves from the root zone through the plant and evaporates from leaf surfaces. That evaporation is the plant's cooling mechanism and is coupled to photosynthesis through the stomata, which is why a grower cannot simply reduce transpiration to solve a humidity problem without also reducing growth. The relationship between stomatal behaviour, CO₂ uptake and moisture release is developed in CO₂ uptake in controlled environments.

The practical consequence: the better the crop is doing, the larger the moisture load. Humidity control in a greenhouse is a consequence of successful production, not a sign that something is wrong.

Estimating the Load from Records You Already Have

The most reliable estimate of a greenhouse's moisture load does not come from a psychrometric calculation. It comes from the irrigation system.

Over a period long enough to average out storage in the substrate — typically a week or more — water is conserved:

Water applied − water drained ≈ water transpired

Growers who log irrigation volume and drain volume, as most fertigation systems do automatically, therefore already have their latent load in litres per day. Divided by growing area, it gives litres per square metre per day, which is the number that sizes equipment.

For orientation, published measurements give a sense of the range. A greenhouse rose crop has been reported at a maximum transpiration rate of about 1.5 L/m² per day in summer. In a study of Cannabis sativa under supplemental lighting, cumulative crop evapotranspiration increased from 39.8 to 70.6 L/m² over the production cycle when supplemental light was added — a reminder that lighting intensity is one of the strongest drivers of moisture load in modern facilities.

These figures are illustrative, not design values. Transpiration varies with crop, cultivar, growth stage, planting density, light integral, temperature and irrigation strategy. The site's own irrigation records are always the better source.

Once the daily volume is known, converting it to an energy quantity is direct. The latent heat of vaporisation of water near ambient temperature is approximately 2,450 kJ/kg, which is about 0.68 kWh per litre. That single number governs the cost comparison in the next section.

The Night Is the Difficult Period

Most greenhouse humidity problems are night problems, and the mechanism is not simply "cooler air holds less water."

After sunset, the crop canopy exchanges long-wave radiation with the glazing, and through it with the sky. On a clear night the effective sky temperature is well below air temperature, so leaf surfaces lose heat radiatively and settle below the temperature of the surrounding air. A leaf that is colder than the air will reach the air's dew point before the air itself does.

The result is a sequence that catches many growers out:

1. Air temperature and relative humidity look acceptable on the climate computer.
2. Leaf surfaces, being colder than air, are already at or below the dew point.
3. Free water forms on leaf surfaces — the precondition for Botrytis and other foliar pathogens.
4. The relative humidity alarm, which is watching air, never triggers.

The condensation condition is a surface condition, not an air condition. The physics and the calculation method are set out in condensation risk and dew point; they are identical whether the surface is a leaf or a busbar.

The same mechanism applies to the glazing itself, which is usually the coldest surface in the house. Condensation there produces drip onto the crop, which is a direct disease pathway independent of leaf wetness.

Transitions are the second difficulty. The change from lit to dark period, or from day to night ventilation strategy, causes rapid changes in both temperature and moisture release. Equipment sized only for steady-state conditions may be unable to hold the target through the transition, which is when the risk is concentrated.

What Ventilation Actually Costs

Ventilation is the default humidity control method in greenhouses because the infrastructure is already there. Its cost is usually underestimated because it is paid in three currencies at once.

Latent. Removing moisture by ventilation means exhausting moist inside air and admitting drier outside air. It works only when outside air has a lower absolute moisture content than inside air. On a humid night, or in a maritime climate, that condition may not hold for long periods — and when it does not hold, ventilation cannot dehumidify at all, no matter how much it is increased.

Sensible. The air being exhausted has been heated. In a heated greenhouse in winter, every cubic metre vented carries heating energy out with it, and the incoming cold air must be heated in turn. The heating cost of ventilation-based dehumidification in cold weather routinely exceeds the notional 0.68 kWh per litre of the latent load itself.

CO₂. In a facility that enriches CO₂, ventilation exhausts enriched air and admits ambient air. The enrichment has to be replaced, and during the ventilation period the crop is growing at a lower CO₂ concentration than intended. This cost is invisible on the energy bill and shows up as reduced yield.

A mechanical dehumidifier changes this accounting in a way that is not obvious. When a refrigerant dehumidifier operates on recirculated air, the latent heat released as vapour condenses does not leave the greenhouse — it is returned to the space as sensible heat, along with the electrical input to the machine. In a heated greenhouse during the heating season, that heat offsets the heating system rather than being wasted, and the CO₂ and the heated air both stay inside.

In summer the same behaviour is a liability, because the added sensible heat has to be removed by the cooling strategy.

This is why the correct answer is seasonal rather than absolute, and why the decision deserves its own analysis. That comparison — including the outdoor conditions under which ventilation stops working — is set out in ventilation vs. dehumidification in controlled agriculture.

Equipment Routes

Four routes are commonly considered, and they suit different conditions.

Ventilation with heating. Lowest capital cost, uses existing infrastructure, and is genuinely appropriate where outdoor air is reliably drier than inside air and CO₂ enrichment is not in use. Fails when outdoor absolute humidity is high.

Refrigerant-cycle dehumidification. Removes moisture independently of outdoor conditions by cooling air below its dew point. Returns latent and electrical heat to the space. Capacity falls as air temperature falls, so performance at greenhouse night conditions is materially lower than a catalogue figure rated at warmer conditions — a point examined in what a daily water removal rating means. Suited to the temperature range most greenhouses operate in.

Desiccant dehumidification. Removes moisture by adsorption rather than condensation, so it remains effective at low temperatures and can reach low dew points that refrigerant equipment cannot. Requires a regeneration heat source and produces a warm, dry supply air stream. Usually specified where the target dew point is low or the operating temperature is below the refrigerant range — drying rooms and post-harvest applications more often than the growing environment itself.

Air-to-air heat recovery. Recovers sensible heat from exhausted air, reducing the heating penalty of ventilation-based dehumidification. It reduces the sensible cost but does not change the fundamental limitation: it still cannot dehumidify when outdoor air is not drier than inside air.

Size by Load, Not by Floor Area

Sizing rules expressed in litres per square metre of floor are unreliable in greenhouses because the moisture load is set by the crop and the light, not by the building. Two houses of identical area with different crops, densities and lighting can differ by a factor of several in daily transpiration.

A defensible sizing sequence:

1. Establish the daily moisture load from irrigation minus drain, in litres per day, at the most demanding period of the production cycle — not the annual average.
2. Establish the design operating condition: the air temperature and humidity at which the equipment must actually deliver that capacity. For a greenhouse this is usually a night or early-morning condition, which is cooler than most catalogue rating conditions.
3. Read capacity at that condition, not at the rating condition. Manufacturers rate at standard conditions that are typically warmer and therefore more favourable; the derated figure is what the project gets.
4. Add the ventilation and infiltration load if the house will ventilate while dehumidifying.
5. Check the transition periods, not only steady state.
6. Decide the number of units from air distribution, not only from total capacity — see below.

Steps 2 and 3 are where most oversizing complaints and undersizing failures originate.

Air Distribution Decides Whether Capacity Is Usable

A dehumidifier removes moisture from the air that passes through it. Air that does not reach the machine is not dehumidified, and a canopy with stagnant zones will have wet microclimates regardless of installed capacity.

Three consequences for layout:

Reach the canopy, not the ridge. Warm moist air rises. Equipment that draws from and discharges into the upper volume can report excellent conditions at the sensor while the canopy — where transpiration occurs and where disease starts — stays humid.

Distribute rather than concentrate. Several smaller units usually produce more uniform conditions than one large unit of the same total capacity, particularly in long or compartmented houses.

Coordinate with existing air movement. Horizontal airflow fans, and polyethylene distribution tubing where fitted, already establish an air pattern. Dehumidification equipment introduced without regard to that pattern can reinforce dead zones instead of eliminating them. Tube and fan design, and how it interacts with humidity control, is covered in greenhouse air-distribution tubes.

Sensor placement follows the same logic: the measurement that matters is at canopy level, where the crop is, not at a convenient mounting height in the aisle.

Integration with the Rest of the Facility

Dehumidification equipment in a modern greenhouse is one actuator among several, and it needs to be part of the climate strategy rather than an independent appliance.

Climate computer. The control interface, available signals and protocol should be established at design stage. Equipment that cannot be sequenced by the climate computer will fight the vents and the heating system rather than working with them.

Heating. Because refrigerant dehumidification returns heat to the space, the heating control needs to know when it is running. Otherwise the two systems oscillate.

CO₂ enrichment. Dehumidifying without ventilating preserves enriched CO₂. The control sequence should take advantage of this rather than defaulting to vents.

Lighting. Supplemental lighting raises both sensible and latent load, and the moisture load follows the lighting schedule closely. Equipment sizing and control should be referenced to the lighting regime, not to an unlit baseline.

Irrigation. Since irrigation volume drives transpiration, changes to irrigation strategy change the humidity load. Where both are logged, the relationship is visible and useful for verification after commissioning.

Information to Prepare for a Project Discussion

• Facility: structure type, glazing or covering, floor area, gutter height, compartment layout and whether compartments are controlled separately.
• Crop: species and cultivar, production system, planting density, growth stages present, and the production calendar.
• Load data: irrigation and drain volumes over a representative period, ideally covering the most demanding stage.
• Lighting: supplemental lighting type, installed intensity and daily schedule.
• CO₂: whether enrichment is used, target concentration, and source.
• Climate targets: day and night temperature and humidity targets, or VPD targets, by growth stage, and which variable the facility currently controls on.
• Current equipment: heating, ventilation, screens, horizontal airflow fans, air-distribution tubing, and existing dehumidification if any.
• Site conditions: location and outdoor design conditions, summer and winter, with coincident humidity.
• Observed problems: where and when condensation occurs, disease pressure and its timing, and any periods when targets cannot be held.
• Constraints: available mounting positions and clearances, structural capacity, power supply, condensate route, and the climate computer's control interface.
• Verification: how performance will be assessed after commissioning, and what data will be logged.

Records covering a full production cycle are more useful than spot measurements, because the demanding condition is usually seasonal.

Discussing a Greenhouse Project

Yakeclimate designs and manufactures industrial dehumidification equipment for complex climate applications, with a focus on agriculture and energy projects.

We co-develop application-specific dehumidification equipment around the operating conditions, interfaces, and integration requirements of the wider project or system.

Explore agriculture humidity control, review industrial ceiling dehumidifiers, or contact Yakeclimate to review the operating conditions for your facility.