Ventilation vs. Dehumidification in Controlled Agriculture: The Test That Decides It

Outside air at 90 % relative humidity can dry a greenhouse. Outside air at 85 % relative humidity can make it wetter. The difference has nothing to do with the humidity readings and everything to do with temperature — and getting this backwards is the most common reason a ventilation strategy quietly stops working.

This article sets out the test that decides between ventilation and mechanical dehumidification, with the arithmetic to run it on your own site.

Ventilation Has One Physical Precondition

Ventilation dehumidifies by exchange: moist inside air out, outside air in. It reduces the moisture content of the space only if the incoming air carries less water per unit of dry air than the air it replaces.

That is a comparison of absolute moisture content — humidity ratio, in grams of water per kilogram of dry air — not relative humidity. Relative humidity describes how close air is to saturation at its own temperature, and since inside and outside are at different temperatures, comparing their relative humidities compares two different scales.

Get this wrong and the ventilation system runs all night importing water.

The Test, With Arithmetic

Humidity ratio is calculated from temperature and relative humidity in two steps.

Saturation vapour pressure:
es = 0.61094 × exp( 17.625 × T / (243.04 + T) )        [kPa, T in °C]

Actual vapour pressure:
e  = es × RH / 100

Humidity ratio:
w  = 0.622 × e / (P − e)                                [kg/kg dry air]
P  = atmospheric pressure, 101.325 kPa at sea level

Case 1 — outside looks wetter but is drier.

Outside air at 90 % RH holds 2.2 g/kg less water than inside air at 80 % RH. Ventilation works, and works well.

Case 2 — outside looks similar but is wetter.

Here outside air carries 2.4 g/kg more water. Ventilating adds moisture to the greenhouse. No increase in vent opening will help; opening further makes it worse faster.

Both cases involve outdoor relative humidities in the 85–90 % band. The readings give no indication of which situation the grower is in. Only the absolute comparison does.

How Much Moisture Ventilation Actually Removes

Once the difference in humidity ratio is known, the removal rate follows directly:

Water removed = ρ × V̇ × Δw

ρ  ≈ 1.2 kg/m³   (air density)
V̇  = ventilation rate, m³/h
Δw = w_inside − w_outside, g/kg

Using Case 1 above, with a ventilation rate of 10,000 m³/h:

= 1.2 × 10,000 × 2.2
= 26,400 g/h
≈ 26 litres per hour

In a colder condition — outside at 5 °C and 80 % RH, giving 4.3 g/kg against the same 11.7 g/kg inside — the difference is 7.4 g/kg and the same ventilation rate removes about 89 litres per hour. Cold outdoor air is a powerful desiccant, which is why winter ventilation works well and summer ventilation often does not.

This also shows why ventilation capacity alone is not the answer to a humidity problem. If Δw is small, no realistic vent opening produces meaningful drying. If Δw is negative, the system is working against itself.

What the Cold-Air Route Costs

Cold outdoor air dehumidifies effectively, but it arrives cold and has to be heated to the target temperature. That heating is the real price of ventilation-based dehumidification in the heating season.

Heating required = ρ × V̇ × cp × ΔT
cp ≈ 1.005 kJ/kg·K

Taking the 5 °C case, heating 10,000 m³/h from 5 °C to 20 °C:

= 1.2 × 10,000 × 1.005 × 15
= 180,900 kJ/h
≈ 50 kW

Against 89 litres per hour removed, that is roughly 0.57 kWh of heating per litre of water removed.

Running the same calculation on Case 1 (15 °C outside, 26 L/h removed, 5 K rise, about 17 kW) gives roughly 0.66 kWh per litre.

The pattern holds across typical heating-season conditions: ventilation-based dehumidification costs on the order of 0.5–0.7 kWh of heat per litre, whether the outside air is cold and dry or mild and moderately dry. For comparison, the latent heat of the water itself is about 0.68 kWh per litre — so the heating penalty is roughly the same size again as the thermodynamic minimum.

A mechanical dehumidifier draws electrical energy instead, and returns both that energy and the latent heat of condensation to the space as sensible heat. During the heating season that heat displaces the heating system rather than being exhausted. The comparison is therefore not "electricity versus free air"; it is "electricity that stays in the house versus heat that leaves it."

The Cost That Does Not Appear on the Energy Bill

In a CO₂-enriched facility, ventilation exhausts enriched air and admits ambient air at roughly 400–420 ppm. Two things follow.

The enrichment has to be replaced, at whatever the CO₂ supply costs. And during the ventilation period the crop photosynthesises at a lower concentration than the strategy intended, so the enrichment investment is partially wasted at exactly the moment the grower is paying to replace it.

This cost is real but invisible: it appears as yield that did not happen, not as a line item. In facilities where enrichment is a significant part of the production strategy, it frequently dominates the comparison — and it is the reason many enriched operations move to mechanical dehumidification even where the energy comparison alone would be marginal.

The interaction between CO₂ concentration, stomatal behaviour and moisture release is developed in CO₂ uptake in controlled environments.

When Ventilation Is the Right Route

Ventilation remains appropriate, and should not be replaced for its own sake, where:

• Outdoor air is reliably drier in absolute terms during the periods when dehumidification is needed. Run the humidity ratio comparison against a year of local weather data, focusing on the difficult periods rather than the annual average.
• CO₂ enrichment is not in use, or is used only during periods when ventilation is closed.
• The heating penalty is acceptable — either because the climate is mild, because heat is inexpensive at that site, or because the facility is not heated to a tight target.
• The humidity problem is occasional rather than a structural feature of the production system. Solving an intermittent problem with capital equipment is rarely justified.
• The infrastructure already performs adequately and the observed problems are traceable to control strategy or air distribution rather than to a genuine capacity limit.

When Mechanical Dehumidification Becomes Necessary

The case changes where:

• Outdoor absolute humidity is frequently equal to or above inside conditions during the critical period. This is the decisive condition: when it holds, ventilation cannot dehumidify at all, and no control refinement changes that.
• CO₂ enrichment is central to the production strategy. Preserving enriched air has direct value that ventilation cannot deliver.
• The heating cost of ventilation is material and the returned heat from mechanical dehumidification has real value during the heating season.
• The facility requires tighter or more repeatable control than a weather-dependent method can provide — for example, holding a VPD target through the night rather than reacting to excursions.
• Condensation on leaves or glazing is a recurring problem, particularly during transitions, when ventilation response is slowest.
• The facility is moving toward a closed or semi-closed design, where the ventilation route is being deliberately reduced for energy or biosecurity reasons.

Most facilities that reach this point do not abandon ventilation. They add a route that works when ventilation cannot, and sequence the two.

Testing It on Your Own Site

This decision can be settled with data most facilities already have, before any equipment is specified.

Step 1 — Log inside and outside temperature and humidity together, at the same interval, for at least one full production cycle including the difficult season. Most climate computers already record both.

Step 2 — Convert every reading pair to humidity ratio using the formulas above. A spreadsheet column is sufficient.

Step 3 — Plot Δw over time. Where Δw is positive, ventilation can dehumidify. Where it is zero or negative, it cannot.

Step 4 — Count the hours. The number of hours per year during which Δw is inadequate while dehumidification is needed is the size of the gap that mechanical equipment would fill. If that number is small, ventilation is adequate. If it covers most of the critical season, ventilation is structurally unable to solve the problem.

Step 5 — Cost the heating. For the hours when ventilation does work, apply the heating calculation above to establish what that route actually costs across the season.

This produces a site-specific answer rather than a general preference, and it produces it from records that already exist.

Sizing the Gap, Not the Whole Load

Where mechanical dehumidification is added alongside working ventilation, the equipment does not need to cover the entire transpiration load. It needs to cover the load during the hours when ventilation cannot.

That distinction matters commercially: sizing for the full load when ventilation handles most of the year produces equipment that is oversized, cycles poorly, and costs more than the problem required.

The sizing sequence — establishing the load from irrigation records, reading capacity at the actual operating condition rather than the catalogue rating condition, and deciding unit count from air distribution — is set out in greenhouse dehumidification.

Information to Prepare

• Logged inside and outside temperature and relative humidity covering a full production cycle, with logging interval.
• Site location and outdoor design conditions, summer and winter.
• Crop, production system, planting density and production calendar.
• Irrigation and drain volumes over a representative period.
• Supplemental lighting type, intensity and schedule.
• Whether CO₂ enrichment is used, the target concentration and the supply cost basis.
• Current heating system, fuel and approximate energy cost.
• Existing ventilation capability: vent area, forced ventilation capacity, screens and control strategy.
• Climate targets by growth stage and which variable the facility controls on.
• Observed problems, with timing: condensation locations, disease pressure, and hours when targets cannot be held.

Discussing a 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.