VPD and Dehumidification: A Practical Indoor Farm Guide

VPD combines temperature and humidity

Vapour pressure deficit describes the gap between a reference saturation vapour pressure and the actual vapour pressure of the air. An air-based value uses air temperature for that reference. An estimate of leaf-to-air VPD uses leaf temperature instead. Both are expressed in kilopascals, or kPa.

Michigan State University explains why growers use VPD alongside relative humidity when interpreting the evaporative environment. Crop stage matters, so a propagation room and a finishing crop should not automatically share a target. MSU’s VPD guide.

Use the target agreed with the crop specialist as a design input, then check whether the facility can maintain it across the growing area. A single number on a dashboard cannot describe leaf temperature, leaf wetness, root conditions and airflow at the same time. Treat departures as a reason to inspect the wider record.

A cooler leaf changes the calculation

The FAO saturation-pressure equation provides a transparent basis for the example below: es(T) = 0.6108 × exp[17.27T / (T + 237.3)]. With temperature in °C, the result is in kPa. Actual vapour pressure is estimated here as es(air temperature) × RH / 100. FAO meteorological data guidance.

At an air temperature of 24 °C and RH of 65%, the calculated air VPD is approximately 1.04 kPa. If a measured leaf is 22 °C, the estimated leaf-to-air difference is about 0.70 kPa. At 20 °C, it is about 0.40 kPa. These are Hamfy calculations using assumed conditions, not measurements from a facility or crop recommendations.

Turn water release into an equipment requirement

Create an hourly moisture balance for the room. Record water added, water leaving through drain or discharge, water retained in the system and any measured condensate. Distinguish water circulating through irrigation pipes from water actually evaporating into the room. A pump flow rate is not a transpiration measurement.

Account for evaporation from exposed water and wet surfaces as well as the crop. Include the effect of ventilation and infiltration in the room balance. Agree on a peak operating scenario: mature crop, irrigation schedule, light programme and outside conditions. A daily average may hide a short period that determines the required capacity.

As an order-of-magnitude calculation, removing 10 kg of water vapour per hour corresponds to about 6.8 kW of latent heat at a latent heat of vaporisation of 2.45 MJ/kg. This is thermal load, not the electrical input of a dehumidifier. The value of 2.45 MJ/kg is given in FAO’s guidance; the conversion is our calculation.

Ask the HVAC designer to explain the whole heat path. For a self-contained unit returning condenser heat to the same room, moisture removal also adds sensible heat to the room air. Systems rejecting heat elsewhere behave differently. Coordinate cooling, reheat and condensate recovery around the selected system instead of adding catalogue capacities independently.

Specify capacity at the conditions you intend to run

A useful dehumidifier enquiry includes room temperature and humidity at design duty, required water removal per hour, airflow, electrical demand, heat rejection arrangement and control range. Request the manufacturer’s performance data at those conditions. A headline litres-per-day rating without its test conditions is not enough to select equipment.

Add service access, drainage, sensor placement and a recovery plan after an interruption. Record which controller has authority when cooling and dehumidification demand different actions. Ask what happens during a sensor fault, a full condensate drain or loss of communication. These are part of the operating specification.

Review climate traces by zone and by phase of the light cycle. The sensor data analyser can help examine a CSV export for gaps and excursions. It cannot establish that the original sensor was calibrated or located in a representative place.

Commission the climate where the plants are

The controlled-environment measurement guidance by Both and colleagues recommends calibration and independent checks of environmental measurements. Environmental measurement guidelines. In a commissioning plan, record each sensor’s position, height, identity and calibration history so future comparisons refer to the same measurement.

Hamfy proposes testing a representative canopy, an edge position and a suspected poorly mixed zone, then repeating the check when the crop structure changes. Capture the lights-off transition and recovery after irrigation. Record both the magnitude and duration of departures from the agreed conditions.

An acceptable handover should leave the grower with a zone map, trend data, verified alarm delivery and a clear explanation of equipment limits. Use the VPD calculator to explore the arithmetic, and the dew-point tool when comparing air moisture with a measured surface temperature.