VPD and Dew-Point Control in Commercial Grow Rooms: An HVAC Guide

Most growers know their VPD targets. Fewer have rooms that actually hold them, especially in late flower and right after lights-off. The gap is almost always in the HVAC system: separate pieces of equipment chasing temperature and humidity on their own, instead of one system controlling both.

This guide explains what VPD, relative humidity and dew point each tell you, why holding VPD takes coordinated cooling, dehumidification and reheat, and what to give your designer so the system is sized right the first time.

VPD, RH and dew point: three views of the same air

Relative humidity (RH) is how close the air is to saturated at its current temperature. Because warm air holds more water than cool air, RH changes whenever temperature changes, even if no moisture is added or removed.

Dew point is the temperature at which the air's moisture would start to condense. It tracks how much water is actually in the air. If dew point holds steady, the moisture in the room is under control, whatever the thermostat is doing.

Vapor pressure deficit (VPD) is the difference between the vapor pressure at the leaf surface and the vapor pressure of the room air. It's measured in kilopascals (kPa). It's the number that matters to the plant because it drives transpiration: the higher the VPD, the faster water moves from the plant into the air. The Resource Innovation Institute (RII) describes a good general range as 0.8 to 1.2 kPa. Below 0.8 kPa transpiration slows, which slows water and nutrient flow and growth. Above 1.2 kPa plants can become water-stressed. Targets also shift by growth stage and photoperiod [1].

A quick worked example

Take a flower room at 80°F and 60% RH. Its dew point is about 65°F. If the leaves run about 2°F cooler than the air, leaf VPD is about 1.2 kPa, right at the top of RII's range.

Now the lights go off and the room drifts down to 72°F, but the moisture in the air stays the same. Dew point is still about 65°F, but RH climbs to about 78%, and VPD falls well below the range. Nothing went wrong with the dehumidifier. The temperature moved and the moisture didn't. (These figures come from the standard vapor-pressure formula, not field data.)

That's the core point: you can't hold VPD by controlling temperature and humidity separately. Every time one moves, the other has to move with it.

Why VPD needs cooling, dehumidification and reheat working together

In a sealed flower room, the plants put back almost all the water you feed them, and the lights add a large heat load. To hold a VPD target, the system has to do three jobs at once:

1. Remove heat (sensible cooling) to hold air temperature.
2. Remove moisture (latent cooling) to hold dew point.
3. Add heat back when needed (reheat), so the room isn't overcooled when the system is mainly pulling moisture, which happens most at lights-off and in late flower.

When those jobs are split across a mini-split, a few standalone dehumidifiers and maybe a heater, each one runs on its own sensor and its own logic. The AC cools, RH rises, the dehumidifiers kick on and dump heat, the AC works harder, and the room swings around the setpoint instead of sitting on it.

An integrated HVACD system does all three in one unit under one controller. Air passes over a cooling coil that pulls it down to the target dew point, then over a hot gas reheat coil that warms it back to the target temperature using heat the refrigeration circuit already made. The controller holds temperature and dew point together, and VPD follows.

Control it by dew point, not just RH

Because RH moves with temperature, a system that only watches RH will chase its own tail at every light change. Controlling to a dew-point setpoint (plus a temperature setpoint) keeps the actual moisture in the room steady, so RH and VPD land where you want them when the temperature does. Yield Logic systems use dew-point control with a built-in PLC for this reason.

Good control also depends on good data:

• Sensor placement. Put sensors at canopy height in representative spots, away from supply diffusers and walls, so the controller sees what the plants see.
• More sensors in bigger rooms. RII recommends adding sensors across your cultivation spaces to get more accurate trends [1]. One sensor in a large room hides hot and wet spots.
• Field checks. Handheld hygrometers and infrared thermometers for canopy temperature are a cheap way to verify that installed sensors and the system are doing what the design said [1].

Lights-on vs lights-off: plan for both

The hardest moment for VPD control is the first hour after lights-off. The heat load disappears almost instantly, but the plants keep transpiring. A system sized only for the lights-on cooling load will overcool the room at night and lose control of humidity.

Give your HVAC designer separate lights-on and lights-off targets. RII recommends setting temperature, RH and/or VPD targets for both periods, and for the cultivars you grow, so capacity is estimated accurately [1].

What to give your designer

RII notes that experienced engineers need two of these three values to size and select equipment: target VPD, dry-bulb temperature and RH [1]. In practice, send:

• Room dimensions and canopy area for each room
• Lighting type, fixture count and wattage
• Watering volumes per day (or plants and pot sizes)
• Lights-on and lights-off targets (temperature plus RH or dew point, or VPD)
• Stage schedule: veg, early flower, late flower, dry

RII also warns that sizing around a VPD that's too high, or temperatures and humidity that are too low, can oversize the system, causing short cycling, wasted energy and shorter equipment life [1]. Realistic targets save money up front and every month after.

What operators get out of it

Rooms that hold VPD through every light change mean steadier plants, fewer late-flower mold scares and less babysitting of equipment. Growers in Yield Logic rooms see consistent VPD and temperatures and higher yields, even with less experienced staff. Results depend on each operation's SOPs and cultivars, and no specific yield increase is guaranteed.

Want a second look at your VPD targets or room plans? Send us your room sizes, lighting plan and setpoints and we'll review the loads with you. Call (303) 710-8616 or email Info@VisionsContractingLLC.com.

Sources

1. Resource Innovation Institute, "HVAC for Cannabis Cultivation & Controlled Environment Agriculture" best-practices guide: resourceinnovation.org