Do you understand how vapor pressure difference (VPD) affects indoor cannabis cultivation?

One very important factor that cannabis growers must account for in the design and operation of their indoor grow facilities is Vapor Pressure Difference (VPD). While it is as important as temperature and humidity, it is often overlooked. Specifically, VPD is the difference between the amount of water vapor the atmosphere can hold at any given temperature minus the amount of water vapor already contained in the atmosphere or what is commonly known as relative humidity. Warmer temperatures hold higher humidity than cooler temperatures. VPD is measured in kilopascals (kPA).

Think of VPD as a benchmark for the evaporation of potential water into the grow facility atmosphere and also as the deficit between the pressure exerted by the existing moisture in the air and the pressure at the saturation point. Indoor cannabis growers rely on VPD measurement as a way to monitor water stress on their plants.

Why VPD is important is its role in the plant growing process. Plants rely on VPD to help in their ability to extract nutrients and minerals through their roots for supply to the rest of the plant. Plants use the pores on their leaves which open and close to facilitate the plant transpiration process; the release of moisture into the air. Think of it like “evaporative cooling” which is used to cool homes and businesses in dryer climates. It is believed that cannabis grows best when the VPD is maintained at optimal levels to help the growing process while reducing potential mold or other plant diseases which thrive in high humidity. Similarly, by managing VPD levels, low humidity conditions, which hinder plant growth, can also be avoided.

A properly designed and maintained dehumidification solution can support the ideal VPD for optimizing healthy plant growth and cost-effective facility operations. To achieve the optimum climate conditions, growers should first start by selecting the ideal VPD and then the highest temperature they want in their facility. From these two figures, the ideal humidity level will be determined. Growers should not be concerned that the humidity level may seem higher than they expected. Instead, they should realize that the VPD is the driving factor for what the humidity should be for their plants’ specific stage of growth. VPD will differ for a facility’s lights on or light off status.

By having their grow facility’s dehumidification solution designed based on these specific parameters, along with configuring and locating it properly (i.e., horizontally or vertically, indoors or outdoors mounted rooftop or elsewhere), cannabis cultivators give their plants the greatest opportunity to thrive, while achieving maximum energy efficiency. Today’s sophisticated wrap-around dehumidification technologies are designed to provide 80% pre-cooling, condensation, and dehumidification by a process of regenerative thermal exchange which delivers significant energy savings, as well as other important operational benefits.

Want more information? Visit

Published by Walter Stark Consulting, Inc.

Walter Stark has spent his professional career serving the HVAC industry, and the past thirty years specializing in dehumidification, making him one of the leading experts in the field. He is best known as the inventor of the MSP Dehumidifying Coil, which saves up to 70% energy. He also holds a number of other HVAC related patents. An award-winning visionary and entrepreneur, Mr. Stark has been the recipient of the Governor’s Award for Energy Excellence in New York, and a grant from the New York State Energy Research and Development Authority. Popular Science magazine has honored his invention among their Best of What’s New. Mr. Stark has served as President of his local chapter of the American Society of Refrigeration Air Conditioning Engineers. His clients include such large firms as Walmart and Hasbro. Mr. Stark is active in the field of dehumidification and the emerging field of indoor plant-growing, providing HVAC and dehumidification equipment, design. In 2018 he sold MSP Technology to his long time manufacturing partner, KCC manufacturing, and his technology is now offered exclusively through the Trane organization. In an effort to put his extensive knowledge in dehumidification and indoor agriculture climate control to use, Mr. Stark is now offering consulting services to help owners, operators, contractors and design professionals optimize existing facilities and plan for expansion.

Get in Touch

I'm Interested in a Free VPD Calculator

12 + 6 =