When I first walked into the world of restorative drying, I was eager, full of energy, and—if I’m honest—a little too confident in my understanding of Psychrometrics. Like many before me, I thought I could lean on a few trusty numbers and formulas, convinced that the air’s temperature or a quick “grain count” would tell me all I needed to know. It didn’t take long for reality to set me straight.
Let me take you back to one of my earliest projects—a water loss in a stately old home. The insurance company wanted quick answers, the homeowner wanted her floors saved, and I wanted to prove myself. I strode in, meter in hand, ready to conquer the problem with what I thought was a solid grasp of the science. But as the days unfolded, I realized that the numbers on my meter were only as good as my understanding of what they truly meant.
The Dry Bulb Dilemma
On that first day, I cranked up the heat, thinking, “More warmth equals faster drying.” I’d read somewhere that higher air temperatures increased evaporation rates. What I failed to consider was that the temperature of the air meant little if I didn’t also know the temperature of the materials I was trying to dry. The hardwood floors, thick and cold, stubbornly held onto their moisture. It was a humbling lesson: air temperature alone is only half the story. Only when I began measuring both the air and the material temperatures did I start to see the real picture.
The Grain Counting Trap
Next, I turned to the humidity ratio—what some call “grains per pound” or GPP. I’d been taught to watch the numbers drop and celebrate every grain of moisture removed. But I soon learned that simply tracking GPP wasn’t enough. The real value came in understanding “grain depression”—the difference in humidity ratio between two air samples. Only by knowing the volume of air exchange could I determine the rate of moisture removal. Anything less was just chasing numbers for their own sake.
The Redemption of Relative Humidity
Relative humidity (RH) often received a negative reception in the industry. Numerous mentors of mine disregarded it as “inconsequential,” claiming it was too easily influenced by temperature variations to be useful. But as I dug deeper, I discovered its true value. RH, or “relative vapor pressure,” is a force—a measurable energy potential. It’s the reason why, according to the US Department of Forestry’s Wood Equilibrium formula, vapor pressure in materials seeks to equalize with the surrounding air. The chart by Cassens made it crystal clear: the final moisture content of materials is dictated by relative vapor pressures, not by GPP or other measures. RH became my daily companion, not as a lone indicator, but as a vital part of a bigger picture—always considered alongside air and material temperatures.
Dew Point: The Unsung Hero
There was a winter job where condensation became my nemesis. Cold window panes sweated, dripping onto sills and threatening to ruin the woodwork. Here, the dew point temperature became my guide. By measuring it, I could decide whether to dry the air, warm the glass, or isolate the window. Dew point wasn’t about drying strategy for hygroscopic materials, but it was invaluable in preventing collateral damage.
Vapor Pressure: The True Force
It took years before I truly appreciated vapor pressure. Unlike other measures, vapor pressure is a property of both the atmosphere and the materials we’re trying to dry. Calculating the “delta vapor pressure”—the difference between the vapor pressure in the air and in the material—became my compass. It told me if my drying plan was working, if the force was in my favor. Software tools helped with the math, but the principle was simple: vapor pressure drives the process. Ignore it, and you’re flying blind.
Wet Bulb Wisdom
Wet bulb temperature was another measure I’d overlooked. It’s rarely used in the field, but knowing it helped me understand whether I was in the “constant rate” or “falling rate” phase of drying. When a material’s temperature neared the wet bulb, I knew I was still in the constant rate phase—evaporation was steady. As the gap widened, I shifted strategy, adjusting airflow to coax out the last stubborn moisture.
The Air Volume Myth
For years, I believed the industry standard: “A pound of air is about 14 cubic feet.” But that’s only true in a narrow temperature range at sea level. Our dehumidifiers and heaters operate across a much wider spectrum. I learned to calculate air volume accurately, correcting my moisture removal rates and ensuring my equipment was performing as expected.
The Maturing Standard of Care
With every job, every challenge, I came to see that responsible psychrometry isn’t about chasing single numbers or relying on old myths. It’s about integrating all the measures—air and material temperatures, RH, GPP, vapor pressure, dew point, wet bulb, and air volume—into a cohesive strategy. It’s about understanding the science and respecting its limits.
As the industry matures, so must our practices. The leaders in restorative drying are those who embrace the full suite of psychrometric tools, using them responsibly, always seeking to improve the standard of care. We owe it to our clients, our colleagues, and ourselves.
That’s the story I share with every new technician who joins our team. Don’t just read the numbers—understand them. That’s the measure of a true drying leader.
Source: Ken Larsen, Et al. Leadership in Restorative Drying 2014
Lorne McIntyre CR WLS FLS CLS ERS CSDS
Lorne McIntyre is a recognized expert in the cleaning and restoration industry, known for solving complex problems and raising the bar across the field. As Principal Technical Advisor at Verify Appraisal & Consulting, he’s a trusted resource for clarity, credibility, and results.
A Technical Trainer for the Restoration Industry Association (RIA) and an IICRC Approved Instructor with the Restoration Science Academy, Lorne holds all three IICRC Master Certifications and every RIA pillar designation—including the prestigious Certified Restorer (CR).
He also contributes to industry advancement through active roles on several RIA and IICRC committees.
To connect with Lorne, reach out at [email protected]
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