What is Psychrometry?
The science of moist air—critical for efficient drying and water damage restoration.
The Science of Drying
Psychrometry is the branch of science that deals with the study of moist air—specifically, the thermodynamic properties of mixtures of dry air and water vapor.
It is a critical field in HVAC, meteorology, and water damage restoration, where it allows professionals to measure, calculate, and control the moisture content of air to dry materials efficiently.
Main Psychrometric Properties
Dry-Bulb Temperature (DBT)
The ordinary air temperature measured by a standard thermometer.
Wet-Bulb Temperature (WBT)
The lowest temperature air can reach by evaporative cooling. The difference between DBT and WBT indicates humidity.
Dew Point Temperature (DPT)
The temperature at which air becomes saturated (100% RH) and condensation begins. A direct measure of moisture content.
Relative Humidity (RH)
The ratio of actual water vapor to the maximum amount the air can hold at that temperature (%).
Absolute Humidity
Actual mass of water vapor (grains/lb).
Vapor Pressure
Pressure exerted by water vapor.
Restoration Applications
In restoration, psychrometry guides drying by calculating how much moisture air can hold and how to manipulate temperature, humidity, and airflow for efficient evaporation.
- Goal: Lower RH to <60% and dew point below surface temperatures to prevent mold.
- Classify Damage: Determine the severity of water intrusion.
- Monitor Progress: Use psychrometers to track drying efficiency.
The Four Principles of Structural Drying
IICRC-certified technicians apply four fundamental principles on every drying project. First is water extraction — physically removing as much liquid water as possible with truck-mounted or portable extractors, because extraction removes water hundreds of times faster than evaporation. Second is evaporation, accelerated by high-velocity air movers positioned to sweep air across wet surfaces and break up the boundary layer of humid air that clings to them.
Third is dehumidification. As moisture evaporates into the air, it must be removed or it will simply condense elsewhere in the building. Low-grain refrigerant (LGR) dehumidifiers and desiccant units pull water vapor out of the air, which technicians measure in grains per pound (GPP). Fourth is temperature control — warmer air holds more moisture and speeds evaporation, so technicians manage temperature to keep the drying system working at peak efficiency without damaging materials.
Throughout the project, technicians record temperature, relative humidity, and moisture content readings — often daily — and compare wet materials against a dry standard until drying goals defined under the ANSI/IICRC S500 standard are met. This documentation protects homeowners and supports insurance claims.
Why This Matters for North Bay Homeowners
When drying is guessed at rather than measured, the results show up months later: cupped hardwood floors, musty odors, and hidden mold colonies inside wall cavities. Mold growth can begin within 24 to 72 hours of a water loss, so the window for correct drying is short. In Sonoma, Marin, and Napa counties — where winter storms, Russian River flooding, and coastal humidity are facts of life — psychrometric drying is the difference between a full recovery and a recurring problem.
Every NBRA Premium Restoration member employs technicians trained in psychrometric drying under IICRC standards. If your home or business has suffered water damage, use our member directory to find a certified professional near you in Santa Rosa, Petaluma, Novato, Napa, or anywhere in the North Bay.
The Chart
The psychrometric chart plots these properties graphically. Restorers use it to trace processes like:
- Heating/Cooling
- Dehumidification
- Evaporative Cooling