Psychrometry for Structural Drying, GPP and Vapor-Pressure Math

Why this matters

A structural-drying technician who reads only "RH and temperature" off a psychrometer is missing the actual driver of moisture movement. Two rooms can show identical 60 percent RH and feel completely different to a drying setup: one might be at 55 F and 60 GPP (grains per pound), the other at 85 F and 134 GPP. The second room has more than twice the absolute moisture load, and a dehumidifier sized for the first room will run forever in the second. This manual covers the psychrometric math that every senior tech should be able to do on a job site without an app.

Grains per pound, the working unit

A grain is 1/7000th of a pound (the unit comes from grain-weight of cereal kernels, historical but standardized). Grains per pound (GPP) is the absolute moisture content of air, mass of water vapor per mass of dry air. A psychrometer gives you GPP from temperature and RH; a true digital thermo-hygrometer (the Protimeter Hygromaster, Tramex Hygro-i, or any other reasonable IICRC-spec instrument) gives GPP directly.

Why GPP matters: a dehumidifier removes water based on the inlet GPP, not the inlet RH. A unit rated for 130 pints per day at AHAM conditions (80 F, 60 percent RH, about 99 GPP) does not pull 130 pints per day at 60 F, 60 percent RH (about 47 GPP). It pulls about half. The colder, lower-GPP air has less water available for the coil to condense.

Reading the chart

The psychrometric chart plots dry-bulb temperature on the X axis, humidity ratio (which scales linearly to GPP) on the Y axis, with curves of constant relative humidity sweeping across.

Five points to memorize for site work:

• 70 F, 50 percent RH = 55 GPP (dry, comfortable indoor air)
• 70 F, 60 percent RH = 65 GPP (the IICRC drying-goal upper bound for most jobs)
• 80 F, 60 percent RH = 99 GPP (AHAM dehumidifier rating condition)
• 90 F, 80 percent RH = 170 GPP (saturated post-flood Gulf-coast summer conditions)
• 55 F, 80 percent RH = 60 GPP (cool basement after a January pipe burst)

The cool basement case is the trap. The 80 percent RH reading looks alarming, but the absolute moisture is the same as a comfortable 70 F room at 50 percent RH. The dehumidifier deployment for that basement needs to either warm the air first (which raises its capacity to hold vapor and increases evaporation from wet substrates) or accept that LGR (low-grain refrigerant) dehus are required because conventional refrigerant dehus are inefficient below about 65 GPP inlet.

Vapor pressure and why evaporation happens

Air at any given temperature has a maximum partial pressure of water vapor it can hold (saturation vapor pressure). At 70 F, saturation is about 0.36 pounds per square inch (psi). The actual partial pressure of water vapor in the air is some fraction of that, set by the RH. The difference between the saturation pressure at the wet surface (assume 100 percent RH at the substrate face) and the partial pressure in the surrounding air is the driving force for evaporation.

In practice the technician does not calculate psi. The working substitute is the GPP gradient:

If surface GPP is higher than air GPP by 30 grains or more, drying is fast. If the gradient is under 15 grains, drying has slowed to the point that more aggressive intervention (warmer air, faster airflow, lower-grain dehu) is needed. If air GPP exceeds 65 to 70 in an occupied building, secondary damage risk (mold germination on adjacent dry materials) becomes real.

The three drying parameters: humidity, temperature, airflow

Drying speed is the product of three variables. Adjusting one without considering the others wastes equipment-hours.

1. Humidity (air GPP). Lower air GPP increases the gradient to wet surfaces, accelerating evaporation. Dehumidifiers do this work.
2. Temperature. Warmer air holds more water (a 10 F rise roughly doubles air's vapor-holding capacity) and energizes water molecules at the substrate, accelerating evaporation. Targeted heat (cuts the drying time on the densest substrates, particularly hardwood and dense framing.
3. Airflow. Moving boundary-layer air off the wet surface replaces vapor-saturated air with drier room air. Air movers do this. Without airflow, a still pocket of saturated boundary air sits on the substrate and stops further evaporation no matter how dry the room is overall.

The IICRC S500 drying goal: air GPP at the affected area is at least 30 grains lower than affected-substrate moisture content (translated through a wet-surface psychrometric estimate), at sufficient airflow to maintain the gradient.

Sizing a dehumidifier from GPP load

Step 1: measure ambient air GPP at job arrival in the affected area.

Step 2: measure affected wet-substrate moisture content (Tramex, Delmhorst, or pin meter as appropriate). Convert to estimated surface GPP using the substrate's published moisture-to-vapor relationship; for general practice, treat saturated wood at room temperature as roughly 150 GPP at the boundary layer.

Step 3: calculate the moisture load you need to remove in the working drying period. A 2,000 cubic-foot affected area drying from 100 GPP to 60 GPP over 72 hours, with continuous air infiltration replacing one room volume per hour, is removing approximately:

• Air-mass-to-vapor conversion: 2,000 ft^3 air at 70 F holds about 150 pounds of dry air. 40 GPP drop = 40/7000 lb water per lb dry air = 0.00571 lb per lb air = 0.857 lb water per air change.
• 24 air changes per day x 3 days x 0.857 lb = 62 lb water from air alone.
• Substrate-bound water released by drying is added separately, estimated from substrate moisture-content drop and total substrate mass.

A 130-pint-per-day AHAM-rated refrigerant dehu pulls about 65 pints per day in low-grain field conditions (50 to 60 GPP inlet), which is about 68 pounds per day. One dehu handles the air load above, but as soon as you add the substrate release, you are at minimum two dehus for the same room.

Inside vs outside conditions

If outside air at the project is drier (lower GPP) than inside air, controlled ventilation (open windows, fans) is faster than dehu. If outside air is wetter, sealing the building and running dehu is the only correct approach. Walk outside at job start, take a GPP reading, and compare it to the affected-room GPP. The decision is binary and obvious once you have both numbers.

This is why a Florida summer flood project and a Minnesota winter flood project look completely different. Florida summer outdoor air is 130 to 170 GPP; opening windows guarantees mold growth. Minnesota winter outdoor air is 5 to 15 GPP; opening windows for an hour drops indoor GPP by 30 grains and saves a dehu run-day.

LGR vs conventional dehumidifier selection

References

• ANSI/IICRC S500 Standard for Professional Water Damage Restoration, 4th Edition (2015), Section 12 (Structural Drying).
• IICRC Reference Guide for Professional Water Damage Restoration (2015), psychrometric chapters.
• ASHRAE Handbook of Fundamentals, current edition, Chapter 1 (Psychrometrics).
• AHAM (Association of Home Appliance Manufacturers) Standard DH-1 governing dehumidifier rating conditions.