What Is Psychrometry? The Drying Science Every Water Mitigation Tech Uses

Psychrometry is the study of moist air: the properties of air, the water vapor carried in it, and how those properties shift as temperature or moisture changes.

Psychrometrics is the same subject under its engineering name.

In restoration writing, the two words point at the same body of knowledge, and both describe what a water mitigation technician is actually managing on a drying job.

The connection to the work is direct.

Drying a structure means moving water out of wet materials, into the air, and out of the building.

Warm air holds more moisture than cool air, and moisture travels from wetter materials toward drier air.

Psychrometry is how a tech measures those conditions and steers them with air movement, temperature control and dehumidification, all judged by readings rather than by guesswork.

The trade’s governing standard puts the subject on its component list.

The ANSI/IICRC S500 — the IICRC’s Standard for Professional Water Damage Restoration — is currently in its fifth edition, published in 2021.

The IICRC lists psychrometry and drying technology as one of the components the S500 describes procedures for, alongside building and material science, equipment and instruments, antimicrobial technology, structural restoration, HVAC restoration and contents.

On a job, psychrometry shows up as readings: temperature, relative humidity, grains per pound and dew point, taken with meters and written into the drying log.

The psychrometric chart plots those properties against each other.

The IICRC describes the WRT as covering how to identify and change drying plans based on the Science of Drying — its name for exactly this working knowledge.

If you are new to the role itself, the water mitigation technician career guide covers the whole job: duties on a loss, certifications, schedules and where the career leads.

A psychrometric reading is the set of air measurements a tech records in the work area at a moment in time.

Four values carry it, and each answers a different question about the air.

• Air temperature. The starting point, because temperature sets how much water the air is able to hold: warm air holds more moisture than cool air. Techs read the air temperature in the work area, and surface temperatures matter too, since a cold surface pulls moisture out of the air onto itself.
• Relative humidity (RH). How close the air is to saturation at its current temperature. The National Weather Service notes that relative humidity is a function of both moisture content and temperature, so RH by itself does not directly indicate the actual amount of moisture in the air. That limitation is the reason restorers track the next two readings as well.
• Grains per pound (GPP). The actual weight of water vapor mixed into a pound of dry air. The unit comes from NIST’s standard weight tables: one grain is 64.79891 milligrams and one pound is 0.4535924 kilograms — 7,000 grains per pound, which is where the name comes from. GPP lets a tech compare the real moisture in the air between rooms and between visits, whatever the temperature is doing.
• Dew point. The temperature to which air must be cooled to reach saturation, with air pressure and moisture content held constant. A higher dew point means more moisture in the air, per the National Weather Service’s definition. Its practical edge on a job is condensation: a surface cooler than the dew point collects moisture out of the air. EPA’s condensation-prevention advice follows the same logic — raise surface temperatures with insulation or air circulation, or lower the moisture in the air by dehumidifying.
• Vapor pressure. The push behind the whole exchange: water vapor moves from where its pressure is higher — wet materials, humid air — toward where it is lower. Air movers and dehumidifiers exist to keep that difference pointed out of the structure.

Together these values describe the air the drying equipment has to work on.

A reading without its date, room and equipment context is just a number.

Logged visit after visit, the same values turn into the trend line a drying plan is steered by.

Psychrometry earns its keep in the decisions it drives: where equipment goes, and what the job is trying to reach.

The S500 lists establishing dry standards and drying goals among the minimum initial-response activities in its section 10.6, as the IICRC quotes it.

The S500’s own definitions of a dry standard and a drying goal sit in the paywalled standard, and our research did not find them published, so take the exact wording from the standard itself.

On the job, the drying goals are what the monitoring readings get checked against.

The goals then shape the equipment.

Air movers speed evaporation off wet surfaces, dehumidifiers remove the moisture that evaporation loads into the air, and the mix and placement come from the readings, the affected materials and the drying zones the crew sets up.

As the structure dries, the setup gets adjusted rather than left alone.

One honest limit: no public source we checked publishes an S500 formula for how many air movers a wet room gets, or a target GPP to chase.

Those specifics are the stuff of the paywalled standard and of course and manufacturer material, and companies teach their own approaches to them.

What the readings give a crew is a defensible basis for the placement decisions it makes and documents.

The loop closes on the monitoring visits.

Readings moving toward the drying goals confirm the setup; readings that stall send the crew back to change something — reposition or add equipment, tighten a drying zone, look for moisture that has not been found yet.

The IICRC builds that loop into the WRT itself: identifying and changing drying plans based on the Science of Drying.

The daily discipline is what turns psychrometry from theory into the record of a drying job.

It has a rhythm: baseline readings when the drying plan goes in, monitoring visits while the equipment runs, and a written log that ties every reading to a place and a moment.

The tools are straightforward.

A thermo-hygrometer reads the air’s temperature and relative humidity; a moisture meter reads moisture content in the materials themselves.

From those air readings, the GPP and dew point values follow: both are derived from temperature and humidity.

What goes in the log, visit after visit, stays consistent: the date and time, the room or drying zone, the temperature, RH, GPP and dew point, the moisture readings in the materials checked, and the equipment running in that space.

Pairing the wet areas’ readings with readings from an unaffected part of the same building gives every number a stable reference to be judged against.

The consistency matters more than any single reading.

Same instruments, same points, similar times of day — that is what makes the numbers comparable across visits, and what lets a project manager or estimator read the file as a trend instead of a pile of snapshots.

And if a job runs through an insurance channel, that log is what lets anyone reading the file see the structure moving toward the drying goals, visit by visit.

The log is also where trouble shows up first.

When readings stop moving toward the goals, the crew sees it in the record and changes the setup — which is exactly the adjustment loop the drying science exists to support.

The IICRC’s water certifications are where new techs meet the drying science formally.

The WRT — Water Damage Restoration Technician — is the entry point, and it has no prerequisites.

It runs live-stream as well as in person, in Spanish as well as English, and the IICRC describes it as covering water damage effects and drying techniques, including identifying and changing drying plans based on the Science of Drying.

The ASD — Applied Structural Drying Technician — sits above it.

It requires the WRT first, covers the effective, efficient and timely drying of water-damaged structures and contents within a restorative drying environment, and counts toward the IICRC’s Master Water Restorer designation.

What neither course page publishes is a topic-level syllabus.

The IICRC’s public course listings do not break the WRT or the ASD down into declared psychrometry learning objectives, so this page will not invent a list of what a class or its exam covers.

The readings explained above — temperature, RH, GPP, dew point, vapor pressure — are the measurements a drying plan is steered by.

Treat them as background knowledge for the course, and take the course itself for its authoritative outline.

The classes themselves run through IICRC-approved schools and instructors.

The IICRC is a certifying body, not a school: it owns no schools and keeps no instructors on staff, and instead approves independent schools and instructors that meet its criteria.

Treat the drying science as fair game in an interview for water work, at any level of experience.

Be ready for questions pitched at where you are: a new tech gets asked what the readings mean, an experienced one gets asked what they did when the readings stalled.

A tight definition set travels well.

Be able to say in one sentence each what air temperature, relative humidity, GPP and dew point tell you — and why RH alone does not tell the whole story.

Relative humidity depends on both moisture content and temperature, so it does not directly indicate how much moisture the air actually holds.

That one distinction is the difference between reciting a meter screen and understanding it.

Experience questions run to the decisions: how you took and logged readings, how a drying goal gets set on a job, what you changed when numbers stopped moving.

With field time, walk through a real file’s arc — baseline, adjustment, dry-down — using the readings you logged.

Without it, describe how you would run that loop honestly, and say plainly which parts you have not done yet.

You do not need formulas in the answer; you need to show that the readings, not habit, drive the setup.

When you are ready to put the drying science to work, browse water mitigation technician jobs on this board.

And keep the drying-log habit from your first day on the job — it is the discipline that makes the rest of the file easy.

Career information, not licensing, legal, insurance, health, safety or tax advice. Confirm certification and course details with the IICRC.