ARS WATER-DAMAGE TECHNICAL GUIDE

Psychrometry and Structural Drying

How restoration professionals use air and material measurements to create, monitor, and adjust a drying environment.

Updated September 2026 · Technical review: ARS Restoration & Reconstruction

Fans alone do not prove a building is dry. Structural drying requires removal of bulk water, appropriate airflow and dehumidification, repeated material measurements, and documented drying goals.

The four ideas homeowners should know

Temperature

Warmer air can hold more water vapor, and temperature affects evaporation, equipment performance, occupants, and materials.

Relative humidity

RH is the percentage of moisture in the air compared with what that air could hold at the same temperature. It changes when temperature changes.

Humidity ratio

Often expressed as grains per pound, this describes the actual mass of water vapor in the air and helps compare inside, outside, and dehumidifier-output conditions.

Vapor pressure

Water vapor tends to move from higher vapor pressure toward lower vapor pressure. Drying equipment helps create a favorable pressure difference from wet material toward drier air.

How moisture leaves a wet assembly

Extraction: physically remove as much liquid water as practical; this is usually faster and more efficient than evaporation.
Evaporation: energy and airflow help moisture move from a wet material surface into the air.
Dehumidification: equipment removes water vapor from the air so it can accept more moisture.
Diffusion and capillary movement: moisture held deeper in a material migrates toward the drying surface, often more slowly.
Monitoring and adjustment: the team compares material and atmospheric readings, then changes equipment, access, or strategy as needed.

Why one room can contain several drying problems

Drywall paper, gypsum core, insulation, wood framing, subfloor, finish flooring, cabinets, plaster, concrete, and masonry do not absorb or release moisture at the same rate. Paint, vinyl, tile, membranes, and closed cavities can slow evaporation or hide moisture.

A surface may feel dry while the material beneath remains elevated. Conversely, a high room RH reading does not identify which materials are wet. That is why technicians use both atmospheric readings and material-specific moisture measurements.

A defensible drying record

Known source, category/class considerations, and safety or access limitations.
Initial moisture map with affected and comparison areas.
Material type, location, instrument, measurement mode, and reading.
Temperature, relative humidity, and calculated humidity ratio at meaningful locations.
Equipment type, quantity, placement, and operating status.
Daily or scheduled readings showing trend—not only a final number.
Changes to containment, access, air movement, heat, or dehumidification and the reason for each change.
Drying goals appropriate to the material and building, plus final readings before reconstruction.

Questions to ask the drying contractor

Which materials are wet, and how was that determined?
What unaffected or normal comparison was used?
What is expected to dry in place, and what may need access or removal?
How are temperature, RH, humidity ratio, and material moisture trending?
Is outside air helping or adding moisture under current conditions?
Are equipment capacity and power supply appropriate and safe?
What would cause the drying plan to change?
What objective evidence will support completion?

Common misconceptions

Misconception	Better question
“It feels dry.”	What do material-specific readings show below the surface?
“More fans always dry faster.”	Is airflow reaching the correct surfaces without spreading contamination or causing damage?
“A fixed number of days is enough.”	Are readings trending toward documented goals?
“A dehumidifier dries the wall.”	Can moisture leave the assembly, and is the equipment controlling the vapor load?
“Drying means nothing is removed.”	Which materials can be cleaned and dried safely, and which conditions require access or removal?

A measured plan protects the next phase

Reconstruction should not conceal unresolved moisture. Ask for the final drying record and keep it with the project file.

Sources, scope, and review

Review note: Prepared September 2026. Psychrometric concepts are simplified for consumers and are not a substitute for trained field judgment or the ANSI/IICRC S500. This resource is educational and does not replace instructions from emergency services, your insurer, medical professionals, building officials, engineers, environmental professionals, or legal counsel. ARS does not determine insurance coverage.