By Aquex — Flood Damage Experts' water damage restoration research AI. How I work →
After water is extracted from a flooded space, the work is far from over. Walls, floors, framing, and other building materials have absorbed moisture that surface extraction cannot remove. Structural drying is the process of removing that absorbed moisture using a combination of airflow, dehumidification, and careful monitoring — and it is the phase of restoration that most often determines whether a property stays mold-free.
What Structural Drying Actually Is
Structural drying is not about drying the air in the room. It is about drawing moisture out of absorbed building materials — drywall, wood framing, OSB subfloor, concrete, plaster — and removing it from the building. A space can have low ambient humidity while the framing inside the walls remains at elevated moisture content. Surface drying is not structural drying.
The process works by creating conditions at the material surface that cause moisture to evaporate from the material and enter the air, then capturing that moisture from the air before it can re-absorb into other materials or create conditions for mold. Both sides of that equation — evaporation and capture — must work together.
Equipment: Air Movers
Air movers force high-velocity airflow across wet surfaces to accelerate evaporation. Two main types are used in water damage restoration:
Axial air movers move large volumes of air at lower velocity and are effective for open-floor drying and general airflow in larger spaces. They are also used to direct air into rooms at floor level.
Centrifugal air movers (often called snail or vortex fans) produce a focused, high-velocity airstream well-suited for directing air into wall cavities through small access holes, under cabinets, or into tight spaces. They are the workhorses of structural drying in most residential applications.
Placement follows IICRC S500 principles. Air movers are positioned at angles to wet surfaces to maximise the airflow boundary at the material surface — not just blowing air around the room.
Equipment: LGR Dehumidifiers
Low grain refrigerant (LGR) dehumidifiers are the standard tool for capturing moisture in professional water damage restoration. Compared to conventional refrigerant dehumidifiers, LGR units can remove moisture from air at lower humidity levels — which is important in the later stages of drying when the relative humidity has already dropped.
The moisture removed by dehumidifiers is collected as liquid water in a tank or drained continuously via a hose. Restorers record how much water is extracted daily — a declining extraction volume over successive days is one confirmation that the structure is drying.
Psychrometrics: The Science Behind Drying Decisions
Structural drying decisions are guided by psychrometrics — the study of air properties including temperature, relative humidity, dew point, and grains per pound (the actual mass of water vapour in the air). Restorers use psychrometric data to:
- Verify that the drying system is performing as expected
- Determine whether outdoor air can be introduced to assist drying (this depends on whether outside air is drier than inside air)
- Track progress toward target conditions
Daily psychrometric readings, recorded alongside moisture meter readings, form the documented evidence that drying is progressing correctly.
Drying Goals per IICRC S500
S500 establishes drying goals in terms of target moisture content — the level a specific material must reach to be considered dry. This varies by material: wood framing has a different target than drywall, concrete, or other substrates. Targets are set relative to a dry reference reading from an unaffected area of the same material.
A restorer who declares a job complete because the floor surface feels dry, rather than because moisture readings have reached target, is not following the S500 standard.
When Wall Cavities Must Be Opened
If moisture readings confirm that wall framing or insulation inside cavities are elevated, the cavity must be accessed to allow drying. This is done either by cutting access holes in the drywall (typically near the base plate) and inserting injecti-dry or cavity drying systems, or by removing a section of drywall. Removing wet insulation is usually necessary — saturated fibreglass or cellulose batts cannot be dried in place effectively.
Opening walls is not a sign that the job is worse than expected. It is a sign that the restorer is doing the job correctly.
Content prepared by Aquex, AI research assistant for Flood Damage Experts. Sources: IICRC S500 Standard for Professional Water Damage Restoration.