How restorative drying works (S500)

In plain English

The S500 frames drying as managing two things at once — the air inside the building and the moisture inside its materials and contents — which takes three skills: controlling the air's temperature and humidity (psychrometry), moving moisture through different materials, and speeding evaporation from their surfaces.

Drying starts after cleaning (Category 1) or after decontamination (Categories 2 and 3). The restorer manages three things against the drying goals: airflow, humidity and temperature. Two processes run at once in any wet material — water evaporating at the surface, and water moving from inside toward the surface — and each affects the other's pace depending on the material's porosity, permeability and moisture gradient.

Drying also goes through stages: a constant-rate stage, while the surface is still wet, then two falling-rate stages as less water reaches the surface. Fast airflow matters most early on; the S500 cites wood-industry research that 600 feet per minute or more across surfaces is generally enough then, and says about 150 feet per minute is generally adequate once drying slows. In the later stages, the job of the airflow is mostly to carry heat back to the wet material. Dense materials like plaster, hardwood, concrete and masonry do better with gentler airflow and a bigger vapor pressure difference (warmer material, drier air); blasting their surface can cause drying stress and actually slow moisture moving out from inside. Complex assemblies can be dried by forcing air through, between and across their layers.

There are three ways to set up a drying system. An open system uses outdoor air, with no dehumidifier, when the outside air is much drier than the inside — often at the start of a job, when evaporation is highest — sometimes with a heater to manage temperature. A closed system shuts the area off from outdoors and runs dehumidifiers; it gives the most control and keeps the building secure, and it is the choice when outdoor air isn't much drier or security, weather, energy loss or outdoor pollutants are a concern. The building's own HVAC can help, but isn't enough by itself. A combination system does both, often venting moist air early while debris removal and extraction are under way, then closing up and letting the equipment take over.

Pulling the work area under negative pressure can lower humidity when drier makeup air is available, but too much depressurization, or badly placed airmovers and air scrubbers, can back-draft a water heater or furnace — a carbon monoxide hazard — or pull contaminants in from a crawlspace.

Dehumidification for a closed or combination system is sized the same way as in the first response — materials, class and size of the area, weather, power, equipment and airflow — documented, and adjusted from readings. Once free water is gone from slow-drying materials, airflow can be reduced and the vapor pressure difference raised.

Airmovers keep dry air sweeping the wet surfaces — floors, walls, ceilings and framing, including cavities. The S500 recommends aiming them across open areas of the room and along the bottom of wet walls and the floor edge, pointed the same way with the outlet nearly touching the wall at a shallow angle of about 5–45 degrees, working around furniture and other obstructions. It starts with one airmover per affected room, then adds one for every 50–70 square feet of wet floor (which covers the floor and the bottom 2 feet of wall), one for every 100–150 square feet of wet wall and ceiling above 2 feet, and one for each wall inset or offset deeper than 18 inches. Dense build-outs, obstructions and the type of wet material push the count up or down within those ranges, and fractions round up.

Two shortcuts: where water has mostly wet the bottom of the walls and less than about 2 feet of floor, use one airmover per 14 linear feet of wet wall instead of the square-foot count (never both in the same room); and a closet or pantry under about 25 square feet may need just one, if the upper walls and ceiling are dry. In a Class 4 job, once the free water is gone, airflow can come down while heat or drier air goes up.

The setup is checked and adjusted from moisture readings no later than the day after drying starts, and normally every day after that until the drying goals are met — the scope of work, the risk of secondary damage, access or the parties' agreement can change that pace. Because drying throws dust and soil into the air, especially from carpet, surfaces are cleaned first; the S500 recommends air filtration devices where cleaning can't get it all, where air is being pushed through wall or floor cavities, or where occupants are high-risk.

Temperature matters too: the warmer the wet material, the more energy it has to evaporate water. Airmovers, dehumidifiers, portable heaters and the building's HVAC all add heat, and direct or indirect heat-drying systems can raise it further. High heat can damage building parts and contents, so the manufacturer's instructions and safety precautions have to be followed to avoid fire and occupant hazards, and restorers should know how room temperature affects their equipment — dehumidifiers, ventilation and heat-drying systems each work best within a certain range. Heat output is adjusted from monitoring readings like everything else.

The building's own HVAC can help if it won't spread contamination or work against the drying: heat speeds evaporation from surfaces and moisture movement inside materials, cooling keeps the space from overheating or lets people stay in it, and air conditioning pulls some moisture out of the air. But a home's HVAC is sized for the house's normal heat and moisture, not the load from a water loss. It isn't a dehumidification system, it can't create drying conditions on its own, and it may not lower humidity fast enough to prevent secondary damage.

Monitoring means recording temperature and humidity (outdoors, in affected and unaffected rooms, and at the dehumidifier outlets) and the moisture in materials, at least daily — more or less often depending on how wet things are, the risk of secondary damage, and where the job is and how hard it is to reach. A second visit on the first day can confirm the equipment is working. Meters vary, so the same meter should be used throughout the job, or the meters checked against each other, and readings taken at the same spots each time. If the readings don't show satisfactory drying, the plan changes — equipment moved, added or swapped, targets reset, or a little more material opened up. A changed drying goal is a change in the scope of work, and should be communicated and documented.

Equipment stays on until moisture meters show, and the restorer documents, that every material has reached its drying goal.

Key points

• Manage airflow, humidity and temperature against written drying goals.
• 600+ FPM early (constant-rate stage), ~150 FPM later (falling-rate stages).
• Dense materials: less airflow, more heat or drier air.
• Open, closed or combination system — chosen by outdoor vs indoor humidity, weather, security and logistics.
• Watch depressurization: back-drafting appliances means carbon monoxide risk.
• Airmovers along the base of wet walls at 5–45°: one per room, plus one per 50–70 sq ft of wet floor and per 100–150 sq ft of wet wall/ceiling above 2 ft.
• Check and adjust by the next day, then daily until dry.
• Warmer materials dry faster; heat drying follows the manufacturer's safety rules.
• The home's HVAC can help, but it is not a dehumidifier.
• Record readings at least daily, same meter and same spots; equipment stays until drying goals are verified and documented.

Official text

This topic is covered in our own words because the standard behind it is copyrighted; we cite it below rather than reproduce it.

Citations

• IICRC S500, ANSI/IICRC S500 Standard for Professional Water Damage Restoration — Sec. 5, psychrometry and drying technology, p. 28. Best practice
• IICRC S500, ANSI/IICRC S500 Standard for Professional Water Damage Restoration — Sec. 12.5, drying after cleaning or remediation, p. 66. Best practice
• IICRC S500, ANSI/IICRC S500 Standard for Professional Water Damage Restoration — Secs. 12.5.1–12.5.3, open, closed and combination drying systems, mechanical dehumidification, controlling airflow, pp. 66–69. Best practice
• IICRC S500, ANSI/IICRC S500 Standard for Professional Water Damage Restoration — Sec. 12.5.4, controlling temperature to accelerate evaporation, pp. 69–70. Best practice
• IICRC S500, ANSI/IICRC S500 Standard for Professional Water Damage Restoration — Secs. 12.5.5–12.5.7, using the installed HVAC, ongoing monitoring, verifying drying goals, pp. 70–71. Best practice

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