LAS VEGAS — Understanding psychrometrics — the science of air and its relationship to moisture — is fundamental to effective structural drying and is the scientific foundation of the IICRC S500 standard. Here is a practical overview of the key concepts that drive drying performance.
The key psychrometric principle in structural drying is that warm, dry air can hold more moisture than cool, humid air. By introducing warm, dry air into a wet structure and removing the moisture-laden air, restoration contractors create conditions that accelerate the evaporation of moisture from wet materials.
Relative humidity (RH) is the ratio of the actual moisture content of the air to the maximum moisture the air can hold at a given temperature. When RH is high — above 60 percent — evaporation from wet materials is slow. When RH is low — below 40 percent — evaporation is rapid. The goal of structural drying is to maintain low RH in the drying environment.
Dehumidifiers remove moisture from the air by cooling it below its dew point, causing moisture to condense on the evaporator coil and drain away. The efficiency of a dehumidifier is measured by its Liters Per Day (LPD) rating, which specifies how much moisture it can remove from the air under standard conditions.
Air movers create airflow across wet surfaces, increasing the rate of evaporation by replacing the humid air layer at the surface with drier air. The placement of air movers — directed at a low angle across wet surfaces — is critical to maximizing drying efficiency. The IICRC S500 standard provides guidance on air mover placement for different drying scenarios.
