What is the term for the force exerted by water molecules in the air on...

Vapor pressureis defined in the IICRC WRT body of knowledge as the force exerted by water vapor molecules in the air against surrounding surfaces. It represents the energy level of moisture in the air and is a key driver of moisture movement.

The WRT manual explains that water vapor moves from areas of higher vapor pressure to areas of lower vapor pressure, whether between materials and air or between different air masses. This principle governs evaporation, condensation, and moisture redistribution within a drying chamber.

Relative humidity describes a percentage relationship, humidity ratio measures moisture mass, and dew point identifies saturation temperature—but vapor pressure quantifies the actualdriving force. Because vapor pressure is directly influenced by both temperature and humidity ratio, it is considered one of the most precise indicators of drying potential.

Effective drying systems focus on lowering air vapor pressure relative to wet materials, ensuring continuous moisture migration out of structural components.