Which mechanism describes moisture movement due to vapor pressure differences across materials?

• A. Airflow
• B. Capillary action
• C. Diffusion
• D. Condensation

Answer: (C)

Moisture movement caused by differences in vapor pressure across materials is diffusion of water vapor. Water vapor molecules migrate from regions of higher vapor pressure to regions of lower vapor pressure, traveling through or across materials even without bulk air movement. This vapor diffusion is driven by the concentration (vapor pressure) gradient and is described conceptually by Fick’s law, where the moisture flux follows the gradient in vapor pressure.

This differs from airflow, which would carry moisture with moving air; capillary action involves liquid water moving through pores due to capillary forces, not vapor diffusion; and condensation is the phase change from vapor to liquid, which can occur when vapor pressure exceeds local saturation but does not describe the transport mechanism itself.