Building scientists talk about several different moisture transport mechanisms. Most of these mechanisms — for example, water entry due to a roof leak — are easy to understand. Other transport mechanisms, like vapor diffusionMovement of water vapor through a material; water vapor can diffuse through even solid materials if the permeability is high enough.
, aren't quite as intuitive.
First, some basic definitions. Water vapor is water in a gaseous state — that is, water that has evaporated. It is invisible.
Water vapor diffusion is the movement of water vapor through vapor-permeable materials. Vapor diffusion happens through a solid material even when the material has no holes.