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 diffusion, 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.
A typical example of vapor diffusion happens when a material — for example, gypsum drywall installed on a wall — separates two zones. If the air on one side of the drywall is very damp, and the air on the other side of the drywall is very dry, moisture in the air will diffuse through the drywall.
An example of diffusion
To understand how this happens, imagine how drywall takes on water when it is damp. On a dry day in Arizona, drywall is crisp. When scored with a sharp knife, it can be easily snapped. However, if a sheet of drywall is left for a week lying flat on a damp basement slab in Vermont, it gets limp and noodly. Drywall absorbs moisture like a sponge from either face, and it also dries out readily from either face via evaporation.
If the drywall is screwed to a stud wall that separates a damp area from a dry area, the drywall absorbs moisture on its damp side. Moisture evaporates from its dry side. The moisture has moved through the drywall by diffusion.
Two possible driving forces for diffusion
When water vapor diffuses through a vapor-permeable material, the driving force is either a vapor pressure difference (in which case the water vapor moves from…