Vapor Diffusion

7/15/2020 (Permalink)

The movement of water vapor through vapor-permeable materials is the definition of Water Vapor Diffusion. Vapor diffusion happens through a solid material even when the material has no holes. A typical example of vapor diffusion occurs when a material separates two zones, such as sheetrock. Moisture will diffuse through drywall if one side of the wall is arid, but the other side is very damp. Sheetrock acts like a sponge if it is left in a highly moisturized area. If a room is dry, a sharp knife could easily cut through sheetrock and snap it; however, if a room is humid or damp, the sheetrock absorbs that moisture and becomes limp, much like a noodle. The difference in vapor pressure on either side of the wall could be the driving force for vapor diffusion. Moisture will look for dry areas to invade by diffusing through sheetrock from a damp region to a dry area on the other side of the sheetrock.