Vapor pressure describes the pressure associated with a vapor. Differences in water-vapor pressure can drive vapor diffusion through a material or assembly.
What this means at your property
Room temperature and moisture conditions influence how a wall or floor interacts with water vapor. This process is different from liquid water passing through a crack, even though both can contribute to a moisture problem.
How does this affect a repair decision?
Ask whether the investigation identified liquid entry, air leakage or vapor movement and how the proposed layers address that mechanism. A product described as waterproof may not answer every question about drying paths and vapor control.