A Vapour Barrier refers to any material, typically a plastic or foil sheet that resists diffusion of moisture through wall, ceiling and floor assemblies of buildings. Technically, many of these materials have varying degrees of permeability. A plastic membrane is laid under the slab to improve its performance against rising damp. This is also called a 'moisture barrier' because it stops the moisture from the soil migrating into the concrete slab. This must be placed under the slab for all areas where there are to be habitable rooms.
Water vapour moves into building cavities by two mechanisms: diffusion through building materials and by air transport (leakage), which is usually far more significant and problematic.