Polythene sheets are used during building construction to prevent structural damping that occurs when warm, moist air penetrates inside a concrete slab, wall, roof or floor structure reaches the dew point, and then condenses into liquid water.The warm, damp air may penetrate a wall from the inside of a heated building in a cold climate or from the exterior of an air-conditioned building in a warm climate..

This condensation may allow mold growth, rotting of timber, corrosion of metal components and reduction in the thermal insulation’s effectiveness.

The resulting structural damage may occur without visible indications until significant damage has occurred.

The most effective method of controlling damping is using an impervious vapor-barrier on the warm side of the structure.

It has an extremely low water permeability rating of typically 1.7SI Perms.

It can be used in between the insulation material and structure surface to act as a damp-proof membrane.

On site concrete slabs employ a 1000 guage (250 microns) polythene sheet laid underneath the concrete to prevent the moisture from being absorbed.