Vapor Barrier

What is Vapor Barrier?

Definition of Vapor Barrier in Construction

A barrier that is designed to stop the migration of moisture from a warm space into a cold space or cold system. Warm air is capable of holding more moisture than cold air. Therefore when warm air saturated with moisture encounters a cold surface, or cold air, the moisture in the warm air condensates out. This condensation results in water build up on the colder surfaces. It is essential when building and installing insulation and vapor barriers that the barriers are placed on the warm side of the system. The vapor barrier is designed to stop the water contained in the warm air from meeting the cold surface or cold air. If the water cannot reach the cold air, then there will be no condensation, no mold and no water buildup. Polyethylene is the most common vapor barrier used for this purpose, although different composites have been manufactured to also serve this purpose. Vapor barriers of various products, created by various manufacturers, are available to contractors, architects and engineers. The proper placement of the vapor barrier is extremely important to its functional success and errors in placement have destroyed buildings. Closed cell, spray foam, of a minimum of 2 inches is a vapor barrier unto itself. However, open cell spray foam is not considered a vapor barrier and a second application is required of a vapor barrier element to complete the proper assembly of the system. Vapor barriers are found everywhere in construction, from under the slab on grade, to the interior surface of an exterior wall, prior to the sheetrock. T he proper placement of a vapor barrier as well as the proper creation of an air space within the system is essential for the elimination of moisture and the build- up of condensation.