A vapor barrier is an impermeable membrane which blocks the flow of air through the building envelope. Water vapor moves into building cavities by two main mechanisms i.e. diffusion through building materials and by air transport. Vapor barrier is very important part of building envelop and resists the passage of the water vapor that is contained in air. Technically, most materials are only vapor retarders as they have varying degrees of permeability for vapors. For maximum output both vapor barrier and insulation should be installed as both affect each other. Vapor barriers are recommended to keep moisture from penetrating and condensing in the insulation. Excessive moisture can damage framing, siding or drywall. An insulation that fills the wall cavity completely (such as cellulose or foam) can help prevent moisture problems. Most buildings require a vapor barrier for external wall. Vapor barriers are very important for a building as they resist moisture build up inside building and stops rotting of materials and avoid growth of harmful organisms like moss, mildew and molds.