A vapor barrier is an impermeable membrane primarily used to resist water vapor transmission from the soil to the concrete slab. The term ‘vapor barrier’ is often used interchangeably with the term ‘vapor retarder’ to describe all membranes used to resist water vapor transmission.
However, vapor retarders only retard the transmission of water vapor, whereas, vapor barriers are impermeable to water vapor. Therefore, the most important criteria used when specifying a vapor barrier is resistance to water vapor transmission, also known as its permeance value.
Vapor barriers are traditionally specified by architects and engineers to limit the amount of moisture that migrates into and upward through concrete slabs. Moisture infiltration through concrete slabs has been known to cause flooring system failures, damage to the concrete slab, and growth of mold and mildew due to higher humidity levels within the building.
More recently, vapor barriers have been used at brownfields redevelopment sites. For sites where the source of soil or groundwater contamination cannot be completely eliminated, vapor barriers are used to prevent vapor intrusion of volatile organic compounds (VOCs) into newly constructed buildings. Additionally, vapor barriers are used in high radon potential areas to prevent the migration and accumulation of radon gas in buildings and homes.
Given the many applications, a vapor barrier is one of the most critical building components used to prevent indoor air quality issues and minimize moisture-related concrete slab and flooring system failures. Additionally, installation of a vapor barrier may help to contribute to LEED credits for buildings seeking to be certified under the U.S. Green Building Council (USGBC) for Leadership in Energy and Environmental Design (LEED).
Vapor Barrier Materials
For the purpose of this discussion, vapor barrier materials will be limited to sheets of membrane materials, though a vapor barrier can be any unbroken surface that is impermeable to water vapor such as spray-applied asphalt/latex. Membranes are most commonly made from high density polyethylene (HDPE) or other polyolefin-based resins. These materials have high tensile strength and high puncture resistance. HDPE tends to have the highest chemical resistance among polyolefin membrane.
Location of Vapor Barrier
Studies have shown that vapor barriers can affect the behavior of the concrete slab and significantly influence finishing time, cracking, and strength. Architects, engineers, and contractors therefore often disagree on whether concrete should be placed directly on the vapor barrier or on a granular base placed over the vapor barrier. There are risks and benefits associated with both options, and they depend primarily on the water-cement ratio of the concrete mix.