Three control layers regulate airflow, water penetration, and water vapor diffusion. An air barrier is a building control layer that resists air leakage between indoors and outdoors. A water resistive barrier (WRB) is a control layer that prevents water intrusion from outdoors. A vapor retarder is a material that resists the diffusion of water vapor through a building assembly.
Air leaks carry water vapor through wall cavities causing condensation during the winter. Warm, moist air from the home can carry moisture rapidly from a moisture source into the building cavities where damaging condensation can occur. Air barriers are important to stopping the flow of moist air into building cavities. Airflow through leaks is a much bigger moisture problem than water-vapor diffusion.
Also, air barriers reduce hot and cold air from affecting indoor comfort and energy consumption. Air barrier materials include plywood, OSB, and drywall. Brick, concrete block, and fibrous insulation are very porous to airflow. Using air barrier materials and sealing all penetrations in the surface of the building creates an effective air barrier
Vapor barriers or vapor retarder’s have a different function — they resist vapor diffusion, which is a slower process than moisture leaking in and out with moist air. We say that polyethylene is a vapor barrier because hardly any water vapor can penetrate. Brick and concrete block let water vapor flow through them quickly. Then there are a variety of materials that create significant resistance to water vapor diffusion depending on the need. For example, different types and thicknesses of foamboard insulation provide a wide variety of resistance to water vapor diffusion. Vapor barriers can trap moisture so builders only use them in cold climates and sometimes in very warm and humid climates.
Many new homes employ polyethylene as a vapor barrier in court climates. Vapor barriers stop almost all water vapor diffusion. Energy specialists call materials like plywood and OSB vapor retarder’s in these materials resist vapor diffusion rather than stopping vapor diffusion completely.
The building codes require a water resistive barrier on the exterior of wood and steel-frame building assemblies. The water-resistive barrier can serve as both the air barrier and water barrier. Air barriers and water resistive barriers, like Tyvek, are vapor-permeable and allow water vapor to escape from building cavities to keep those building cavities dry.
The exterior air barrier, like Tyvek®, is a good option for existing buildings when you replace siding. Don’t neglect flashing around windows and doors, which if properly installed prevents water leakage into the wall cavity. Window flashing flaws have cost mistakes billions of dollars in damage to modern homes.
For more information on air barriers, vapor retarder’s, and water-resistive barriers see the following publications and online courses.