Air Barriers and Air/Vapor Retarders
Air barriers are used to block random air movement through structural cavities while at the same time allowing the moisture in the air to diffuse back out. Some common materials used as air barriers are fibrous spun polyolefin (commonly called house wrap), gypsum drywall board, plywood, and foam board. Air barriers are installed to the outside of the structure, with all joints sealed with tape.
House wrap materials react poorly with the wood lignin in certain types of exterior wood sidings, such as cedar, redwood, and manufactured hardboard siding. Substitute 30-lb impregnated paper for the house wrap in these applications. It will not react to the wood lignin.
Air/vapor retarders are materials designed to control simultaneously both air movement and water vapor transmission. Air/vapor retarders are very common in regions with hot, humid weather, such as that common in the southern United States. An air/vapor retarder can be a single material, but it is more commonly a combination of polyurethane plastic sheets, builder’s aluminum foamboard insulation, and other exterior sheathings. Like the house wrap, it is applied to the exterior of the structure, with all joints carefully sealed.