Air barriers are intended to restrict the flow of air through openings in the walls, ceilings, and floors of a building, while vapor barriers are designed to restrict the flow of water vapors through the same openings. In general terms, air barriers attempt to block incoming air while vapor barriers attempt to block outgoing air. Since air typically carries a significant amount of moisture, air and vapor barriers actually work together in an effort to keep moisture out of houses, buildings, and other structures.
Air typically enters a house or building through gaps in walls, ceilings, and floors. During warm weather when air conditioning is in use, this moist incoming air can become trapped in cavities between exterior and interior walls resulting in undesirable moisture condensation on sheathing and framing members. Air barriers are typically used to prevent this unwanted condensation from occurring. An air barrier may be composed of a continuous sheet of plastic, a spray-type foam insulation, or even a combination of latex caulk and paint.
Water vapors, on the other hand, are usually present on the inside of a house or building and do most of the work during cold weather. Since the air is usually much drier outside, this moist air attempts to escape to the outside by means of diffusion through walls, ceilings, and floors. Diffusion is a term used to describe the transfer of moisture through minute pores in various types of building materials.
As these moist vapors move through the pores of the building materials, condensation occurs in the cavities between the exterior and interior walls. A vapor barrier is needed to prevent this method of condensation from occurring. Vapor barriers are only marginally effective however, since they are only slightly less porous than the building materials they are intended to protect.
Although many local building codes require the use of both air barriers and vapor barriers in new construction, some building codes do not make any distinction between the two. There is however, some controversy among researchers as to whether a vapor barrier is actually even necessary. Some studies have shown that air leakage is often far more responsible for transferring moisture than diffusion is. Many experts believe that the presence of a good air barrier may be more effective against unwanted moisture transfer than a vapor barrier. Until a definite conclusion is reached regarding this subject, a combination of both of these types of barriers may offer the best overall protection against undesired moisture transfer.