In order to choose the appropriate barriers for building projects, the following guidelines apply:

Water-Resistant Barriers are critical for most climates to prevent bulk water intrusion. They are typically installed behind the cladding, towards the exterior side of the building envelope. If the water-resistant barrier is vapor permeable, it will not interfere with the condensation control strategies.

Air Barriers are critical for most climates to prevent heating and cooling energy loss, as well as prevent moisture transported by air currents. The location of an air barrier within the building envelope is not important from the standpoint of controlling air leakage, because an air barrier can be located anywhere in the wall assembly. Air barriers are located towards the exterior of the building envelope for ease of constructability and continuity. If the air barrier is vapor permeable it will not interfere with the condensation control strategies. A single membrane is generally used to perform both air barrier and water barrier functions.

Vapor Barriers should only be used in those climates where diffusion into the wall cavity occurs predominantly in one direction. The vapor barrier should be located on the high vapor pressure side of the envelope. In climates dominated by heating, this means that the vapor barrier should be located toward the inside, and in cooling climates toward the outside.

A single membrane can perform all three functions, and serve as an air, water, and vapor barrier. When a membrane is intended to be used as a vapor barrier, in addition to an air and water barrier, it must be determined if the typical location of air and water barrier, such as on the exterior of the building envelope, also meets the condensation control criteria for a vapor barrier, that is, if the vapor barrier is on the high vapor pressure side of the building envelope.

Every time that a vapor non-permeable membrane (such as vapor non-permeable peel and stick or fluid applied membrane) is used with insulated stud cavity wall design, conditions for condensation are created in most North American climates. In predominantly heating climates, a vapor non-permeable air and water barrier installed on the exterior side of the building envelope (the preferred installation for air and water barriers) creates conditions for condensation all year round. In U.S. climates with both heating and cooling seasons, this creates conditions for condensation during the heating season. Section 502.4.3 of the ICC, Sealing of the Building Envelope, requires a moisture vapor-permeable wrapping material, in order to avoid potential condensation. The requirements and properties of vapor barriers must not be confused with those of air and water barriers.

Factors to consider when selecting barriers for projects include the following:

1. Water and air barriers are critical for controlling the most important moisture sources in buildings and should be used in most climates. As long as these membranes are vapor permeable, their typical location, at the exterior of the building enclosure, and under the cladding, will not interfere with the condensation control strategies.

2. Diffusion is essential for drying, and a diffusion pathway must be allowed. The preferred diffusion direction is climate specific. Many U.S. climates require diffusion pathways in both directions, toward the inside and toward the outside.

3. Vapor barriers should only be used in climates where they do not interfere with diffusion drying, such as predominant heating or cooling climates. In many U.S. climates, the vapor barrier would be located on the wrong side of the wall assembly during the heating or cooling season.

4. If an air and water barrier is also a vapor barrier, its location in the building envelope is determined by the vapor barrier function and must follow the vapor barrier requirements for condensation control. In many U.S. climates (except for predominantly heating or cooling climates) such membranes should be avoided. Air and water barriers should be vapor permeable.