SYSTEMS/STUD WALLS
Air Barriers and Vapor Retarders
An air barrier system is required in metal stud infill walls to control air leakage. The air barrier can
be located on either side on the studs. If the air barrier is located on the inside of the studs, e.g.,
the interior gypsum is the air barrier, the air barrier must be sealed to the floor slab, windows and
other penetrations to maintain continuity. This approach has several advantages including that the
air barrier is kept at a relatively constant temperature and is therefore less susceptible to opening of
cracks and degradation of sealants due to temperature cycling. An interior air barrier is also easier
to inspect and repair during construction. However, an interior air barrier is more susceptible to
puncture during the installation of services in the wall. An air barrier located outside of the studs will
be protected from such construction activities and can be carried continuously over the floor slab.
On the down side, long term maintenance and repair of an exterior air barrier is almost impossible.
Therefore, high quality materials and construction must be employed. Unless an additional layer of
insulation is included outside of the air barrier, the air barrier material will be outside of the
insulation and subjected to outdoor temperature cycling. The positioning and water vapor
permeability of an outer air barrier must be considered with reference to the whole wall's water
vapor transmission characteristics.
The vapor retarder location must be based on consideration of the climate and the total wall design,
as it needs to be kept relatively warm to control condensation. In heating climates the vapor
retarder needs to be inside of most of the insulation, and an interior air barrier can also serve as the
vapor retarder. If an outside air barrier is used, it must be sufficiently permeable to water vapor so
as not to constitute a second vapor retarder. In cooling climates, the vapor retarder needs to
located outside of most of the insulation and can be readily combined with the air barrier system.
Care needs to be exercised in cooling climates when using low permeability interior finishes, since
these surfaces will be cold and will act as vapor retarders. A much lower permeability vapor
retarder needs to be used outside of the insulation, and air leakage must be controlled on the warm
side of the insulation.
Wall details that depict the installation of a continuous air barrier system for brick veneer systems
are shown in the following section. Many of these concepts can be applied to metal stud walls with
other exterior finishes.
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