Architects note that installation is key, especially in residential applications. The energy benefit of the house wrap is to minimize air infiltration along sheathing joints and into framing cavities. Literally unfolding the roll around the house in a full nine-foot length provides a seamless wrap that then has door and window openings cut into it. "I get frustrated by contractors who insist on cutting up the membrane into small pieces. This not only defeats the purpose, it creates the opportunity for air and moisture infiltration at each seam," says Arsenault, who has been involved in energy efficient-housing since 1980. "Even if it is seamed properly with 18-inch overlaps and seaming tape as the manufacturer suggests, that approach is more costly and still prone to installation errors. This is a sophisticated product that warrants proper installation and attention to detail to be effective and avoid unnecessary problems."

When selecting an air barrier it is important to understand how it will perform in each system and under specific environmental conditions. While the air barrier must protect against air leakage, it should also enhance the ability of wall assemblies to manage incidental moisture intrusion. There are two key considerations to keep in mind when selecting an air barrier system for a building enclosure: wall assembly design, and climate type.

Wall Assembly Design

There are basically two types of wall assembly designs, based on the location of thermal insulation within the wall assembly.

Interior vs. Exterior Insulation

The interior insulation wall design has the thermal insulation within the wall cavity, that is, the batt insulation within the stud cavity. In the exterior insulation design, the thermal insulation is placed outside the wall cavity, with rigid insulation outside of the exterior sheathing. The exterior insulation wall, which is more commonly used in cold climates, provides two main advantages: it eliminates the thermal bridging (which is significant in steel stud construction) and it moves the dew point outside the interior wall cavity, thus avoiding condensation within the interior wall. A combination wall design includes both batt insulation and rigid exterior insulation.

Climate type

Climate is a very important design factor when considering air barrier selection. While the building physics principles are universal, they must be applied to local environmental conditions, which are climate specific. North America has eight climate zones that could be considered as the intersection of the three major climate types, dry, humid, and marine, with eight thermal bands that progress northward from very hot to very cold.

Many condensation and moisture problems may be the result of importing building practices from one climate to another, without considering the consequences of the local climates, such as changes in moisture loads and vapor pressure drive. Building practices must address climate-specific loadings on the building enclosure. An extensive discussion on the climate specific design is beyond the scope of this article. However, the wall assembly design and climate type affect the choice of air barrier.