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Another example would be an exterior insulation wall assembly, located in the same climate. An important characteristic for this type of wall design is that the wall cavity is considered part of the conditioned interior space, therefore it relies on HVAC, and not on vapor diffusion, for removal of incidental moisture. The air barrier in this assembly is typically sandwiched between the rigid insulation and the exterior sheathing. Therefore, it would be located on the warm side of the insulation. As a result, this assembly design could use either vapor permeable or vapor non-permeable air barrier membranes.

Exterior insulation wall with air Barrier

BEST PRACTICES

The previous examples are in cold climates; however, a similar analysis must be performed for other climates. A common mistake is importing building practices from one climate to another, without a fundamental understanding of building physics.

For example, vapor barriers have been introduced in Canada to prevent interior moisture from migrating into the wall assembly. “While this is a good practice for cold climates where the inside air is warmer and wetter than the outside, the use of vapor barriers in mixed or hot and humid climates has probably contributed to moisture problems in recent years,” says Spinu, noting that this is further aggravated by the practice of using vinyl wall paper as an interior finish in these climates. Such practices often lead to double vapor barriers. “Fundamental physics must be understood in order to avoid moisture problems. Vapor barrier use in mixed or hot and humid climates has been challenged, and the codes are being reviewed,” Spinu adds.

The 2006 IBC code will delete section 1403.3 from the 2003 IBC version (on Vapor Retarders). The vapor retarder requirements will be referenced to the IECC code (International Energy Conservation Code). The major change is that vapor barriers will no longer be required in certain constructions, for climate zones 1, 2, 3, 4A, and 4B.

DOE proposed climate zones. The climate zones 1,2,3,4A, and 4B will no longer mandate vapor barriers (below the black line).

An extended discussion of all climate zones is beyond the scope of this article. However, as a general rule, vapor permeable air barriers can and should be used in all climates and all wall designs. Vapor non-permeable air barriers, which are air and vapor barriers, must be located on the warm side of the thermal insulation, and their use should be climate specific.