The incorrect use of vapor barriers is a serious issue in building design and construction. A vapor barrier is a Class I vapor control layer; it prevents the migration of water vapor past the point of its placement in an assembly. This can stop assemblies from getting wet (water vapor can’t get in past the vapor barrier), but it can also stop assemblies from drying (water vapor can’t get out past the vapor barrier).
The articles in this section discuss where and how to use vapor barriers effectively. Specific topics include the distinction between air barriers and vapor barriers, the critical importance of climate in wall design, and the different classes of vapor control layers in the International Building Code.