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.