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.
Low-permeance vapor barriers are widely used on the interior of wall and roof systems in large parts of North America. Many codes and standards imply or even state that low-permeance vapor barriers should be used in all cold regions as well as many moderate climate zones. The influence of vapor barriers on the hygrothermal performance of wall and roof systems is a function of exterior climate, interior climate, solar absorptance, rainwater absorption, and the vapor and thermal resistance of all of the layers in the system. In many practical situations, a low-permeance vapor barrier will not improve hygrothermal performance and may in fact increase the likelihood of damaging condensation or trap moisture in the system. This paper will examine the role of vapor barriers on hygrothermal performance with the aid of simple and transparent diffusion calculations supported by measurements from full-scale natural exposure monitoring. The phenomenon of summertime condensation, the drying of roofs and walls, and multiple vapor barrier layers will be explored. The importance of properly assessing both the interior and exterior climate will be discussed. Vapor diffusion control strategies will be presented.
