Masonry Magazine July 2007 Page. 44
Moisture Management
The WRB, flashings and the drainage system should be considered as a system of various components that work together to prevent water penetration through the exterior walls.
The IECC defines a vapor retarder as: "A vapor-resistant material, membrane or covering such as foil, plastic sheeting, or insulation facing having permeance rating of one perm (5.7 X 10⁻¹¹ kg/PA.s.m²) or less when tested in accordance with the desiccant method using Procedure A of ASTM E 96. Vapor retarders limit the amount of moisture vapor that passes through material or wall assembly."
Although vapor permeance does not play as significant of a role in moisture control as air movement, the use of a vapor retarder in an exterior wall assembly is required by many model building codes, including IBC [Required by IECC, which is referenced in IBC]. In its 2006 version, the IECC requires the use of vapor retarders in cold climates with certain exceptions [Exceptions include Climate Zones 1 through 3, cases where moisture or its freezing do not damage construction materials, and where other means of condensation control in unventilated walls is provided).
It should be noted that a vapor retarder also can act as an air barrier if all of its penetrations and terminations are sealed properly.
Weather-resistive Barriers
WHILE VAPOR RETARDERS and air barriers are intended to control moisture condensation within wall assemblies, weather-resistive barriers (WRBs) are intended to prevent the penetration of liquid water through the exterior walls. As such, they typically are made of materials that can resist water and are not damaged by exposure to water.
With the exception of certain types of walls, such as solid concrete or masonry walls, the IBC requires the installation of a WRB in exterior wall assemblies with drainage provisions. Such WRBs should be integrated with the mandated flashings to provide for "a continuous weather-resistive barrier behind the exterior wall veneer."
There are many types of WRBs, with the most traditional type being asphalt-saturated felt installed in a shingle fashion. Many of the air barriers available on the market today also can act as a WRB.
The WRB should be placed within the drainage cavity of a wall - typically attached to the back-up material - and integrated properly with the flashings and drainage system at various locations within the wall; the proper integration is critical for providing good resistance to water penetration. The WRB, flashings and the drainage system should be considered as a system of various components that work together to prevent water penetration through the exterior walls. One method to envision proper installation of the weather-resistive system is to consider the exterior cladding as only a decorative layer, such that if the exterior cladding material is removed, water still could not penetrate the exterior walls.
In masonry cavity construction with CMU back-up, incorporation of a WRB on the back-up is important. While the IBC does not state clearly that CMU back-up is required to have a weather-resistive layer, it is the authors' opinion that CMU is not a weather-resistant material and can absorb large quantities of moisture that in turn can harm other building components, including interior sheathing and insulation materials.
Proper Locations for Air Barriers, Vapor Retarders and Weather-resistive Barriers
LOCATING THE AIR BARRIER, vapor retarder and WRB within a wall assembly should be performed by the designer. The proper location of these components is critical in thermal and moisture performance of the building envelope. Given the changing requirements of model building codes, building envelope designers should be thoroughly familiar with the building code requirements and industry standards regarding these components.
In addition, designers should be aware of the different performance characteristics of various materials that are marketed as air barriers, vapor retarders and WRBs. As previously mentioned, there are many materials that can function as all three, or two out of the three.
In order to properly evaluate the performance of exterior walls that incorporate air barriers, vapor retarders and WRBs,