Masonry Magazine July 2007 Page. 46
Moisture Management
Sophisticated computer modeling tools can be used to better evaluate the potential for condensation within wall assemblies, and to predict moisture accumulation.
Simple dew point analysis may not suffice. More sophisticated computer modeling tools can be used to better evaluate the potential for condensation within wall assemblies, and to predict moisture accumulation. The use of such sophisticated modeling tools is more critical when specifying new wall assemblies without long-term track records, or when specifying building envelope assemblies for special-use buildings, such as cold storage facilities or swimming pools. Buildings located in severely cold, or hot and humid climates also should receive special attention during the design phase.
Although there are countless variations of exterior building assemblies that can perform satisfactorily in various climate conditions, the authors recommend considering a particular wall assembly that can perform well in all climate conditions, depicted in Figure 1. As shown, a material that can perform as an air barrier, vapor retarder and WRB- many products such as self-adhering rubberized asphalt sheets or liquid-applied membranes and some mechanically-attached sheet membranes can perform this function - is installed on the exterior face of the back-up material and is integrated with all wall flashings. Moisture-resistant insulation, such as extruded polystyrene, is then placed on the outside of this layer. The drainage layer and cladding are then placed on the outer side of the wall assembly.
While this wall assembly can add to the overall thickness of the wall, it presents several advantages. One major advantage is that the vapor retarder, WRB and air barrier are all combined into one material. Also, this wall assembly does not pose any risk of moisture condensation or accumulation regardless of the climate zone.
As previously mentioned, many other types of wall assemblies can provide adequate moisture performance. However, they are more susceptible to moisture condensation than the example provided. For instance, in cold climates, the vapor retarder typically is placed on the warm side of the wall, directly behind the interior finishes. During cold weather, warm and moist interior air is driven toward the exterior, but is stopped by the vapor retarder. Since the vapor retarder is on the warm side of the insulation, the moist air will not condense. On the other hand, during warm weather, the warm moist exterior air is driven toward the interior. Again, the vapor barrier stops the movement. However, since is it now on the cold side of the insulation, condensation can form within the wall. In this case, it is important that any weather-resistive barrier or air barrier placed over the exterior sheathing be permeable (not act as a vapor retarder), to allow water vapor to escape the wall cavity.
For these reasons, proper placement of the vapor retarder, air barrier and WRB in a wall is critical.
Summary
WITH THE EMPHASIS ON thermal performance of building exterior walls, the proper design, selection and installation of vapor retarders, air barriers and WRBs is important. While these three components are intended to serve three, distinct functions, they can be combined into one or two materials depending on the properties of the materials. However, please note that improper selection or placement of these materials can lead to moisture failure.
The proper placement of air barriers, vapor retarders and WRBs requires an analysis and understanding of the moisture movements through various wall components. The appropriate selection of the materials for these components also requires a thorough understanding of each material's physical properties.
Kami Farahmandpour is the principal of Building Technology Consultants PC. His expertise is concentrated in the evaluation and repair of building envelopes, including various types of exterior walls, waterproofing systems and roofs. Among his many professional activities, he is currently serving a three-year term as an associate director of the Sealant, Waterproofing and Restoration Institute (SWR Institute). He also is the co-author of "A Practical Guide to Weatherproofing of Exterior Walls, developed by SWR Institute.
Joshua J. Summers is a principal structural engineer at Building Technology Consultarits PC. He has evaluated and developed repair designs for numerous masonry building components. These projects have included both solid and cavity wall construction with brick, CMU, terra cotta, limestone and clay tile materials.