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Advances in Moisture Control
Unless the specified cladding or wall system is designated as a true barrier system, we must anticipate and plan for the inevitability of moisture intrusion. Remember, a barrier wall assembly is dependent upon the weather-tight integrity of the cladding as well as construction joints to prevent moisture intrusion. These systems are generally precast concrete panels or intricate glazing systems that can be expensive to construct and to maintain. When these systems fail, typically at the joints, there is no backup plan for moisture intrusion. A breach in a barrier system can result in costly repairs to the exterior and interior components as well as structural components that may have been compromised.
Cavity wall systems continue to be the most versatile, popular, and cost-effective wall systems. As with all wall systems, un-anticipated moisture intrusion is the greatest threat to their sustained performance. Over the years there has been a number of Water Resistive Barrier products specifically developed to prevent damage to the building structure as a result of moisture intrusion.
From the veritable felt papers and grade D papers, to house wraps and enhanced house wraps, to fluid applied air/moisture barriers, the industry continues to develop products that out perform their predecessors. Depending on the type of climate and the exterior cladding, grade D papers may be perfectly acceptable, but if the climate is generally described as wet or humid, and wind-driven rain is the norm rather than the exception, more robust WRBs should be considered.
Cladding types should also be taken into consideration. Wood sidings and/or wood systems are highly susceptible to moisture penetration and absorption. Stucco and stone are somewhat porous and also susceptible to some moisture penetration, whereas vinyl sidings are non-absorbent but can trap moisture that comes between the siding and the WRB.
Regardless of the type of WRB that is specified, there must be a means of allowing moisture to drain from the cavity. Remember, we all know that if there is moisture present, and there always is, it will eventually find its way into the cavity of the wall assembly.
The rainscreen systems have proven to be the most effective method of providing enhanced drainage capabilities in a cavity wall system. Over the years, the British Columbia Wall and Ceiling Association (BCWCA) and the Northwest Wall and Ceiling Bureau (NWCB) have commissioned and performed extensive testing on traditional stucco assemblies as well as rainscreen stucco assemblies. The two groups have concluded that although the traditional two layers of grade D paper creates a drainage space and performs adequately in most environments, a rainscreen that provides a defined drainage or air space is more likely to afford additional protection and value to the project. Initially these rainscreen systems utilized 1 x 4 wood furring strips or light-gauge metal Z-furring to create the space.
Although rainscreen systems performed quite well, they came at a cost. If not mandated by code, the rainscreen methods often became a victim of value engineering. In addition to the added cost of the rainscreen furring method there were also concerns over the potential failure of wood products and for thermal breaks when using light-gauge metal furring.
In response to the need to create a defined space at a more reasonable cost, the industry introduced what is referred to as a drainage medium or drainage mat. These come in a variety of designs such as dimpled polyethylene, channeled polystyrene, or polymer entangled filaments, and in varying thicknesses from a nominal 4.75mm or 3/16-inch to 3/4-inch or 10mm to meet the CBC. These products are more cost effective and provide a continuous or un-interrupted cavity that promotes enhanced air circulation and drainage capabilities. Although these products are relatively new to the wall and ceiling industry, the design community has recognized their value and are specifying their use on more and more projects.
As with any system, these products are only as good as their installation. All of the manufacturers provide concise installation instructions and detailed drawings for the application of materials around doors and windows, at pipe or vent penetrations, and at the head and base of wall. If the use of drainage mat products are new to you, insist that the installation instructions and CAD details accompany your purchase and then follow the instructions to the letter.
I was recently asked to perform an inspection on a 14,000-square-foot home and was pleased to learn that the design had included a drainage mat. I was then surprised when the contractor told me that some of the walls on this six-year-old home were rotting away. As the saying goes, the devil is in the details. In Figure 1 below you can get a general idea of the design. The wall assembly was standard 2 x 6 with plywood sheathing, housewrap, drainage mat, and traditional three-coat stucco. A number of stone-like architectural features were utilized as well as cultured stone at the base of the stucco assembly.
Figure 2 below shows the base of the stucco wall as it terminates above a very nice EPS architectural stone feature. As you can see, there is an appropriate gap between stucco and stone but the problem is that there is no flashing to divert the water away from the exterior or to prevent wind driven rain from getting into the gap or being siphoned into the gap by negative air pressure. To complicate matters more, the WRB terminated at the base of the stucco and plywood was visible in the gap.
Had the contractor or the designer reviewed the CAD drawings available by the manufacturer, they would have noted that a rigid flashing is required at the base of the wall or between differing elements. The problems arise from wind driven rain, or negative pressure eliminated on the back of the home. In Figure 3, you can see that the gap between the bottom of the stucco wall and the architectural stone is non-existent. In fact the gap has been sealed with caulk. Again, the devil is in the details. By sealing the space between architectural stone and stucco, the contractor has eliminated any avenue for moisture to drain from the wall assembly.
If the contractor had complied with the manufacturers’ details, the water, entering at the head of wall from an improperly installed parapet cap, could have drained from the system without compromising the structure. As it is, we have a project that took into consideration the potentially devastating effects of water intrusion and specified a proven product to mitigate the effects of water intrusion only to have the system fail.
Drainage mats are an excellent product that will benefit not only the stucco industry, but the wall and ceiling industry in general. The reputation of the industry and your livelihood depends in great part on the individual contractor’s performance.
Do your part by adhering to good construction practices, questioning poor or nonexistent details, and adhering to the manufacturers’ installation instructions and details for their products.
