By Christopher Bupp
As masonry wall design continues to evolve into the 21st century, terms like air and vapor barriers, steel stud assemblies, rigid insulation, semi-rigid insulation, batt insulation, ventilation with weeps and vents, mortar collection devices, clear air space, and oversize masonry units are all employed to describe today’s masonry wall. Add to that list things like “green,” “high-performance” and “sustainable,” and well … masonry has gone through quite a transformation. But the constant over time has been the concern over the damaging effects of moisture infiltration. Today, we use the phrase “moisture management in the building envelope” as a total system approach to designing, detailing and specifying products that work together to prevent moisture intrusion and effectively direct any moisture within the wall to the outside. The brick exterior of a cavity wall system is viewed much more so today as strictly the veneer of the wall assembly as opposed to an actual barrier. With this in mind, we must look at cavity wall design and construction much differently than we have in the past. The concept of rain screen design is where the “true” barrier in most wall systems today is actually the outer face of the back-up wall. With this in mind, controlling all types of moisture in the wall assembly takes on an even greater importance.
One of the main weapons in fighting moisture infiltration in today’s wall systems includes air barriers, vapor barriers and weather-resistive barriers. These various barriers perform distinctly different functions in the wall. Moisture-laden air can pass through the exterior wall assembly from both the inside and outside. The temperature change that occurs between inside and outside air can convert that moisture into actual liquid or condensation. According to the U.S. Department of Energy, this air movement can account for up to 40 percent of the heating and cooling costs of a structure. Vapor diffusion passes much smaller amounts of moisture through many common building materials, but with similar, potentially damaging effects. Add to that the possible infiltration of wind-driven rain, and you can see why the proper selection and installation of these barriers is critical to the performance of the entire structure.
Requirements for energy-efficient exterior walls involve the use of more insulation to increase overall R-values, which can in turn, result in increased potential of condensation within the wall assembly. The type and location of insulation within the wall can determine which type of barrier will be most applicable – along with geographic and climate considerations. No matter which type of barrier is used, it is absolutely necessary that the installation create a totally continuous system that is effectively tied into door/window openings, roofing and below-grade materials. These barriers must be able to handle pressure differentials that will normally occur within the wall, potentially causing it to separate from the substrate. These pressure changes can occur in both directions, requiring proper design and installation. This is a major problem with mechanically attached barriers. Cavity walls also require the barrier to successfully seal around anchoring systems and be compatible with flashing products.
Another major aspect within the building envelope is the thru wall flashing system. More and more today, we are correctly looking at flashing as a complete system that includes numerous potential components, including drip plates, termination bars, mortar collection devices, pre-formed corners, plus weeps and vents. Technology and innovation has brought us many new improved flashing products that are able to withstand high temperatures and ultraviolet exposure. New non-asphaltic adhesives provide distinct advantages over the older rubberized asphalt counterparts of the past while making it easier for the contractor to install in the field. Copper flashings remain a very popular choice today among designers, and once again, improvements have been made with clear protective coatings over the copper materials, plus newer self-adhesive copper materials. With the Brick Industry Association stating in its Technical Notes, “It is imperative that flashing be extended at least to the face of the brickwork,” UV-resistant membranes are becoming a more popular choice. Still, the masonry industry recommends the use of a drip plate to extend the flashing beyond the outer face of the brick. This is normally accomplished with a stainless steel drip plate that extends approximately 3⁄8-of-an-inch beyond the face and turned down on a 30-degree angle with a hemmed edge. Aesthetics become a major problem with these drip plates, so today, newer drip plates are being manufactured with UV-resistant plastics that come in various colors to match a mortar or brick color. As steel stud wall assemblies continue to become more commonplace, flashings are being adhered onto sheathing materials, requiring the use of a termination bar with a sealed top edge preventing moisture from migrating behind the flashing material.
When discussing thru wall flashing materials, one of the most important criteria should be the “installability” of the material with the success of the system largely related to the installation in the field. Certainly, a good, high-quality material is essential, but I would argue that just as essential is the ability for the contractor in the field to be able to work with the material and create a total watertight system, which includes numerous detail points at inside/outside corners, head and sills of window openings, elevation changes and other transitions. While the tried and true products of the past still can provide us with quality installations, we should not ignore the many improved, user-friendly products on the market today.
The final aspect of moisture management is actually getting the moisture out of the wall. We accomplish this with the use of weeps located at every flashing line. Plastic tubes and cotton cords were installed 16 inches on center, and are giving way to full height weeps made of honeycomb plastic, aluminum or synthetic mesh spaced 24 inches on center. These more common products are installed into the vertical head joint of the brick course on top of the flashing. Many of these new materials come in various colors to match a mortar color. The cavity area must be kept open and unclogged at the flashing line to allow any accumulated moisture to be able to fall to the weeps via gravity and exit out of the wall. The mason contractor placing a wooden board at the bottom of the cavity had accomplished this in the past and after a number of rows of brick were installed, this board would be raised up and out of the cavity with any excess mortar droppings being removed to keep the cavity area clean. Today’s mortar collection devices provide a similar level of protection with the use of mesh or grooved materials that will collect the excess mortar droppings while still providing a pathway for moisture to exit the wall. These collection devices must be placed at every location within the wall assembly where flashing has been installed. The key factor in a quality product is something that will not clog or potentially create a dam or blockage within the wall. Woven mesh products perform this function quite well.
Another major development in our industry related to moisture management is the concept of ventilating the cavity air space through the use of vents at the top of a cavity wall section in addition to the weeps at the bottom of the section. This combination allows for the circular movement of air in and out of the cavity to allow materials within the wall to dry out, plus the added benefit of more effective movement of moisture out of the cavity. The masonry industry now recommends a 2-inch air space behind the brick to provide a clean, open area for any moisture to more easily work its way to the bottom of the cavity, onto the flashing material and out through the weeps. Ventilation can be accomplished through the use of similar weep-type materials or open head joints. This ventilation concept also lowers the air pressure differential between the outside air and the air with the wall assembly thus slowing down the moisture drive into the wall. Some veneer products, especially stone, can have varying thicknesses, making it virtually impossible to maintain a consistent 2-inch air space over the entire wall. Options such as woven mesh material or drainage panels installed over the entire wall can provide a clear pathway for moisture in these applications.
Obviously, masonry wall design has become increasingly more complex with so many components coming together in the wall assembly. Designers and specifiers must look at all of these materials and how they interact with each other to provide a complete system approach that can accomplish all the requirements of the 21st century structure. Component manufacturers must be keenly aware of how all of these products must effectively function together, and not just understand their own particular product. Contractors have the ultimate responsibility of constructing these complicated wall systems to manage moisture and give the owner a building that perform up to today’s high performance standards.
Christopher Bupp is head of the Educational and Architectural Services Division at Hohmann and Barnard in York, Pa.