Importance of UV stability
If wind, rain, and snow can get through the open joints, it goes without saying UV light can also do so because the wall assemblies are exposed. This increases the risk of degradation of wall components by UV light. It is important to note that regular membranes and even building paper might provide the desired water holdout but the aging process will accelerate due to excessive UV exposure. The assembly might only perform as expected for a few years, resulting in significant loss of performance over the life of a new building. However, design professionals can protect the exterior wall assembly by employing materials with UV-stabilized properties.
In the case of long-term UV exposure, there is no ASTM standard on which a manufacturer may base their claim of acceptable exposure time. There is a durability component to the International Code Council-Evaluation Services’ (ICC-ES’) acceptance criteria (AC) 38, “Water-resistive Barriers,” but since it was designed to test any WRB, not just UV-stable membranes, it is not a meaningful measurement tool for membranes specifically designed for long-term open-joint exposure. However, using a UV-stable membrane, along with best practices in understanding and preventing excessive UV exposure, will help to prevent impact on performance. For example, one of the open-joint cladding WRBs available in the market are manufactured with special acrylic coatings to make them UV-resistant. These materials may be used in systems with up to 50-mm (2-in.) openings comprising up to 40 percent of the overall façade.
Durability and tear resistance
The ideal barrier for open-joint cladding is resistant to tears and can withstand the day-to-day environment of a construction site because the WRB is left uncovered prior to the installation of the cladding material. The durability is also a critical consideration for open-joint designs because some elements of the barrier will be left exposed even after the cladding is applied, as previously discussed.
The WRB is a part of the wall system but its location makes it quite vulnerable and exposed. Therefore, it is a poor idea to designate the membrane at this location as the primary air barrier. Though both the air and water-resistive barriers are expected to be durable and tear resistant, it is important to note that their location is also critical to their long-term performance.
As building industry experts John Straube and Joe Lstiburek recommend in the Perfect Wall concept, critical performance layers, such as the air barrier, should be placed in a protected location to prevent damage. Barriers—exposed or actually performing as cladding—such as those seen in an open-joint design, would not be considered protected in the Perfect Wall concept. Therefore, the designated air barrier should always be placed behind the ci, to the interior, to prevent damage.
Adds to innovative design
Most architects and designers choose open-joint cladding to enhance the overall visual appeal of their buildings. When not using a WRB directly behind the cladding, there can be concerns around the appearance of the wall elements. For example, if the uncovered insulation shows through in such designs, a beautiful concept might miss the mark. However, when the right barrier is employed in the proper manner, it can provide a 3D effect to enhance the visual appearance of the building. This concept
is applied by the world’s most advanced building professionals designing some of the most cutting-edge buildings.
Bringing the concept to life
Recognizing the need for protection and getting ahead of building challenges were the drivers behind the development of the Collaborative Life Sciences Building (CLSB) in Portland. The project brought together three of the state’s top universities to create a landmark facility. The project combined the resources of the Oregon Health & Science University (OHSU), Oregon State University (OSU), and Portland State University (PSU) to provide 46,452 m2 (500,000 sf) of instructional and research space for life sciences, pharmacy, medical, and dental programs.
As you state it is critical for an exposed WRB/AB to be; UV stable, water, and wind-resistant, though in most cases a hidden WRB/AB, as in the Life Science Center building’s interior of the insulation peel-n-stick, is used as the primary barrier. The exterior exposed membrane hides the continuous mineral wool insulation and provides an umbrella that limits moisture and air infiltration.
I would add that it is critical that the exterior exposed membrane must be highly vapor permeable to allow diffusive drying of interstitial moisture. This is especially the case for Portland’s zone 4c rainy climate, where discontinuities of the WRB/AB installation provide water intrusion that can be trapped.