September 12, 2022
By Eric Atkins, CEP, AIA
Water, wind, and ice—while their force and formidable power continue to threaten even the best laid structures, building science and cladding products has come a long way in the defense against weather damage. Moisture causes issues like mold, rot, and long-term structural decay. For these reasons, it is critical to defend against potential moisture issues that can cause harm to a building’s aesthetic and structural integrity, posing a threat to occupant health and safety.
The case for moisture management
The American Society for Testing Materials (ASTM) reports, “about 90 percent of all building problems are associated with water in some way.” This statistic highlights the need for deliberate and comprehensive moisture management strategies for exterior wall assemblies and products that can stand up to the elements.
Moisture management strategies start with control measures to mitigate the intrusion of moisture. It is important to analyze the various states of moisture (i.e. liquid, ice, and vapor), and to be aware of the areas where water can infiltrate the building envelope.
The four major transport mechanisms of moisture into the building envelope are wind-driven precipitation, water vapor in the air (including interior air), built-in or stored moisture, and liquid-bound ground water driven by capillary action and gravity. Other drivers include gravity, pressure differential, stack effect, air leakage, and the location of building components within the structure’s envelope. In some situations, there could be competing strategies for managing these that create new problems (trapped moisture). This must be avoided.
The key to effective moisture management is to limit water penetration and to provide an escape route for moisture that gets in the building envelope. The building envelope must be designed with this in mind. From a cladding perspective, a rainscreen assembly is a practical option to achieve a high-performing building envelope.
A rainscreen is an exterior wall system where the visible cladding acts as the primary moisture barrier and stands off from the second moisture resistant surface applied to the sheathing. This arrangement creates a capillary break and allows drainage, evaporation, and drying of moisture which penetrates the cladding to become trapped in the wall.
Rainscreens are typically recommended for areas that receive at least 1016 mm (40 in.) of rain per year, but they add value in all geographic areas by mitigating the risk of moisture intrusion by creating a high-performing exterior wall assembly.
Rainscreen walls offer two lines of defense against moisture penetration into the building envelope by minimizing moisture passage into the building envelope, and intercepting and dissipating the water behind the cladding screen. This is achieved through the interaction of four basic components, each designed to complement the functions of the others.
The outermost component of the rainscreen system is exterior cladding. In addition to providing the desired appearance, a building’s cladding protects
the enclosed structure against the stresses of climate, directing rain downward and away from the underlying elements of the envelope. This component must endure the stress of wind and building movement derived from temperature and moisture changes without causing damage to the cladding. Depending on the material and construction details, moisture may pass through this layer. Another crucial consideration for cladding is combustion, so architects must carefully follow NFPA code requirements for each jurisdiction and building type.
The next component is the air gap behind the exterior cladding. This gap, sometimes referred to as the channel or drainage cavity, must be ventilated at both the top and bottom. Its primary function is to provide a runoff for moisture passing through the cladding or is trapped within the interior wall or assembly. This is achieved with furring strips, drainage mats or weather-resistant barriers with integrated drainage technology. Typical assemblies require a minimum 9.7 mm (0.38 in.) cavity for efficient drainage.
The weather-resistive barrier (WRB) is located behind the air gap. The 2018 IBC section 1402.2 states that a building’s exterior walls must be covered with a weather-resistant exterior envelope. A water-resistive barrier covers the building envelope to prevent the accumulation of moisture within the wall assembly.
For a drainage plane to be effective, it must be continuous and integrated with flashings. The most common material for this layer is a building wrap, which resists the penetration of water from the exterior into interior sheathing. It also allows the passage of vapor/trapped moisture from the interior into the gap. A WRB also functions as an air barrier and prevents external air from passing through the envelope into the interior. Building felts and various troweled materials and foam insulators are also suitable for this layer. The capacity of this layer compensates for inadequacies in the cladding. In an ideal system, this layer never gets wet. In the worst case, it should only be subjected to a small amount of moisture under low pressure.
The final component is flashing, thin pieces of impervious metal installed over openings in the wall, around the inside and outside corners, and where cladding styles change to prevent the passage of water into the structure. Flashing is intended
to direct water to the outside of the cladding. Additionally, flashing helps to prevent water penetration at windows, doors, vents, pipes, and other openings to make a building more durable and reduce or prevent moisture management problems. Flashings must be carefully sealed into the WRB to function properly. Metal flashings include aluminum, copper, stainless steel, and galvanized metals. Fluid-applied flashings are alternative products that provide continuous coverage around windows, doors, penetrations, and transitions within the building envelope.
Rainscreen cladding options
There are many options for rainscreen cladding, and the individual performance of each material in this role varies. It is important to classify materials into two separate groups as it relates to how they handle moisture—hydroscopic
Some building materials capture water molecules from the surrounding air and collect them on the surfaces of their inner structures in what is known as an absorbed state. Hydroscopic materials absorb water. Some traditional building materials such as wood, concrete, brick, and plaster are hydroscopic. Hydrophobic materials repel water. Examples include glass, metals, and plastics, including wood-plastic composites.
Materials with less vulnerability to water are gaining popularity among construction teams looking for high-performing, aesthetically pleasing, and sustainable products. This category includes metal cladding options like galvanized steel, aluminum, and copper, which are recognized for their ease
of maintenance. Another compelling option is wood-composite cladding. Designed to emulate real wood, composite cladding is resistant to impact, freeze-thaw cycles, ultraviolet (UV) degradation, fading and staining, cracking, splintering, peeling, decay, and insect infestation.
Taking a closer look at wood-plastic composites
The popularity of WPCs has increased substantially year over year and, in some cases, has become the cladding of choice for architects thanks to the material’s performance characteristics, sustainable construction, durability, and life-cycle attributes.
In terms of performance, WPCs have a durable core and never needs sanding, staining, or painting, which makes it a great choice for difficult-to-reach applications. Unlike wood and stone, WPC cladding deflects the water it encounters. It also resists fading due to sun exposure (wood does not), as well as freeze-thaw cycles, while fiber-cement, wood, and sometimes brick and stucco
are affected. When installed correctly, WPC rainscreen cladding also provides a low-maintenance, aesthetically pleasing facade.
Often made from sawdust, scrap wood, and other products, WPC cladding is also a highly sustainable material. Binders are often low in volatile organic compounds (VOCs) and there is minimal production waste.
There is considerable evidence showing how moisture—in particular, rain—can be extremely destructive to building envelopes and interiors, and why managing it is critical to building durability and occupants’ health.
There is a parallel body of evidence demonstrating how rainscreen walls are highly effective in managing moisture and in ensuring the durability and integrity of building envelopes.
Thankfully, there are options when it comes to selecting the right rainscreen cladding material. Specifiers no longer need to choose between form and function, they can achieve an organic, timeless look without the upkeep or vulnerability other traditional materials face. Materials like WPC siding means architects can achieve the impossible feat—a building that looks and feels forever young.
Eric Atkins is the East Coast and international cladding manager for Fiberon. With a strong background in architectural support and fabrication, Atkins facilitates and supports the work of architects, designers, and contractors, providing pragmatic answers and solutions to complex problems.
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