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By Jonathan D. Hill
June 2011 -
Windows & Exterior Walls Article Use Policy
As the complexity of facade design continues to evolve, modern architecture requires wall cladding systems to have clean, distinguishable features with complex geometries that strive to achieve a high-performance system with limited maintenance. Although rain screen systems can meet these aesthetic and performance requirements, it is important to understand the fundamental concepts behind this type of wall system to produce and maintain this potentially complex cladding assembly during the life of the building.
Open-joint rain screen systems are a variation of the conventional drainage wall where moisture is allowed to pass beyond the surface of the cladding and drain within the panel cavity. While conventional drainage walls typically place the cladding material tight to the air and water barrier, occasionally impeding the drainage plane, open-joint rain screens create an air space that promotes drainage and ventilation. The design approach and choice of materials within an open-joint rain screen wall have important impacts on how the cladding will interact with the wall assembly and other building components, how the wall will resist air and water infiltration, and what level of performance and durability is achieved.
Contemporary open-joint rain screen assemblies generally consist of interchangeable panels held off the face of a sheathed and insulated structural backup wall via an engineered support system. The open joints between panels and the air space created between the panels and the face of the sheathing promote air movement, which allows the rain screen to function.
The intent is to have the solid panels deflect bulk water while only allowing incidental moisture to reach the panel cavity. Eventually, in some systems, limited moisture can also reach the primary water barrier. Water that reaches the cavity is contained and drained to the exterior at predesigned intervals using flashing elements. The void created by the air space minimizes the amount of moisture that reaches the water barrier, and also creates a space for active ventilation. With open joints between panels, air can circulate within the cavity, effectively drying the wall components after wet weather subsides.
The presence of air flow behind the panel systems creates an opportunity for more sophisticated high-performance closed-joint rain screen assemblies that are commonly marketed as "pressure-equalized" systems. The concept of pressure equalization is that, as varying air pressures act on the face of the cladding, the joints in the panels create compartments allowing air to enter, equalizing pressures within the cavity with exterior conditions. In theory, because the interior cavity pressure is the same as the exterior pressure, water is not driven into the cavity and instead flows down the face of the panels.
For these systems to be effective, they must accommodate varying design pressures and installation tolerances. The air pressure acting on the panels is directly related to the actual wind pressure, which changes constantly with time and location around the building. Generally speaking, design wind loads are smallest at the base of the building and increase with height; design pressures also increase at corner conditions where both positive and negative pressures interact.
For each panel, the size of the compartments will dictate the effectiveness of the pressure equalization under a particular load. To create optimal performance, the size of the compartments should reflect the varying pressures around the building. Theoretically, this would result in different size panels or compartments throughout the facade. If the compartment sizes and seals are designed to allow for full pressure equalization under a particular load, the actual layout and installation of the system must accurately reflect the calculated design.
The practical application can be further complicated since typical cladding assemblies installed over a support system will require shimming to accommodate variations in the exterior wall framing to properly align the face of panels. Normally this shimming occurs within the cavity along the edge of the panels. If not adequately designed, the shims may eliminate the compartmentalization by allowing air to pass between zones.
The architecture, engineering and construction industry is actively addressing these issues in an attempt to create high-performing cladding systems that may not require additional waterproofing protection; however, currently available systems typically do not sufficiently eliminate the risk of leakage to the degree that it is advisable to eliminate a backup water barrier.
In addition to knowing the design fundamentals, it is important to understand the advantages and disadvantages of cladding assemblies in order to confidently enter the decision-making process, whether it is new design, rehabilitation, pre-purchase review, or setting up a maintenance program.
Some advantages of both open-joint and pressure-equalized rain screen systems are well recognized. A cladding system comprised of panels that can vary in size, shape, color and material provides an almost endless opportunity to express architectural vision. The panelized assemblies currently available create clean, crisp lines with a modern appearance. From a performance standpoint, the air gap provided between the panel and the wall framing allows air to circulate within the cavity. While the joints between non-pressure-equalized panels will allow water to reach the panel sub-framing and water barrier, the ventilation promotes drying which can prolong the service life of the system components, although the materials still need to be selected properly.
Because the panels stand off the face of the wall framing, shadow lines are easily controlled while providing the opportunity to conceal most flashings and sealant joints. This may create a clean look; however, it can lead to several disadvantages when examining the performance and maintainability of the cladding system.
The tendency to hide primary flashing elements behind the cladding components limits the opportunity to conduct routine maintenance or evaluate how materials are performing without removing panels. This is of particular concern with perimeter sealant joints and flexible flashings, which are often used to seal between the air and water barrier and adjacent systems like windows or other claddings. If there is no regular inspection of sealant joints, or evaluation of flashing components after extreme weather events, the risk of material failure and water intrusion significantly increases.
For typical open-joint systems, even though the cladding components benefit from the ventilation created through the open joints between panels, exposing these same components to exterior conditions creates inherent performance concerns. The design intent suggests the air pressure acting on the face of the panels passes through the panel joints and circulates within the cavity. This theoretically exposes the cladding components, the air and water barriers, and any flashing materials to full design pressures. The air and water barriers must be designed to withstand these pressures.
The sizing of the open panel joints also directly affects how the exterior elements interact with the cladding’s structural and waterproofing components. Large joints will allow more water to reach the water barrier, which requires the design to use a robust air and water barrier able to withstand and drain moisture. Depending on the dimensions of the joints and the air space between the panels and water barrier, direct sunlight may reach the waterproofing. Most air and water barriers are not UV stable and will degrade over time if exposed to anything other than incidental sunlight. There are several membranes available that will allow permanent UV exposure; however, many of them, particularly the fluid-applied products, do not possess the necessary performance history to determine if they will reliably perform when left unprotected throughout the service life of the building.
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