Rainscreens are growing in usage as a reliable building component that shields against rain, snow and ice, preventing potentially harmful moisture from bleeding through a building’s exterior walls.
The dynamics of rainscreens’ drainage, deflection and drying
As a building envelope support mechanism, a rainscreen’s primary function is to shed water downwards and outwards out of the building. To do this, the exterior cladding is placed away from the backup wall, creating an air cavity that becomes a weather-resistant barrier. By design, any moisture that does migrate through the cladding will easily drain down the cavity, away from the building. Air that flows between the cladding and the wall accelerates evaporation of any residual moisture. Additionally, rainscreens provide enhanced shading and UV protection benefits, and when combined with continuous exterior insulation will provide significantly improved thermal performance.
Two Rainscreen Types
There are two basic types of rainscreen systems, as classified by the American Architectural Manufacturers Association (AAMA): drained, back-ventilated (AAMA 509-09) and pressure-equalized/pressure-moderated (AAMA 508-07). According to Zeke Miller, president of The MillerClapperton Partnership Inc., Austell, Ga., pressure-equalized rainscreens rely in compartmentalizing all the compartments behind the attachment system and are the least common.
“The main difference between the two is a drainable assembly uses a drain plane/ventilation chamber to control the flow of moisture,” says Kenny McMann, architectural manager, Morin Corp., Bristol, Conn., “and the pressurized system is supposed to be weathertight, installed in zones (like a gird) and sealed with a weathertight sealant.” Miller claims it’s an architect’s personal choice whether they want a sealed system or an open joint system, explaining it’s dictated by the complexity of the wall shape, adjacent materials and types of materials.
Drainable assemblies are much simpler to install, and the detailing is more straightforward and familiar. “A true pressure-equalized system is much more sophisticated, as compartmentalization of the cladding must be done to effectively pressure equalize the dynamic pressure differentials on the building’s façade,” says Brian Nelson, general manager, Knight Wall Systems, Deer Park, Wash. “Generally speaking, a one-size-fits-all approach is not feasible for pressure-equalized systems, thus making the design and install much more tedious and cumbersome. Today, we mostly see drained and back-ventilated rainscreen façade systems in the market.”
Jonnie Hasan, PE, director of engineering and business development at IMETCO, Norcross, Ga., believes that there are no significant differences between the two and explains their basic layers are the same: air/vapor, thermal, framing/airspace, cladding. “The difference is dependent on how each manufacturer tested their system for the type of rainscreen it was to market. Some manufacturers will face seal their systems for pressure-equalized systems, while others will only provide venting at bottom and not the top. On the other hand, most back-drained ventilated systems will have open joints and top/bottom venting.”
“Drained and back-ventilated assemblies are not reliant on specific compartments and calculations of those compartments to be able to prove that they is pressure equalized or not,” Miller says. “With AAMA 508, there is a calculation that is dependent on the size of the compartment and the depth of the compartment, and the project’s wind loading all has to go into calculating is it pressure equalized or is it not. It’s the biggest fool’s gold sale ever to say that you’re pressure equalized because your laboratory testing isn’t always a job-site condition. So if you have passed in a laboratory setting, it doesn’t necessarily mean that if you do the same sort of techniques on the job site, it will be so. It’s all dependent on so many other factors, it’s impossible to know whether or not it is pressure equalized.”
Rainscreen Details
McMann claims the most distinguishable detail of a rainscreen is the drain plane/ventilation chamber. “This space between the cladding and the substrate provides much needed moisture control, supplying both a route for the moisture to drain and air flow, which dries the surface,” he says. Miller says flashing at horizontal terminations makes sure the water gets out and, “That’s usually at base conditions or window-heads and a lot of times, it’s open at your head conditions to allow air to get into the backside of the cavity to allow for drying of the cavity.”
Nelson says one important rainscreen detail to consider is its natural movement from temperature change: expansion/contraction. “When alloys such as aluminum are used, the rate of expansion in service is quite high because aluminum has a very high expansion coefficient. Conversely, using steel or stainless steel, the expansion rate is much lower and thus results in less movement. Since the aluminum rails or girts move so much, the connections of these rails must be made using a very specific layout of sliding versus fixed connections. This results in a more complex and burdensome installation.”
“Another detail that is often overlooked is the floor deflection joints of the building,” Nelson adds. “If the floor slab of a building is expected to have any movement up or down, the stud wall is generally designed to accommodate this movement with a deflection track. This movement of the floor slab and wall studs up or down will directly impact the façade and rainscreen system, as it will move in unison with the floor slab and stud wall it is connected to. Thus, a commonly overlooked detail is how the rainscreen system and façade will accommodate this movement, and it must accommodate the movement at the same location as the stud wall does.”
Every rainscreen system has multiple layers, and their combination and location may vary from manufacturer to manufacturer. Designers must ensure each product specified for each of these layers is compatible with each other. “Unfortunately, cladding manufacturers that provide the basis of rainscreen design (most of the time) do not have a say in what the other layers are and hence the overall system may not be like how it was tested,” Hasan says. “Designers calculate and specify the venting space thickness and venting air exchange (the vent strip perforation rate). They also need to do an overall hygrothermal modelling to understand vapor and moisture balance and specify the proper perm-rated air water barrier. In turn, this design info will need to be translated into properly coordinated details for the wall envelope assembly for contract documents.”
In terms of rainscreen fastening, McMann claims the best fastening method when using concealed fastener panels is clips. “This allows for the panel to move freely thus addressing the concerns of thermal expansion and contraction. This method also allows for a little more separation from the panel and the substrate, allowing moisture to drain more freely. Obviously when using an exposed fastener panel, your only option is to fasten through the panel, directly into the substrate. When using single skin, there are two options, an exposed fastener system or a concealed fastener. With an exposed you install by fastening through the metal panel into the substrate. If using a concealed fastener you have two options, use a clip to secure the panel to the substrate or fasten directly through the fastening flange into the substrate. Metal composite material (MCM) panels usually incorporate some sort of a grid utilizing channels to secure into the substrate.”
Understanding Rainscreens
Exterior cladding systems are typically installed toward the completion of most projects. But Joe Mellott, vice president, IMETCO, says this, “presents a unique level of anxiety among installers and owners as cladding is last to the dance.” He says designers and installers have to work quickly to provide proper installation, exactly dimensioned panels—on somewhat variant construction practices of prior trades—and sequential delivery based on specific plans of attack. “In typical rainscreen installations, the building is made airtight and water tight first, including all penetrations, followed by insulation, framing and ultimately the surface cladding.”
One of the drivers behind rainscreen façade attachment systems is the ever-changing energy codes. These codes have only become more and more stringent over the last decade and will continue to do so over the next. “Therefore, it is important for the entire industry to understand these systems are generally being used for code compliance,” Nelson says. “So, understanding the impact of using properly thermally isolated systems versus conventional metal girts is important. Too many times, installers plan to use a non-thermally isolated system on a building, believing they can substitute one for the other. They then later learn the substitution cannot be allowed due to code compliance. This has been a repeated problem across the industry.”
To educate the industry on this and other rainscreen variables, the Rainscreen Association in North America (RAiNA) was established this year. A national nonprofit trade organization dedicated to serving the rainscreen market in North America, it aims to create awareness and an understanding of the building science behind a properly designed, detailed and installed rainscreen assembly for opaque façades in the construction of high-performance building envelopes. Nelson says this member-run organization will develop into a wonderful resource for the entire industry to lean on.
