Tips from the experts on best practices for metal rainscreen installation
Rainscreen wall systems are designed to manage moisture and energy transfer through a wall assembly. Typically made up of an air barrier, insulation, framing and cladding. There are two types of rainscreen systems: drained/back-ventilated and pressure-equalized.
Drained/back-ventilated systems rely on the ventilation cavity to drain and dry out the residual water, while pressure-equalized systems employ drainable compartmentalization to limit water penetration during periods of pressure disequilibrium and to facilitate rapid pressure equalization.
Pressure-equalized rainscreens are governed by AAMA 508, Voluntary Test Method and Specification for Pressure Equalized Rain Screen Wall Cladding Systems. Additionally, AAMA 509, Voluntary Test and Classification Method for Drained and Back Ventilated Rain Screen Wall Cladding Systems, tests for air and water infiltration, along with the entire system’s ability to ventilate any air and moisture that gets into the wall system.
Here are some tips from the experts on how to install a rainscreen system.
Along with a basic understanding of the concept, purpose and function of a rainscreen wall, preparation is vital to a successful installation. “Knowing a rainscreen is designed to breathe, allow air to vent and any moisture to escape are some of the important points,” explains Brian Nelson, general manager and product developer, Knight Wall Systems, Deer Park, Wash. “In addition, always remembering the rainscreen cavity is a moist environment must be understood as this can affect both how and why items and details are specified and required in the manner they are. Any field changes that may be required must always take these basic points into consideration.”
Installing a rainscreen system successfully requires an entire team-made up of a designer, waterproofing contractor, sheet metal contractor and manufacturer’s representative-working together. “Without a united team approach, the installation can become quite convoluted,” says Tom Diamond, PE, senior product manager at The Garland Co. Inc., Cleveland.
“A pressure-equalized rainscreen system only works if installed exactly the way it was tested under AAMA 508-07,” says Jonnie Hasan, PE, director of engineering and sales support at Innovative Metals Co. Inc. (IMETCO), Norcross, Ga. “It is essential the installer follows the exact assembly as outlined in the shop drawings and testing documents.”
Diamond offers up the following tips for installing a pressure-equalized rainscreen system:
- Ensure that the substrate is clean and properly prepared to receive the air/water barrier, in accordance with the manufacturer’s guidelines.
- A waterproofing contractor is typically best suited for the application of the air/water barrier. Fluid applied air barriers should be installed by someone who has experience with liquid-applied systems. Membrane air barriers also require experienced installers to ensure all laps and penetrations are sealed properly.
- Windows and other penetrations can pose many challenges. Sealing the membrane around these penetrations, in the proper sequence is needed to maintain an airtight and watertight system.
- After the air barrier is installed, the insulation package (if exterior of the air barrier, which is recommended whenever possible) is installed.
- The framing system and cladding are then attached to the wall, through the insulation package. It is always better, if possible, to utilize a framing system that attaches over the insulation, with only fasteners penetrating the insulation, as opposed to attaching framing directly to the substrate with the insulation surrounding it. A continuous insulation package, without being interrupted by framing members, is code compliant and much more energy efficient.
- The cladding system is then installed according to the manufacturer’s guidelines. The pressure equalization occurs from the cladding system’s unique design feature.
There are two framing systems available for rainscreen assemblies. The hanging method, in which the exterior cladding is attached to a light-gauge steel section that is in turn screwed through the insulation into the substrate behind. “This method relies on the compression of the insulation for structural integrity,” Hasan says. “Over the years, this has become extremely popular due to little-to-no thermal bridging, ease of installation and the overall simplicity of the system. Installers must ensure that the fasteners are properly attached to the substrate and must use the right type of fastener.”
On the other hand, Hasan says the sub-girt system is the more traditional framing system with a Z, hat or other light-gauge framing system nestled between the insulation. The cladding system is attached directly to the framing in this case, and he says this method can have as much as 20 percent loss of R-value due to thermal bridging. Diamond notes that some systems require the insulation to be installed around the framing, resulting in significant loss in effective R-value.
“This method of installing framing first and the insulation around it does not technically comply with the code required continuous insulation layer,” he explains. “Properly designed systems allow for the insulation to be installed in a continuous layer, then the framing installed over it so that only the fasteners penetrate the insulation. This results in a code-compliant continuous insulation layer that is much more energy efficient. “
Nelson adds that using a standardized framing system supplied by specialty manufacturers help streamline the design and installation process, cutting down on instances of design, detailing and installation issues.
When installing rainscreen systems, Hasan says it should not have any caulking as it should be an open-jointed system. “Fasteners for panel attachment must be designed by the manufacturer and clearly called out on the shop drawings,” he recommends.
Also, it’s important to not use impact drivers on the fasteners. “The additional torque and large load placed on these fasteners may fracture or fatigue the fasteners in such a way that long-term durability may be jeopardized,” Nelson explains.
But the most important characteristic to consider with fasteners for the rainscreen is corrosion resistance. “The rainscreen cavity is a moist environment,” Nelson says, “so having a 1,000-hour-plus salt spray rating will help ensure a durable connection. Electro-plated fasteners (such as common zinc-coated screws) should also be shied away from since they not only provide poor corrosion resistance, but the electro-plating process may inhibit other issues with the fasteners, such as embrittlement.”
Location is Key
The location of the building can play a role in the components of a rainscreen system. According to Nelson, the most important aspect would be the wind load acting on the system. “The location of the building directly impacts what the wind load will be, which will directly impact how the system is designed and installed,” he says. “If this is ignored, budgets can blow up very quickly as more material may be required to structurally support the system and resist the wind loads.”
Diamond says it is important to keep in mind that air barriers and vapor barriers are two very different things, but can be addressed using the same air barrier material. “Air barriers are always beneficial and should be used throughout the building enclosure,” says Diamond. “The vapor profile of the air barrier layer, however, should vary depending on the warmth of the climate and the general humidity levels. Cold climates generally require a vapor-closed profile, where as warm climates allow for a vapor open profile. It is typically beneficial to use a vapor open profile, as this will allow for more drying potential of the overall wall assembly.”
“However,” Diamond continues, “some scenarios require special consideration and will not allow for variations in the humidity levels within the building or the building enclosure assembly. Some facilities that require vapor barriers are swimming pools, gymnasiums, museums and data centers, especially in cold climates where the vapor drive is very strong in the winter.”