Rainscreens offer many benefits for exterior wall performance and maintenance. This reliable building envelope resists the elements, and protects against air and water infiltration-provided their connectors are specified correctly.
Important factors for specifying the connection details of rainscreens
For many years, a rainscreen connector was simply specified as an accessory to the panel, and would only contain one or two lines in the existing panel specification. But the complexity and requirements of this critical building envelope component have grown substantially in recent years. Now this piece of the complex building envelope puzzle can include warranty requirements, engineering requirements, testing and thermal performance, all of which can be easily contained and organized in its own specification.
Photo courtesy of Knight Wall Systems
Selection Questions
Early on in the rainscreen connector selection process, some research and decision making is required. Brian Nelson, general manager at Knight Wall Systems, Deer Park, Wash., says the following rainscreen connector selection questions should be asked during design:
Will we need exterior insulation on this project to meet the energy code? “More often than not, the answer is yes,” Nelson says, “especially from the Northwest stretching across North America to the Northeast. With modern building codes and lessons learned from the past, many changes to the design of the rainscreen façade have occurred over the past decade or so. Exterior insulation has almost become the design standard in colder climate zones from North Carolina up the East Coast over to the Pacific Northwest. One of the most significant considerations we have seen for specifying the connection of rainscreen panels to the building is the emphasis on the effectiveness of the exterior insulation, as this design approach is so popular due to building codes.”
How will the panels connect to the building and have the least amount of impact on the insulation’s effectiveness (if required)? “Here is where we start to sort out this connection detail, what it will look like and what it will be made out of,” Nelson says. “Insulation serves little purpose if it is not effective when installed.”
Is the connection detail being used behind multiple rainscreen panel types? “If so, it might be best to specify the connection (attachment) system in its own specification section,” Nelson says.
What is the desired life span of the rainscreen connection? “Many commercial projects are built with goals of 50-plus year life spans,” Nelson says. “Therefore, these connection details need to be specified with products that can last this long. There are a wide variety of coatings and material approaches that can aid in ensuring a long-lasting product is used.”
Image courtesy of Charles F. Bloszies
Behind the Scenes
It’s important that the rainscreen connector be easily adjustable so workers in the field can easily fine-tune the installation to achieve precision. “The beauty of a rainscreen is that the waterproofing is taken care of behind the scenes-the rhythm of the rainscreen panels is independent of the waterproofing joints,” says San Francisco-based architect Charles F. Bloszies, FAIA. “The connector system needs to be adjustable so the rainscreen panels may be aligned accurately.” Christopher Costanza, RA, AIA, LEED AP, architect at 9X30 Design Architecture LLP, Rochester, N.Y., agrees the frequency and alignment of rainscreen connectors must be able to be adjusted accordingly. “I am more concerned with the structural components beneath the panel,” he says. “The quality of the connections you don’t see are often more important than the finished surface you do see.”
Connection Considerations
How does a rainscreen’s panel type and size affect its connector specification? At his firm, Bloszies uses single-skin terra cotta, cement board and custom metal rainscreens. “The terra cotta system includes an aluminum connector system,” he says. “Cement board marries best to an adjustable metal system; one that can also accommodate rigid insulation. Custom rainscreens typically require connectors specifically suited for the particular circumstance.”
Similarly, how does a building’s size and function affect your rainscreen connector selection? “In our experience, building size affects the connector system type more than building function,” Bloszies says. “The rainscreen principle has been successfully adapted to small-scale, wood-framed residential buildings. In this case, wood-board siding or even shingles are simply nailed through a separator membrane that allows the wall to breathe. Larger buildings, sheathed in heavier materials with larger rainscreen panels, require a substructure connecting the rainscreen panels to the primary structural components.”
On larger buildings, Costanza prefers using more robust connections so there will be fewer connections installed in the field. “With small projects, the flexibility of multiple, small connections allow contractors to adapt easier to the inconsistencies of real-world construction,” he adds.
Metal’s Weatherability
Open-joint rainscreen systems are designed to let limited moisture in, so architects must pay close attention to metal’s weatherability when specifying their connectors. A combination of dissimilar metal materials can cause cathodic corrosion. Tyler Bruton, product development and technical specialist at The Miller-Clapperton Partnership Inc., Austell, Ga., stresses that highly corrosive environments are going to require a considerable amount of attention to dissimilar material interactions as compared to environments that aren’t as conducive to enabling galvanic corrosion. A corrosive reaction will degrade the less noble material.
“Aluminum weathers well, as long as it’s not placed up against another dissimilar metal,” says John Barbara, AIA, LEED AP, senior associate at LEO A DALY, Washington, D.C. “If steel and aluminum are used together, a coating is needed to separate the two. Steel-only connectors need to be either coated or galvanized. A good example of weatherability failure is in the East Wing of the National Gallery of Art in D.C. They just finished taking all the stone off at great cost to replace the steel anchors that should have been high-grade stainless steel, but were not, and were corroded and failing.”
When specifying his rainscreen connectors, Richard Turlington AIA, project manager at Pelli Clarke Pelli Architects, New Haven, Conn., says he almost always uses stainless steel fasteners and plates. “This addresses most dissimilar metal contact points and, if the project can afford Type 316 stainless steel, then rust and streaking are greatly reduced,” he says. “We have found that Type 304 stainless steel, while a good product, is inclined to rust due to impurities in the steel.”
Location and Weather
Building height, location and region influence rainscreen connector specification. Connectors have to be stronger to withstand wind loads at corners, parapets and higher elevations above streets. “If you are doing high-rise unitized construction, that would steer you down a different path for connector specification,” says architect Alex Terzich, AIA, of Minneapolis-based Hammel, Green and Abrahamson. Coastal locations also have special requirements for hurricanes and typhoons. “A lightweight cladding material (MCM, single skin) installed on a project in a high-wind load region is going to require a back-up framing system that can resist the elevated wind loads and secure the cladding material to the structure,” Bruton says. “Having a tried-and-true tested system minimizes the designer’s uncertain ties in the performance of the cladding system, and increases their confidence in the performance of the specified system.”
Also, “In extreme climates, like in a desert, there are greater temperature shifts over the course of 24 hours,” Barbara says. “Connectors in these climates have to withstand rapid cycles of thermal expansion and contraction. This is especially important to analyze if you’re not using a pre-engineered system designed for the climate you are working in.”
When Turlington has a project in a tropical environment he always considers the rainscreen surface design and how its surface deflects heavy downpours. “Collecting, channeling and dissipating extreme rainfalls poses some interesting structural loads which impacts a connectors job,” he says. “For example, a rainscreen can be used to protect horizontally driven rain as well as vertical rainfall.”
“I want higher-grade stainless steel in a marine environment, and heavier-gauge connectors in a seismic and high-wind exposure zones,” says Costanza. “Although I have not yet had to change a specification due to thermal expansion concerns, I do keep that in mind with dark-colored rigid panels that are used in northern zones that have hot summers and cold winters.”
A Clean Aesthetic
The aesthetics of the panel layout will also play a big role in rainscreen connection specification. Typically the panel’s attachment will run perpendicular to the panel’s orientation. “If the panels are laid out in a very specific manner, they will likely have two layers of attachment to assist in positioning the panels in specific patterns separate from how the building’s structure (studs) are laid out,” Nelson says. “Understanding this is key when specifying the methods of attachment, so the project will turn out as planned with the least amount of detailing issues.”
Defering the Decision
JJ Marks, PE, technical sales support engineer at Laminators Inc., Hatfield, Pa., suggests that architects often defer to manufacturers for systemspecific rainscreen connection requirements, partly because of the ambiguity associated with where connection requirements should be outlined in architectural specifications. “This ambiguity can cause confusion when trying to comply with multiple, conflicting code requirements while maintaining constructability,” he says.
Marks emphasizes that when specifying rainscreen connections, always adhere to the requirements of the International Building Code, some of which are difficult to combine while maintaining code compliance:
- Strength and deflection
- Water and vapor controls
- NFPA 285 fire testing requirements
- ASHRAE 90.1 continuous insulation requirements
Rainscreen connector manufacturers typically have design information and tips on their websites, which design professionals can reference. Some also provide calculators online to help determine connector spacing and display total product assembly ideas.
Andrew Dunlap, AIA, principal at SmithGroupJJR, Detroit, says when dealing with code requirements in rainscreen connector specifications, he approaches it from a performance standpoint. “We review the energy code and determine the required total product assembly thermal performance,” he says. “The energy codes allow for conformance based on a total R-value of the insulation alone or a total product U-factor of the complete wall assembly. We often indicate the total product thermal performance (U-factor) requirement in our specifications. This is critical as most of the major connector manufacturers are going to perform somewhat differently based on the amount of connectors required for a given design.”
Dunlap says that often the connector size, spacing and number are delegated to the manufacturer/ fabricator based on the load of the specific rainscreen material, the wind load and the required thermal performance. However, he does provide in the specification the required wind loads, or at least the criteria to develop the wind load, depending on the zone or terrain. “We require the fabricators to perform the structural calculations themselves and provide us this delegated design in a required submittal that we review,” he says. “Depending on the type of connector used, it can greatly impact how many attachment points are required, especially in a high-wind zone. Note the amount of connectors will have a direct impact on the total product thermal performance. It is a balance that must be verified during the design process in order to be certain that products are available that can meet the requirements.”
Reducing Thermal Bridging
Often, rainscreen panels are supported by a series of horizontal and vertical metal Z- or hat channels. While this is an economical connection method, this method also reduces the rainscreen’s energy performance by providing thermal bridging through the insulation. When specifying rainscreen connectors, architect Nathan Casteel, AIA, LEED AP BD + C, of Chicago-based GREC Architects, stresses that there are now connection systems that allow for a fiberglass spacer or connector that eliminates the thermal bridging normally seen in metal connections. “There have also been advancements in continuous insulated metal panel systems that are able to support single-skin rainscreens while also providing a moisture barrier,” he says.
Terzich says there are a few ways to eliminate thermal bridging when specifying his projects’ rainscreen connectors. “In a perfect world, we would use a fiberglass clip, which is a localized thermal break you attach to a hat channel,” he says. “But these are expensive and it’s often not in the cards for us to do that. So we look at different scenarios. Do we use intermittent Z-girts? These localized points of penetration through the insulation work well thermally, but they require a lot more layout and time. Do we use two lines of Z-girts? [They are] horizontals, with verticals on top of that, and where they cross is the one thermal bridge, which helps reduce thermal transmission. It’s a good option but also more material, more time, more cost. What we usually specify is a conventional, continuous linear Z-girt that’s bridging through the insulation, then we take that thermal bridge into account when we specify the insulation type and thickness to make sure we still meet the code-required system Uvalue requirement for the wall.”
Obviously, there are many details to rainscreen connector specification and many others not discussed in this article. Barbara believes it’s these details that often make or break a project, and bring it to the next level. “It really comes down to getting those details finessed,” he says. “The quality of the design documents is critical, but even more important is the builder’s ability to execute them, so good detailing that is buildable and resolved in the studio is essential.”