The Benefits and Opportunities of the Rainscreen Principle

By Ellen Rogers

Architects want beautiful buildings that are high-performance, use lots of glass, and don’t leak water. That’s how it starts. Now add in bigger spans of glass, minimal sightlines, make sure the glass is as clear as possible and add a rainscreen to help control moisture penetration.

Unfortunately, addressing a request to add a rainscreen isn’t so simple. Not only do buildings today need to look good, they’re also challenged to save on energy costs while ensuring the occupants have a healthy, comfortable work environment. Yet even before that, a building must meet certain performance requirements: air, water and structural. Of those three, water is the one that probably causes the most headaches. Even after testing, buildings still can leak. Caulking and sealing exterior cracks is certainly a solution, though the results aren’t always attractive. As an alternative, the rainscreen principle is becoming more common, but it’s a term that’s not always understood. However, as glazing contractors find they’re taking on more and more of the exterior scope, understanding rainscreens, how they’re fabricated and installed will be essential for future growth and success.

What is a Rainscreen?

A variety of materials, including both glass and metal, can be used to fabricate a rainscreen. But what, exactly, is a rainscreen? If you’ve ever pondered this question, you’re not alone. Probably the most important detail to understand is that a rainscreen isn’t so much a product, as a design principle. These systems consist of an outer wall (usually the aesthetic layer); an air space; and the inner wall, which is typically the structural/supporting wall. This layer also incorporates the air and vapor barrier.

Defined by the American Architectural Manufacturers Association (now the Fenestration and Glazing Industry Alliance) Procedural Guide for Certification of Window and Door Assemblies, a rainscreen is “an exterior wall construction technique consisting of an exterior cladding (outer leaf), a cavity, and an inner leaf. Rainscreens are subdivided into two distinct performance categories, one being pressure equalized and the second being drained and back ventilated.”

Paul Millslagle, vice president of sales and marketing with Americlad Quality Metalcrafts, explains that pressure equalized systems are designed to ventilate, control and drain any moisture that could potentially penetrate the system, but also add compartmentalization to the cavity behind the system to allow rapid pressure equalization. This design can substantially minimize and, in some cases, eliminate moisture from penetrating the system’s wall cavity. Drained and back ventilated systems are designed to allow moisture behind the system, which is controlled and evacuated through guttering, channeling, etc. The system is also designed to allow air into the wall cavity to dry any moisture that may become trapped.

Sto Corp. is a German building products manufacturer with U.S. operations based in Atlanta.

StoVentec is its drained and back ventilated rainscreen system, and it recently launched StoVentec Glass, an open joint, drained and back ventilated rainscreen which features backpainted glass.

According to Brock Osborn, strategic business development and rainscreen expert with Sto, Europeans are ahead of North America by 15 to 20 years when it comes to the use of rainscreen design.

“The increased use in the U.S. brought about the need to increase the insulating value of opaque walls and still maintain aesthetics,” he says, adding that the outer wall is typically there to provide aesthetics and sheds 95 to 100% of water. “Rainscreens always have a back-up, airtight wall and the insulation is in the middle. There are various types of subconstruction that go from the back wall to the front wall and hold up the aesthetic.”

Common Misconceptions

With increasing interest in any type of product or technology, there will certainly be misunderstandings and questions. According to John McClatchy, vice president of sales and marketing for Southern Aluminum Finishing in Atlanta, probably the most common misconception is that a rainscreen prevents moisture from entering the building.

“It is designed to allow moisture to pass though the exterior envelope (much like a screen door). While there is some benefit of using a rainscreen to prevent moisture from reaching the vapor barrier, it’s not designed as the true vapor barrier for the building. Its primary purpose is aesthetics with a secondary benefit of preventing complete exposure of the vapor barrier,” he says. “Rainscreens should greatly reduce the possibility of mold and/or metal corrosion from trapped moisture when fabricated and installed correctly in accordance with AAMA 508 and AAMA 509 (see page 67). The moisture which passes through the outer envelope will drain and evaporate from the system (drained and back ventilated). A pressure equalized system is intended to prevent the pressure on the outside of the rainscreen to be equal to the pressure behind the rainscreen using venting so that the panels aren’t structurally compromised.”

Rainscreens can be constructed with a wide selection of aesthetic options. Architectural metals, terra cotta, stucco, stone, cement and glass are some of the materials available.

“Typically with metal, whether it is aluminum composite material or metal plate Kynar, painted or anodized are the two finishes of choice; we [also] inform customers of the likely possibility of color variance with anodized wall panels,” says Millslagle. “There is also a fast moving trend by paint companies to produce a UV-stable Kynar finish that mimics different natural metals and stones. These color patterns have been around a long time, but in recent years the application has been perfected to reduce pattern repetition and make the finish replicate the real thing more accurately.”

Aesthetic Options

Using glass can also create an attractive façade. The StoVentec Glass panel is made of opaque backpainted or back-printed glass that is permanently bonded to a proprietary carrier board. The prefabricated glass façade panels are manufactured in custom sizes and a range of colors.

“I think what surprises people is they can get the look of glass and the performance they need in the opaque wall,” says Osborn. “There’s a lot of appeal with people who like and want glass.”

Installation Considerations

Adam Boeckmann, president of Architectural Wall Systems located in Clive, Iowa, says his company has been involved with rainscreen projects for several decades and considers itself an envelope contractor, rather than a glazing contractor.

“In our market, the rainscreen and glazing are often scoped together and if not we will typically bid them together,” he says, explaining there have been projects where they’ve handled just one or both segments.

When discussing most any portion of the envelope, Boeckmann points out that water and air infiltration are the biggest concerns. And it’s for these reasons that rainscreen designs are increasing in popularity.

“They’ve become more common as prevention of air and water infiltration has become increasingly relevant,” he says, adding that the design is also helpful in avoiding condensation within the walls.

“When installing the support clips, insulation and cladding over the air barrier, it’s important to seal any penetrations created. When selecting material and finalizing design, we also recommend considering a non-progressive system. That way, if a panel needs to be replaced down the road you can replace just one without removing the whole system.”

McClatchy points out there are two specifications for rainscreens that are important for installers to understand: AAMA 508, for pressure equalized systems, and AAMA 509 for drained and back ventilated systems.

“The difference between the two systems is the amount of moisture and air that is allowed behind the rainscreen to reach the wall behind it containing the building’s vapor barrier.”

He explains that AAMA 509 systems are designed to allow moisture to permeate the outside wall, but subsequently, drain through weep holes. “It allows more moisture behind it than an AAMA 508 system. The measurements for an AAMA 509 system rate the amount of water and air which passes through the rainscreen façade.”

Likewise, he adds that AAMA 508 reduces the mechanical stress on the panels from high winds. “The system adds more ‘compartmentalization’ to reduce moisture and air penetration. This system uses a pass/fail rating.”

Millslagle agrees that testing is an important part of ensuring a quality product and installation.

“There are a lot of different trades going into the scope trying to consolidate the process into a single source, but the big thing to look for is the quality of the manufacturer. A lot of companies can advertise and say they do composite metal rainscreen, but it’s having the ASTM and AAMA testing that ensures the building owner that the products they are getting are of the highest quality and meet the performance testing required to achieve the longevity and overall success of the system.”

New Opportunities

Incorporating a rainscreen into the design of architectural façades offers a number of aesthetic and performance opportunities and benefits. But the concept is still new to many companies in the U.S. Those already familiar and knowledgeable about this design principle have an opportunity to help educate those less familiar.

“Architects like the look of the rainscreen because there are no exposed sealants, caulking, etc. and it’s clean and crisp,” says Millslagle. “But there are certain elements in construction where a barrier or caulk system still come into play. You have to keep an open mind and rely on manufacturer support to design and use the right product for the job.”

These systems can also offer an opportunity for contract glaziers.

“When a project is 70% curtainwall and 30% opaque, I can see the contract glazier easily hanging these systems,” Osborn says. “Outside of that will be a mixture of specialty rainscreen contractors.”

He also points to the trend toward larger and larger sizes of glass.

“The larger the panels get, the more you’ll need [specialty glass handling equipment] such as suction cups, which the glass companies already have, to pick up and install the panels.”

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