Architectural Record BE - Building Enclosure

Extruded Concrete Panels for Rainscreen Assembly

Extruded concrete has become an increasingly popular material choice because it is thinner, offers more design options, and is noncombustible
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Sponsored by Rieder North America
By Andrew A. Hunt

Components and Installation of Rainscreens

As noted earlier, the three main elements of a rainscreen system are the exterior cladding layer (the “screen”), the pressure-equalization chamber (PEC), which has a water-resistant barrier along its sides, and the air gap, which is vented to the outside. Each of these three components has additional features to help keep the water out and to ensure that the system functions properly. It helps to look at the requirements for each element one at a time.

University of New Mexico Cancer Center.

© Ditz Fejer

University of New Mexico Cancer Center

Cladding: Ideally, the cladding is made of a nonporous, durable material, usually formed into panels. The goal of the cladding is to allow water to run off of it—and this means down both sides.

Air gap and vents (also “open joints”): The required air gap provides ventilation between the other two layers. Comments on the recommended gap size are discussed below. On the inside of the gap usually are interlocking dry joints that control the water and air flow through gravity, momentum, surface tension, and capillary action forces. The water-resistant barrier is also usually sealed with caulking, which is protected behind the cladding.

Finally, the pressure equalization chamber (PEC) requires careful design to work with the particular system. The PEC is a series of smaller, airtight chambers, and the size and depth of the chambers dictate how much air can be taken in and how big the given vents need to be. Most PECs also include chamber baffles that resist the wind and pressure loads inside the air gap and are often simply the fasteners that support the external cladding.

In addition to these main components—and their subcomponents—a well-designed rainscreen system also typically includes flashing and drip edges as a means of directing the water out of the air chamber and to the outside. Rainscreen panels may have drainage channels built into them for the purpose of draining the water.

Installation

A rainscreen system is usually installed to vertical furring, which is attached to the wall. The cladding is attached either to the framing or a substrate that can tolerate screws. Furring materials may include steel, wood, or plastic drainage mats, depending on the panel manufacturer’s requirements and builder’s preferences. All of this is done through the furring. Different manufacturers and designers will require that the air gap has a minimum spacing to ensure that the system works properly. However, even a 1/16-inch gap is considered adequate. That said, the reality is that most job-sites will require a gap of at least 1/4-inch to accommodate variations in material thicknesses. Some builders may wish to err on the side of caution and include even deeper air gaps, up to 3/4-inch; this depth is useful in wet climates since it provides more ventilation and thus can reduce drying times.

Builders should note that some rainscreen gaps are easier to work with than others. For example, a 1/4-inch or 3/8-inch gap can make it much simpler to trim out and flash a wall with 1/4-inch furring strips than to do the same job on a wall that has 3/4-inch strips. Another issue is that the air gap distance can dictate (and limit) the furring thickness, especially if it’s being installed over thick rigid foam. Given that rainscreens require adequate furring to be properly attached, the air gap dimension does need to be carefully considered as part of the design process.

Keeping Water Out

Roofing and exterior walls receive bulk water impacts when it rains or snows, or even when conditions prompt exterior condensation, such as morning dew. While roofing systems are designed to shed water downward to the eaves, exterior walls are more exposed to variations in water flow. Moreover, water will react differently depending on its material. For example, a masonry wall will absorb water unless it has been protected with a water-resistant seal, and even then moisture may get in. Because masonry is porous, the moisture will seep through the entire surface, distribute the water, and will eventually dry—unless it encounters a weak spot in the joints, in which case it may leak.

Water that meets nonporous materials, such as metal and glass, hits the surface and can go any direction, depending on the weather conditions. For example, without wind, it may trickle downward, but in rainy, windy weather conditions, it can also push downward, laterally, or even upward under the roof. In windy situations, the water can also work its way into building corners. Again, if there is a weak point in the exterior cladding, the water has a chance to get through.

The rainscreen helps keep water out of the building interior by providing a barrier against the water outside and controlling any water that makes it past the cladding. The internal air gap and water-vapor barrier ensure that any water that does get in is drained through the gap and back outside. Some systems work to force air through the wall cavity/air gap to equalize the pressure between the exterior and interior wall, thus preventing any moisture from entering.

The primary goal of a rainscreen is to manage moisture, whether from a rain-soaked siding or a sheathing that has suffered from accumulated moisture during cold weather. A back-ventilated rainscreen system works to protect the building in several ways. First, the air gap in the system limits wicking between the back of the siding and the water-resistant barrier by providing a capillary break. In addition to preventing wicking action, the space also helps facilitate moisture drainage and airflow, thus protecting the building.

Any water that does accumulate is directed through the gaps provided by the assembly, where it can drain out. Finally, rainscreen walls that have top ventilation employ the stack effect principle, which, in this case, means that when the sun heats a surface (such as a wall), the air within the gap rises. The movement of rising and air escaping from the air gap can quickly dry any accumulated moisture within the wall system.

Risks of Water Entry

A well-built structure should have a thermal envelope that prevents any and all sources of moisture from entering it. Exterior moisture such as snow, rain, or even fog can cause tremendous structural problems, and groundwater entry from flooding can cause lasting damage to the building’s foundation. First and foremost, builders should ensure that the building envelope is designed to prevent any outdoor moisture from entering the building. In environments with extreme humidity and with frequent rain, a dehumidifier can help. However, the first goal is to keep the moisture out.

Unfortunately, in some situations, moisture does enter a structure. Water can enter the building envelope during or after construction, through wet wood or building materials, roof or window leaks, seepage into the basement, through the wall cladding, or any number of other sources. And when it does, bad things can happen. Most professionals in the building industry recognize that water or moisture damage is one of the most expensive problems that can occur to a building, especially if it happens after construction is complete, and even more so if the problem is continual.

Photo of multifunctional building for the Winner Spedition.

© Ditz Fejer

A multifunctional building for the Winner Spedition has been built in Iserlohn. In order to secure growth and quality for the future, the Winner Forum built a meeting point for internal staff training as well as office space for the IT department and central functions at the headquarters. The basic values for the three-story building, with a floor space of 1,200 square meters, are precision, sustainability, and energy efficiency.

 

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Originally published in Architectural Record

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