Building the Ideal Rainscreen: Advantages of Extruded Concrete Panels

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

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 161-inch gap is considered adequate. That said, the reality is that most job-sites will require a gap of at least 14-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 34-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 14-inch or 38-inch gap can make it much simpler to trim out and flash a wall with 14-inch furring strips than to do the same job on a wall that has 34-inch strips. Another issue is that the air gap distance can dictate (and limit) the furring thickness, especially 44 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.

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Rainscreens Keeping Occupants Safe

Aside from rotting and warping wood, attracting insects, and damaging other building materials, water intrusion can create basement puddles, cause mold to develop on and within walls, and set off a chain reaction of many other problems. Mold issues, for example, quickly affect the air quality of the whole building and can affect the health, safety, and welfare of the occupants, usually in the form of respiratory illness.

Damp buildings and the associated moisture that can flow through the building’s airways can result in mold, fungus, and spore growth, which is then spread through the ventilation system and into the building’s air. Even more serious is if certain building materials begin to degrade from moisture exposure; many materials can release volatile organic compounds, which can cause significant adverse health effects with long-term exposure. Often the water intrusion and damage isn’t noticed until building occupants start to experience health problems. It goes without saying that the best practice is to ensure that the building envelope is sound and that moisture does not get into the building in the first place, and any moisture that does is dealt with immediately.

Given the seriousness of moisture in buildings, whether for potential structural damage, material degradation, or occupant health and safety, building professionals need to plan a water-mitigation strategy.

Cladding and facades that repel water and keep the building secure are necessary to ensure both durability and longevity of the building. In environments that have high humidity or exposure to moisture, such as coastal zones, a rainscreen system can be highly effective in keeping water out of the building. Designers should consider their water-mitigation strategy early on in the design process and recognize the value of a multilayered rainscreen system.

Building the Ideal Rainscreen

As part of a water-mitigation system, rainscreens first and foremost need to repel water. However, they also need to be durable enough to withstand other environmental forces, such as the wind, the sun, and extreme heat and cold, and to do so over time. Wood, for example, is porous and loses out on long-term durability. This leads us to stronger materials, such as metals, stone, glass, and concrete, all of which, when used as exterior cladding, can provide long-term protection without being negatively affected by the elements and moisture they are designed to withstand.

Architects and designers have a wide range of options of materials available that can satisfy this core requirement, while also contributing to an aesthetically beautiful building envelope. Some of the more commonly used materials for rainscreens are: aluminum composite materials (ACM), stone, fiber cement, high-pressure laminate (HPL), reconstituted stone, terracotta, timber, ceramic tile, glass reinforced plastic (GRP), fibrous concrete, or, more recently, extruded concrete. For the purpose of this course, we will focus on the commonly used fiberboard, cement board, cast-in-place concrete, and extruded concrete.

 

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Originally published in Architectural Record

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