Physical Attributes, Benefits, and Challenges of Typical Rainscreen Material

The goal of a rainscreen is to prevent moisture from entering the building shell, but also to provide an internal means of water drainage and evaporation if water does get through. 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. Regardless of the rainscreen design, a good rainscreen needs to meet the “ four Ds”: deflect water, drain moisture, dry any water that has penetrated the cladding, and be durable enough to withstand the elements over time without deteriorating.

The physical characteristics of exterior rainscreen cladding tend to follow the four Ds carefully, but with a strong emphasis on the durability. Ideally, the material will deflect water from the building surface or, if somewhat porous, will distribute and dry the moisture before it enters the air cavity. Some materials, such as ceramic tile or different metals, may have a high gloss that can help water shear away from the surface. The trade-off with various materials, however, is that they need to balance water mitigation with weight, strength, and overall durability. From an architectural standpoint, they also need some level of design flexibility.

Commonly used materials include fiberboard, cement board, cast-in-place concrete, and extruded concrete. Here’s how the various materials compare.

Water mitigation: Fiberboard, cement board, cast-in-place concrete, and extruded concrete are all highly moisture resistant. However, fiberboard is not commonly used for siding because of its low fire-resistant properties.

Weight: Material weight is a huge issue for both the shipping of materials to the project site and the installation. One of the main considerations is panel size and thickness, and how those dimensions work to meet a building’s needs. For example, extruded concrete panels are relatively lightweight and can easily be mounted. Larger, thicker panels may be required to handle dramatic temperature changes in extreme climates and strong wind loads. In most cases, extruded concrete panels meet the project needs in ways that fiberboard cannot.

Cast-in-place concrete is a different story in terms of delivery to the project site since it is “delivered” in a ready-mix form and poured on-site. However, by its nature, whatever is produced is very heavy since the thinnest mold tends to be 1-inch thick. Alternatively, extruded concrete is a relatively lightweight material. Panels that are ¹/₂-inch thick do not incur the same high shipping rates as other materials, and the relatively light weight means that the panels are easier for the building team to install.

Strength: Rainscreens, and any other exterior cladding, needs to be strong enough to withstand high winds, debris, or natural elements, such as hailstones. They also must be able to handle freeze-thaw situations without cracking. When we compare the various cladding options, all of the materials stand up fairly well except for the fiberboard, which is not fire resistant. The three concrete materials are all very strong by nature, but over time, cast-in-place concrete tends to degrade if the internal rebar comes in contact with water and begins to rust. And concrete can develop pressure cracks when placed under heavy loads. However, the glass fiber layers embedded in extruded concrete create an incredibly strong material that can withstand almost anything that Mother Nature can throw at it.

Durability: When you’re dealing with exterior cladding that is exposed continually to the elements, durability is critical. Not only must the cladding be able to do its job as a rainscreen, but it must also be able to do so for a long time—if not the lifetime of the building. All concrete products do well in this category, particularly in comparison to wood. Even in climates that experience wintertime freeze-thaw cycles, the panels prove long-term resistance.

Design Flexibility: Given that a building’s exterior cladding is an important part of its architectural design, it is important that the material used for the exterior has some level of flexibility regarding its design, including color, shape, and texture. Fiberboard can be painted and look like wood, thus providing a long-lasting and safe design alternative to real wood. Cement board and cast-in-place concrete can both be polished to have a beautiful shine, sandblasted, or honed smooth for different surface textures, and cast-in-place can be done in a variety of shapes. Again, extruded concrete provides an attractive option; it is made of mineral-based raw materials, giving the panels their unique characteristics: lively surfaces with an interplay of color shades and light cloud effects, rather than dead and clinical surfaces, are characteristic of extruded concrete. Furthermore, it can be formed in flat panels or curves (“U” shapes), and it is through-colored instead of only painted. As a result, architects have a broad range of options for their design when they choose this material.

© Ditz Fejer

© Ditz Fejer

CASE STUDY: AN ELEGANT NEW CONCRETE SKIN FACADE PROVIDES A MODERN BACKDROP TO MONTRÉAL’S ICONIC MOUNT STEPHEN CLUB

© Ditz Fejer

More than 1,000 square meters of extruded concrete facade slabs in cross form as well as window openings of different sizes create an abstract pattern on the facade of the new hotel building behind the iconic Mount Stephen Club in Montréal.

Architect: Lemay Architects
Product: 1,000 square meters of concrete skin facade panels
Color: Ivory
Surface finish: Matte

The Mount Stephen Club, designed by architect William Titus Thomas in the 1880s, is an iconic historical monument of Quebec. The building’s rich heritage lies in its neoclassical architecture, inspired by the classic aesthetic of ancient Greece and Rome. A recent refurbishment project sought to maintain its historical character while bringing in a clean, modern look as a backdrop.

Challenge: Lemay Architects’ design approach for the rehabilitation of the old structure and the new construction of the hotel was to maintain the historical piece as a major element in the foreground while creating a complementary new facade for the 12-story hotel behind it—one that would suit Montréal’s lively, urban setting.

Solution: The facade was designed as a contrasting background to the unique character of the Mount Stephen Club. The aim was to create an abstract pattern that is not in competition with the neoclassical decorations of the historic building, but one that would highlight design elements inspired by the tapestries, wallpaper, and lace fabrics of the landmark.

The architects decided to implement the facade using European-crafted large-format extruded concrete skin panels, which were only 13 millimeters thick and offered a wide scope for the design of individual facades. Within the maximum size of 1.2 x 3.6 meters, the panel format is freely configurable. Due to their formability, the concrete skin panels were able to fit smoothly over corners and edges. The plates were completely dyed with natural color pigments, and the architects were able to choose from 12 colors, each with three surface finishes.

Result: The unique resulting design for the hotel’s building envelope was realized with a modern fiberglass-reinforced extruded concrete skin. The more than 1,000 square meters of ivory-colored, matte-surface finish (which is a honing process) facade panels punctuate the architect’s intention of creating a structure that equally highlights the expression of the elegant historic building in the foreground, while simultaneously contributing to the allure of a contemporary masterpiece. The more than 1,000 concrete skin elements form a cross shape, and the different sizes of window openings provide a spectacular, luminous pattern when viewed in the dark.