Façades Beyond Curtain Walls and Metal Cladding

 In the span of two or three short decades, glazed curtain walls and metal panel cladding have become ubiquitous, gracing the skylines of cities as distant and widespread as Chicago, Sydney, London, Singapore, Santiago and Johannesburg. Building designers and the public alike have become used, or we should say addicted, to these types of façades. Yet, there exists a plethora of other materials or systems that can be considered when designing building envelopes, and which offer an incredible array of aesthetics. The latter need to fulfil a variety of functions, including keeping the weather out, ensuring the safety of the public in and out of the buildings, helping achieve a comfortable living and working indoor environment; and much more. When considering a material or system to use in the design of a building envelope, one needs to bear in mind a number of performance requirements.

 
It thus seems appropriate to carry out a systematic review of the various other façade materials that reside in a designer’s palette, in order to assess the viability of these available choices. As designers, it is absolutely essential that we understand the pros and cons of each alternative, in addition to any possible limitations in terms of their size, availability, formability, cost, etc.
 

FAÇADE PERFORMANCE
1. Aesthetics
Etymologically, the façade is quite literally the face of the building. It defines its identity, and is often what the public would use to describe or to refer to a specific development. Consequently, the aesthetics of the building envelope is a fundamental aspect to consider. It needs to exhibit flair and refinement. Other than the overall shape of the envelope, which ties with the internal space planning and layouts, the architect should be able to work on qualities such as modulations, texture, colour, reflectivity, gloss level, as well as the possibility to incorporate holes or patterns on the surface for aesthetics, daylighting or natural ventilation. All of these parameters, in conjunction with each other, define the appearance of the building.
 

2. Structure
Besides the aesthetics, the design team needs to consider the structural aspects of the façade design. The latter needs to be able to withstand wind load, live load, self-weight, seismic loads, snow load, as well as the building movements under various loading conditions, and all of the above should be accommodated for the full lifespan of the building envelope. Several considerations will influence the size of the building envelope framing members, including the type of system, material strength, testing, design code, modulations, loading and floor-to-floor height, amongst others.
 

3. Durability

Aesthetics alone is not sufficient if the latter does not last for an adequate period of time. The building envelope should be designed using materials and systems that are durable. The façade of a building is susceptible to multiple sources of aggression, including the sun, rain, temperature variations, and other weather conditions. Under the effects of these elements, cladding materials may fade, crack, chip, craze, chalk, debond, warp, become scratched, etc. It should be noted that regardless of how careful one is with the material selection process, no material or system can last for their intended lifespan without proper and regular maintenance.

4. Safety
In addition, the façade should be designed to be safe while in service. Besides strength, which is considered in the structural design mentioned earlier, the building envelope should be resistant to impacts, be they from building users, the general public, vehicles in and around the building, or the building maintenance equipment. In case of impacts of a foreseeable nature, the façade should not be damaged and should remain safe for users and passers-by. Recent unfortunate events have also highlighted the particular importance that one should lend to the fire behaviour of the façade. This involves not only testing and verifying the reaction to fire of the materials used, but also paying careful attention to the detailing and testing of the overall façade systems, since the use of certain materials in combination with each other, and the configuration of the façade elements and fire-stopping systems all contribute to achieving an acceptable behaviour in case of fire.
 
5. Weather Performance

One of the main functions of the building envelope is to shield the occupants from the weather. This includes preventing water penetration, as well as limiting air infiltration and exfiltration so as to ensure proper thermal performance and minimise the risk of condensation and mould growth internally. Restricting the passage of air through the façade also helps avoid whistling noise (sometimes fondly called “ghost noises”), and improves the overall acoustic performance of the façade, limiting the ingress of external noises.

6. Acoustics
The acoustic performance of the façade is an essential aspect to consider as well. Beyond limiting air infiltration, as mentioned above, the building envelope should also be designed to sufficiently attenuate external noise sources, in particular, traffic and mechanical equipment. The STC (Sound Transmission Class) or OITC (Outdoor/Indoor Transmission Class) values of the façade system can be estimated from past data, or preferably accurately determined through laboratory and/or site tests. Besides, the façade should also be designed in such a way as to prevent self-generated noises, such as those that can occur due to the friction of metal on metal parts.
 

7. Thermal
Equally, if not more, important is the thermal performance of the façade. In the tropics, the contribution of the building envelope to the heat load can be as much as 30%. Considering that in commercial buildings, cooling systems can contribute up to 50% of the total energy bill, improving the thermal performance of the façade also contributes to reducing the size of HVAC (Heating, Ventilation and Air Conditioning) systems, and lowering the energy cost of the building while in operation. A similar situation exists in temperate and cold climate, where heating constitutes a major part of the energy consumption of the building and can be significantly reduced by reducing the amounts of convective, conductive and radiant heat passing through the façade.
 

8. Sustainability
Beyond simply looking at the thermal aspects, building envelopes should be designed in the most sustainable manner possible. A variety of measures can be taken towards this, including sourcing sustainable materials such as the use of recycled or recyclable materials (many façade materials, including glass, aluminium and steel, are infinitely recyclable), limiting or altogether avoiding the selection of components with high VOC (volatile organic compounds) content, ensuring that the materials chosen for the façade elements have been produced in ISO14001-certified factories, incorporating building envelope components that can generate energy (from solar, wind or other renewable sources), etc.
 

9. Cost

Last, but not least, cost is a major consideration in the design of a façade and in the selection of materials and components. On a high-rise building, in particular, the cost of the building envelope could be in the range of 20% of the total construction cost of the building, or even more, since for taller developments, the ratio of façade area to Gross Floor Area increases as the building height increases. In many cases, regrettably, the cost is the ultimate deciding factor, beyond aesthetics and other considerations listed above. Judicious material choices and precise engineering can have a dramatic impact on cost and are thus crucial steps in the design process.

FAÇADE MATERIALS
1. Stone

Stone is a natural material that has been used in building construction for millennia. Its use in a decorative, veneer application is much more recent. By virtue of its natural constitution, the properties of stones vary widely, not only between stones of different types but even between blocks extracted from different layers (or benches) of the same quarry. This highlights the need for extensive testing to determine the actual properties of the panels that will be used for a specific project. This will allow the designer to ascertain the specific gravity, water absorption, flexural strength, compressive strength, modulus of rupture, amongst others, as well as the variability of these properties.
Generally, granite is considered the most suitable type of stone for use in external façade cladding, thanks to its very low porosity, high strength, and good consistency. Unfortunately, one of the defining features of granite is the presence of coarse or fine black inclusions (amphiboles and/or biotites), which many architects object to, finding them unappealing aesthetically.
 
Other options include marble, which is visually appealing but is often inadequate for external use, due to the presence of inclusions that will discolour when in presence of water, as well as the outward bowing that characterises wet marble veneer panels.
 
Sandstones and limestones are also very popular due to their consistent appearance and particular grain. However, these types of stones also generally exhibit a very high water absorption, low compressive strength, and other properties that make them unsuitable for external use as façade panels. Exceptions do exist, but they are few and far apart, so careful consideration and extensive testing are required.
 
Finally, travertine frequently comes up in design discussions. The cross-cut version, in particular, is very attractive visually, thanks to its marked lines. On the other hand, travertines are also typified by the presence of voids of various sizes, which affect the consistency and structural strength of this stone. Once again, before electing to use a specific type of travertine, extensive testing should be conducted.
 
Regardless of the stone selected, the designers should consider available volume, factoring in possibly high wastage volumes. One wouldn’t want to reach near the end of the project, only to realise that the quarry has run out of the type of stone required to complete the project.

There exist alternatives to solid natural stones, including reconstituted panels made of crushed stone cast together with a resin or cementitious materials, or sandwich panels which are composed of a very thin layer of stone bonded to a layer of aluminium honeycomb for increased stiffness and flexural strengths. One should only use these materials in the building envelope with extreme caution. In particular, the sandwiched panels are made of two dissimilar materials that are bonded together. That bond is very strong, but it is not possible to assess how long this bond will last when subjected to heat, rain and other external factors. Bond failure would have catastrophic consequences.

2. Architectural Precast Concrete (APC)

APC has been adopted for façade elements for many decades, falling alternatingly in and out of favour over the years. On one hand, as a material, it offers excellent properties in terms of thermal, acoustic and weather-tightness performance. It can also be designed to take on very simple or very complex shapes, with finishes as diverse as plain off-mould, painted, acid-wash, stone veneer and more. On the other hand, due to its thickness and density, architectural precast façade elements tend to be heavier than other options, making transportation and lifting more complex. The substantial weight can also contribute to an increase in the size of structural elements at the periphery of the building.
 
When designing precast concrete façades, a designer should be cognizant of the need to maintain a minimum cover to the steel reinforcement. This can have a significant impact on the thickness of the precast panels when the design calls for grooves or other aesthetic features. Besides simple, minimal geometric patterns, form-liners can be used to achieve highly elaborate surfaces. These consist of thick and dense moulded silicone sheets that are placed inside the concrete forms before casting, creating a three-dimensional stencilled surface for the architectural precast panels. Using such a system, there is virtually no limit to what can be achieved, especially when combined with white cement and pigments to obtain durable coloured surfaces.
 

3. Autoclaved Aerated Lightweight Concrete (AALC)

Based on a more modern process, AALC improves on some of the properties of typical architectural precast concrete. Sometimes also called foamed concrete, it is manufactured following an industrial process that is much faster to cure, and more tightly controlled than standard precast concrete. AALC is produced using a foaming agent which makes this product not only considerably lighter than precast concrete but also contributes to much improved thermal performance (U-value). Nothing comes free, though. These improved properties are balanced by a lower flexural strength, as well as an increased susceptibility to breakage or chipping-off from impact. The range of finishes is also much narrower as compared to architectural precast concrete, being largely limited to painted surfaces or integral pigments.

4. Ultra-High Performance Concrete (UHPC)

Ultra-High Performance Concrete stands at the other end of the spectrum of architectural concrete products. It is characterised by very high strength, which means that for veneer façade applications, it normally does not require steel reinforcement, and can be much thinner than traditional concrete products. It offers high durability and high formability. In particular, it can easily feature holes or surface patterns, since maintaining a minimum cover for steel bars is not required. However, these beneficial properties come at a relatively high cost, so designers have to weigh the pros and cons before deciding to adopt this material.
 
5. Glass-Fibre Reinforced Concrete (GRC/GFRC)

For more refined applications or designs that require finer details, but with similar, solid appearance as that of concrete, GRC represents the ideal choice. Produced using a mixture of cement-sand slurry and alkali-resistant glass fibres (used as tensile reinforcement), it is much thinner and lighter than precast concrete. GRC is either cast or sprayed into moulds, making it easy to obtain very fine details. Typically, GRC is produced using white cement, which is mixed with pigments to obtain various colours of panels. On a square metre basis, GRC is a more expensive material than architectural precast concrete. However, this needs to be weighed against the much finer appearance and details that can be obtained with GRC, as well as the logistical advantages of the latter (easier and faster to transport and erect).
 

6. Fibre Cement Boards

One cannot conclude a round-up of cement-based façade options without discussing the merits of fibre cement boards. Like AALC, this material is produced through an online industrial process, resulting in thin, but strong and light slats of various colours (the choice of which admittedly is rather limited). One clear advantage of fibre cement boards is that the material is naturally very flat by virtue of its manufacturing process. It is also very easy to form into different shapes of panels, as it can simply be sawn to obtain the required configuration. Besides colour variants, it is possible to emboss patterns on the surface of the panels, such as grooves. Fibre-cement-clad buildings look very neat and have a clean appearance. Unfortunately, this material is fairly expensive, so it tends to be limited in its adoption for building envelopes.
 
A summary of the relative properties of the various traditional materials presented above is included in Table 1 below (more stars * is better; when multiple ratings are given, this means that the performance depends on the actual material and/or finish selected):

7. High-Pressure Compact Laminates (HPL/HPCL)

HPL or HPCL refers to decorative high-pressure compact laminate with an integral surface and consisting of a blend of up to 70% natural fibres and thermosetting resins, manufactured under high pressures and temperatures, yielding a highly stable, dense panel with a good strength-to-weight ratio. The outer decorative layer can be supplied in a wide array of finishes, ranging from plain colours to simulated woodgrain. One advantage of this product over other options is that it can be procured in small volumes, making it possible to mix and match colours and textures if the design calls for it. Designers should be careful with fire performance, as it varies substantially from one manufacturer to another, and between different product lines.
 

8. Carbon Fibre Composite (CFC)

Thrown into the limelight with the completion of the column-less Apple Theatre in Cupertino, CFC exhibits an extremely high strength-to-weight ratio, high durability and high formability but all coming at a high cost, because of the relatively low number of manufacturers and low volume produced for use as cladding, at least for the time being. With increased demand, prices are expected to come down, thereby increasing the attractivity of this material.

9. Glass Fibre Reinforced Plastics/Polyester (GRP)
Also in the family of products that comprise of glass fibre reinforcement, GRP is similar in nature to the material used to fabricate the hulls of speedboats. Consisting of very thin shells that are much lighter than APC or even GRC, it is easy to obtain very fine details, a broad gamut of colours and finishes, as well as a very wide range of gloss levels. However, designers should be careful in specifying GRP, as special treatments need to be applied in order to achieve reasonable levels of UV (ultra violet) resistance and fire performance.

10. Terracotta
Terracotta is a traditional material that is now produced using modern techniques on automated production lines. Made from clay that is baked under specific conditions (in Italian, “terra” = earth, and “cotta” = cooked or baked), this cladding material is available in either matt or glossy (glazed) versions, as well as in a variety of plain colours. The terracotta cladding elements can be extruded into virtually any shape, from simple plates or hollow tubes (sometimes called “baguettes”) to fairly elaborate shapes that are only limited by the designer’s imagination and the extruding process. Typically, the length of terracotta cladding elements is limited to 1,500mm or 1,800mm, even though some manufacturers offer lengths of up to 3,000mm based on special requests. One should note that terracotta is a brittle material. Although it is relatively robust, this cladding material should be protected from sharp impacts (from building maintenance cradles, for instance), as pieces may become detached and fall off, should they be broken.

11. Ceramic
Ceramic is another traditional building material that has been updated to meet the expectations of modern buildings. Relatively inexpensive, ceramic tiles are available in unlimited colours, designs and textures which make it a very popular design option for cladding. This material has a very consistent appearance and aesthetic appeal, and can even be manufactured to match the appearance of other materials (e.g. stone, wood or Corten steel).

On one hand, ceramic cladding is very durable, easy to clean and requires only low maintenance. It is resistant to external weather conditions, rain and pollution. Modern ceramic cladding is available in a variety of sizes, with some manufacturers offering up to 1,000mm wide by 3,000mm tall panels. On the other hand, just like terracotta, it is a brittle material, which also is very thin. Although it is relatively robust, this cladding material should only be used in situations where it will not be subjected to sharp impacts, as it could shatter and fall off.
 
12. EIFS

Very popular and commonly used in the US, EIFS (External Insulated Façade System) refers to a prefabricated façade which consists in a sandwich of layers (strong backboard substrate, insulation layer, reinforcing mesh, and finish layers), lending to this material high formability, high strength, very lightweight and good thermal performance. The finished surface can be anything that can be achieved with a stucco finish, but as is the case for many other materials described earlier, fire performance can be an issue and needs to be carefully considered.

A summary of the relative properties of the various traditional materials presented above is included in Table 2 below:

13. Metal Meshes

Metal meshes captivate in the field of architecture thanks to the purist elegance of stainless steel. Typically made of welded or woven stainless steel (or sometimes bronze) strands or wires, metal meshes lend a sheer, ethereal appearance to buildings. By virtue of the materials used in their fabrication, they offer very high durability and variable porosity, which allows designers to play with daylight, outward and inward view, as well as shading.

However, the better products often come at a relatively high cost. Some manufacturers even offer integrated LED lighting systems, directly woven into the mesh, for night lighting. To enable the use of colours in architecture, various surface treatment procedures have also developed for wire meshes. Depending on the location, coloured architectural meshes convey different impressions and appearances. Metal meshes can be assigned to the three groups of cable mesh, PC mesh and spiral mesh based on their production process. Complex graphics on metal meshes for outdoor architecture are applied using the screen-printing technique. Depending on the application, the print format ranges from a few centimetres to several metres.
 

14. Cast Glass
For a different take on the use of glass in building envelopes, designers should consider cast glass. Contrary to the vast majority of architectural glass produced using the float process, cast glass is normally produced on a relative scale, by pouring (casting) molten glass into bespoke moulds in order to achieve complex, organic shapes and surfaces. This allows designers to express their creativity while working with a familiar material. This is also one of the few material options that can be considered when there is a need or desire to allow natural daylight to pass through. However, due to its high cost (unless large repetition is involved), it is relatively seldom used in building envelopes.


15. Polycarbonate

Polycarbonate (sometimes also called acrylic) comes in the form of clear or translucent sheets (or sometimes in the form of extruded tubes) that offer high formability as well as a vast choice of integral colours, all for a relatively modest price. It can also be seamlessly welded into very large and very thick sheets, which explains its use for the walls of large aquaria. It is mentioned here for completeness and because it is sometimes used in building envelopes, but it is also typically not adequate to be adopted as a façade material due to poor UV resistance affecting its durability, greater propensity to be scratched as compared to glass, as well as poor reaction to fire (flame spread and flaming droplets) which makes this material incompatible with most regulations governing the fire performance of façade elements.

16. Solid Surfaces

Like polycarbonate, solid surfaces, such as Dupont’s Corian or LG’s Hausys, exhibit high three-dimensional formability, the ability to be seamlessly welded into large expanses; and a wide range of integral colour. However, contrary to polycarbonate, solid surfaces, as the name suggests, are opaque rather than transparent. Also, they display excellent scratch resistance and good UV resistance, but the latter applies only to the lightest available colours (the only ones that can be used externally). However, due to the high cost of solid surfaces, their use tends to be limited to small areas inside the buildings rather than on the building envelope.
 

17. PVC Membranes

PVC is the most cost-effective membrane material and, therefore, an ideal choice for both temporary and permanent tension structures including architectural umbrellas. The material is soft, pliable and less expensive than PTFE or ETFE (see below). PVC, or polyvinyl chloride, is available as a woven material. This flexible fabric membrane reduces radiant heat gain, keeping interior temperatures cooler. PVC membranes can be infused with UV light inhibitors and anti-soiling fungicides. In addition, PVC coated with non-toxic and flame-resistant TiO₂ (titanium dioxide) produces a photocatalytic membrane that actively neutralises airborne pollutants and odours. This fabric also offers aesthetic features such as partial translucency, which allows natural daylighting, increased textural interest and wide colour selection. Careful attention needs to be given to the durability of the specific product selected, and stringent fire testing should be conducted in situations where public assembly is likely to take place below this membrane. Finally, maintainability and access are often forgotten at the design stage but are essential aspects to consider in order to ensure the extended lifespan of this product.
 

18. PTFE Membranes

PTFE, or polytetrafluoroethylene, is a Teflon^®-coated woven fibreglass membrane that is extremely durable, weather resistant and a highly sustainable building material for roofing applications. PTFE fibreglass membranes can be installed in climates ranging from the frigid Arctic to scorching desert heat with a reasonable project life, provided that a proper maintenance regime is in place. Before installation, PTFE has an irregular off-white or slightly brown colour, which is caused by the manufacturing and fabrication process. Once exposed to direct sunlight, the external surface of the membrane bleaches to a milky white within a matter of days. The low-surface free energy of the material creates a surface which is readily cleaned by rainwater. It is also completely immune to UV degradation. Just like PVC membranes, TiO₂-coated PTFE is a popular choice for designers and architects in sustainable commercial roofing market sector. Some specific products can even be folded to create openable or retractable covers. Fire performance needs to be verified through thorough testing, as with other membrane materials.

19. Textile Façades

Textile façades consist of a woven fibre material that has been coated in a protective and decorative surface. Lightweight and cost-effective, this material is also well adapted to complex geometries. It can be either solid or perforated, with variable porosity in order to achieve various levels of visible light transmission and shading coefficient. This material is often adopted when retrofitting an existing building, as it allows designers to improve both the aesthetics and thermal performance of the envelope, often without the need for structural strengthening, since the material is very lightweight. Further, some models of textile façades can feature drawings, patterns or signage through the use of digital or serigraphic printing on the front face.
 

20. ETFE

ETFE (Ethylene Tetrafluoroethylene) comes in the form of a single layer or as a cushion with double or triple layers, and is used in tensile architecture applications. The material is durable, highly transparent (approximate 85% light transmission in the absence of tints or applied graphics) and very lightweight in comparison to glass structures. ETFE is actually not a fabric but a film used as an alternative to glass. It can be supplied as a single layer membrane supported by a cable net system or commonly as a series of pneumatic cushions made up of between two or more layers.
 
Multi-layer “pillows” or “cushion” are attached to an aluminium perimeter extrusion which is supported by the main structural frame. In the case of ETFE cushions, they are kept continually pressurised by a small inflation system the size of a washing machine which maintains the pressure and gives the foil a structural stability and the roof some thermal insulation properties (low U-value). It can have frits or graphics applied onto it to reduce its light and heat transmission. ETFE is often considered the material of choice for traditional skylight or canopy applications, on long-span structures and for building façades.
 
Due to the lightweight nature of ETFE, substructure support systems and concrete foundations can be designed more efficiently (much reduced framing sizes and weight). ETFE is also acoustically “transparent”, in that it offers little impediment to the passage of sound waves. This is beneficial to reduce internal reverberation (echoing), but can contribute significant noise levels during rain events.

A summary of the relative properties of the various traditional materials presented above is included in Table 3 below:

CONCLUSION

Despite being fairly lengthy, this article only constitutes a brief introduction to the various parameters to consider and the various material options that can be adopted when designing building envelopes. Architects and designers can build up experience gradually by cleverly experimenting on projects over time, learning what works, and progressively becoming more familiar with a variety of materials.

 

This knowledge paper is written in conjunction with DP’s in-house DesignShare talk conducted by DP Façade on 22/06/2018.