External Cladding

Live-loads
The first factor that is normally taken into consideration is structural integrity and safety, which becomes especially critical the larger the cladding units and the higher the building becomes, due to higher live-loads (wind pressures and building movements) applied to the cladding. This factor becomes more complex once security requirements like blast-proof are added to the equation. The higher live loads are applied to the cladding units, the stronger yet more flexible the selected building material needs to be.

Shading & thermal insulation
Another major driver is energy performance and the ability of the building to reduce energy exchange between the internal and external environments of the building. This includes the ability of the façade system/materials to shade the building during intense sunny conditions and to insulate the building during extremely hot/cold temperatures. The latter especially requires materials that are low in thermal conductivity like wood and rockwool. Plastic-based materials are also low in thermal conductivity; however, they should be used with a lot of caution as they are prone to burning or emitting high amounts of smoke if exposed directly to fire. Introducing air gaps or inert gases into façade walls is also common practice, like Insulated Glass Units (IGUs), double walls, hollow blocks, etc. However, if not designed and ventilated properly, façades with cavities can cause condensation and grow mold.

Lighting
Another major factor is the ability of the façade to optimise and balance the admission of natural sunlight; too much sunlight causes overheating and glare, and too little creates a dim and lifeless environment of very low energy indeed.

Weather tightness
This is the ability of the façade to keep rainwater out of the building while controlling (not preventing) air infiltration. This also includes incorporating a proper ventilation mechanism while minimising thermal bridging that could cause condensation, especially during highly humid conditions. It is important to emphasize that no façade system can prevent water and air infiltration 100%. Good façade design entails creating a system that can channel water, that manages to breach or collect behind the first line of defence, out of the building again, while controlling the ability of the façade to breathe. The latter is especially critical when it comes to condensation; some forms of condensation can be avoided by avoiding thermal bridging (contact of cold components with humid air), while other forms of condensation are inevitable during highly humid conditions and high due-point temperatures, therefore, the ventilation of cavities is very critical in this case to avoid the collection of condensed water causing water penetration, rust, mold, and other serious issues.

Acoustics
The facade needs to be able to reduce noise, especially if surrounded by major sources of noise like highways and airports, or if subjected to highly windy conditions. Acoustics can be very tricky and counterintuitive in many ways. For example, if opening areas in a façade screen reach about 10% of the total area of the screen, it becomes almost useless as an acoustic barrier. Similar to thermal conductivity, using materials that are low in acoustic conductivity is a good starting point to reduce noise. Metal for example is one of the worst materials in terms of sound insulation due to its high-density and conductive nature, while double walls and IGUs with incorporated air cavities/chambers tend to be most effective in reducing noise. Also, varying the external and internal wall layers in terms of material and thickness (like combining hollow blocks with bricks with a cavity in between, or by introducing a laminated lite in the IGU) can increase the sound insulation quality of façades.

Durability & maintenance
Façade materials need to resist several environmental elements that could cause components to deteriorate quickly like solar rays, humidity, salination, dust and sand, and other air particles. That said, façade materials should be able to embrace (and not defy) such environmental elements to minimise requirements nt for cleaning and maintenance, and extend the overall service life of the façade. For example, cladding an entire tower with glass in a hot desert environment is counterintuitive on every level; technical performance, user comfort, and last but least cleaning and maintenance. The latter is especially the case as glass façades easily attract dust and sand that stick to their surface, which in return require a lot of water (a scarcity in desert regions) to clean. Furthermore, over-exposure to solar rays accelerates the deterioration of critical curtain wall components like gaskets and IGU sealants. Therefore, a careful study of the building context and maximising the use of indigenous materials that have withstood the test of time in the building environment is a good starting point in that respect. Finally, bird dropping is another major challenge that we face in maintaining building façades, and while many solutions have been applied over the years like spikes, poison, falcons, light reflectors, ultra-sound devices, etc. none has been entirely effective in keeping birds away from building façades. Therefore, attention to geometric articulation should be given to avoid creating unreachable corners where birds can safely nest.