The building façade is the interface between the external and internal environments of a building. Therefore, it has a large impact on occupants’ interface with the surrounding environment; energy efficiency and the indoor environmental quality performance of a building, such as lighting and HVAC electricity loads; and peak load to maintain good lighting level and thermal comfort for the occupants. High performance building façade systems involve selecting and deploying the right materials, advanced technologies, good detailing and installation, all of which must be contextually and functionally appropriate.
Due to the multiple important roles – i.e., aesthetics, thermal comfort, day lighting quality, visual connection to the outdoor environment, acoustic performance, and energy-related performances – building façades, especially glazing systems, have received much attention in research and development. This results in a wide range of products and technologies available to achieve high performance systems.
Solid walls: it was believed that exterior solid walls with high-mass building materials, have better energy performance. The presumption is primarily based on the shifting of peak load conditions or in an reduction in overall heat gain/loss. However, these presumptions have been challenged by recent technology development in material science and thermodynamics – e.g., phase change materials. At present, there are a wide range of high performance solid wall systems – e.g., from cavity insulated walls (150-250mm thick) to composite panels (with insulation materials integrated and thickness as low as 75mm).
To create solid walls with better thermal performance that are thinner, ‘cool paints’ have recently been developed. Compared to conventional exterior surface, cool paints help significantly reduce heat gain through their high solar reflectivity when applied on building façades. The use of cool paints is feasible in hot climatic regions.
Glazing systems: There is a growing interest in glass materials and detailing technologies that result in glazing systems with a high ability to interrupt heat gain/loss while allowing maximum visible light transmission. Figure 1 illustrates the various glazing systems with their respective light transmissions (the percentage of light transmitted through a glazed panel into an interior space). A recently developed material technology involves applying a thin coat of clear metal oxide on a glass surface to enable a reduction in the release of infra reds, resulting in ‘low-emissivity glass’.
Technologies and solutions to enhance the thermal performance of glazing systems include inserting a ‘transparent’ insulator, e.g., inert gas dry air, vacuum, argon, or krypton, between panes in order to provide a good thermal break to reduce thermal conductance. If the width of the air gap is larger, the insulating property of such a double-glazing system is higher. Triple glazing has also been used to achieve even better thermal performance. The additional advantage of double and triple glazing systems is the excellent acoustic performance, which is an additional benefit for buildings located in noise-polluted environments.
Thanks to the availability of different kinds of glass and different combinations, innovative applications have led to the development of smart glazing systems. An example is the glazing system that automatically adjusts its opacity to respond to outdoor lighting conditions, resulting in optimised indoor daylight performance and glare control. Such a system is made possible through the use of photochromic glass technologies.
Another example is a ‘smart window’ with electrified glazing, in which a liquid crystal film is placed between the glazing panes and is controlled by an electrical field to align the crystals so that the window can become clear, or misalign the crystals so that the window can become frosted (Liebard & Herde, 2010). Current research and development of glazing systems also include the integration of thin film photovoltaic, so that a building façade can offer an additional function of generating electricity. However, this technology is still too costly for large-scale market penetration.
One emerging glazing façade system is the double skin façade, consisting of two glazing skins arranged with a ventilated intermediate cavity of 0.2-2m. For a wider cavity – i.e., 0.6m or more, perforated metal catwalks are usually installed for cleaning and maintenance accessibility. Sun shading devices, such as operable blinds, can be installed within the ventilated cavity. The insulated glazing is used as the inner skin. The ventilation in the cavity space can be natural (e.g., wind and/or buoyancy) or mechanically supported, (such as with an exhaust fan). The ventilated cavity serves as a multi-functional space. Besides being used for maintenance access and sun shading, the cavity inlet/outlet can be closed during a cold winter as an additional insulation layer. The cavity can also be used to preheat fresh air intake, before it is supplied to the air handling unit. During a hot summer, natural ventilation can be allowed in to extract the heated air in the cavity. (Liebard & Herde, 2010).
Being contextually appropriate is an prerequisite for high performance façade systems– i.e., designing with local climatic conditions, solar orientation, prevailing wind direction, view opportunity, safety consideration, acoustics, nature of occupancy, and so on. “Since climate and occupant needs are dynamic variables, high performance building façade solution must have the ability to respond and adapt to these variable exterior conditions and to changing occupant needs” (LBNL, 2006). Following are the key application requirements:
Wall to window ratio: is a simple rule for high performance building façade design in response to climatic condition and solar orientation. In temperate climatic regions, it is rational to have a low wall-to-window ratio, as the system will allow daylight to penetrate deep into a building’s internal space and sunlight accessibility during cold winter months. In hot climatic regions, it is less sensible to have a low wall-to-window ratio as sunlight is ample, sky illumination is high, and window/glazing areas are the weak areas for building heat gain. Following the same principle, a high wall-to-window ratio on a west facing façade offers better thermal performance. This is due to the fact that hot afternoon sunlight and radiation are kept away from building’s interior spaces.
Integration of sun shading devices: is essential for glazing systems or glazing areas that are exposed to sunlight. Sun shading devices keep direct sunlight from shining on glazing surfaces, enhance the shading co-efficiency of façades, and result in less thermal transmission through the façade system.
Air-tight but operable: concern about thermal transmittance through building façades has led to the call for air-tight construction. On the other hand, air-tight construction may be detrimental to other building environmental performances, such as natural ventilation and the building’s ability to continue operating during electricity black-outs or HVAC malfunctions. Furthermore, air-tight construction has recently been criticised as a contributing factor to poor indoor air quality and sick building syndrome (Passarelli, 2009). In order to mitigate these issues, it is best to provide operable window/glazed panels as part of an air-tight façade system, giving occupants some level of control. For example, high performance double or tripleglazed operable windows.
Night-time ventilation can be used in double skin façades due to the additional weather protection of the two skin layers and the cavity. It is applicable in hot climatic regions, in summer months in temperate regions and in commercial buildings, which are pre-cooled during the night using natural ventilation. This way, the indoor temperatures will be lower during the early morning hours, reducing the need for and cooling load of air-conditioning (Poirazis, 2006).
Condensation on double-glazing systems. There are three common types of condensation on doubleglazing systems: indoor, outdoor and in-between. Indoor condensation is often caused by high internal humidity together with a low outdoor temperature, which cools the inside glazing surface to below the dew point. Condensation forms on the outdoor surface of glass when the glass’s temperature drops below the outdoor dew point temperature. The use of low-emissivity glass can restrict heat exchange through the air layer between the two panes of glass, thus the inner glass panel is kept warm, which reduces the chances of indoor condensation forming. At the same time, the outer glass panel is not warmed up due to the heat transmission from the indoor and inner glass panel, which reduces the chances of outdoor condensation forming. Lastly, when condensation is formed on the surfaces facing the air cavity between the two glass panels, it is an indication of leakage in the air cavity, where damp air penetrates in the cavity area and forms condensation. The double-glazing system, in this case, does not perform as intended.
Self-cleaning façade solution titanium dioxide (TiO2) can be applied on both the solid walls and glazing system. TiO2 is a type of photo-catalyst. When exposed to sunlight, TiO2 activates its oxygen molecules to decompose germs, bacteria and organic matter. Therefore, by applying a TiO2 coating on external façade surfaces – i.e., aluminium claddings, wall tiles, glass, etc., the façade can perform a self-cleaning function. This helps reduce maintenance and cleaning requirements.
Building envelope commissioning. Since the building envelope is one of the most crucial components determining a building’s thermal and energy performance, it is worthwhile for larger-scale buildings and buildings with complex façade systems to have building envelope commissioning, to safeguard its workmanship, durability and other environmental performance.
Because a building façade is a necessity for every building, large-scale implementation of high performance building façade systems are highly feasible and rely on:
- Designing an appropriate wall-to-window ratio as a cost-effective measure for buildings to be sensible to orientation
- Raising awareness of the importance and benefits of installing high performance building façade systems. The availability of demonstration project(s), from the public or private sectors or both, is particular useful for this purpose. Target groups include building developers, owners, tenants, building-related professionals and the public.
- Making local building codes and regulations related to thermal and daylight performance of building façade systems more stringent over time. It is important to have performance-based rather than prescription-based codes and regulations, so that there is room for new technology development and innovative design. The limit on maximum Overall Thermal Transfer Value (OTTV) or Envelope Thermal Transfer Value (ETTV) is an example of performance-based regulation to control thermal performances of building façades in many local and national governments, e.g., Malaysia, Singapore, many cities in China.
- In places where high performance building façade systems are not used or are unfamiliar, it is useful to first carry out research and development to determine material availability and types of façade systems that are appropriate to the local context, including the climatic conditions, patterns and norms of building occupants’ behaviours defined by local culture and social values, etc. The findings will serve as baselines for further research and development on designs and the implementation of innovative façade systems. Capacity building is then implemented to upgrade the professional’s knowledge and train a workforce with skills for designing, installing, operating and maintaining high performance building façade systems.
Simpler forms of high performance façade systems – i.e., cavity insulated walls, cool paints, double glazing and low-emissivity glass – have already become mainstream in many regions around the world. On the other hand, sophisticated façade systems – i.e., triple-glazing systems, double skin façade systems, the use of photochromic glass and electrified glazing, etc. – have a market limited to high-end buildings. Double skin façade systems are costly and usually applied in high-end commercial projects, as they are aesthetically appealing and project an image of transparency and openness that corporations like to convey to the public.
In temperate regions, both high performance solid walls and glazing systems are common practice and have large market penetration. Cavity insulated walls are found in many residential buildings, while composite panels and double skin façade systems are more popular for application in commercial buildings. In hot and arid climatic regions, solid walls with high thermal storage capacity have been widely used. In hot and humid climatic regions near the equator, the use of low thermal transmission façade technologies and air-tight construction are not popular, due to the appropriateness of natural ventilation in these climate conditions.
High performance building façade systems offer lower heat gain and/or loss and thus reduce the cooling and/or heating loads of a building. This results in electricity saving from HVAC operations and improved thermal comfort for occupants.
Well designed and installed glazing façade systems result in good daylight penetration to a building’s internal spaces without creating a glaze effect. This will also contribute to electricity saving by reducing the use of artificial lighting. Glazing façade systems also offer occupants external views, and enhance the quality of the living or working environment.
Applying a self-cleaning façade solution on the external surface of building façade systems means cleaning is required less frequently. This translates to savings in water and maintenance costs.
Coupling air-tight construction with operable high performance façade systems provides occupants some level of control, enhances indoor air quality, reduces sick building syndrome, improves occupant health, and contributes to occupants’ productivity in commercial buildings.
Because a building façade is a necessity component of a building, the financial requirements depend on the choice of façade system. For example, in general, the cost of a solid wall is lower than that of a glazing system. However, this may not be true for high-end light-weight and super-insulated sandwich panel claddings (typically consisting of two aluminium skins with a mineral wool core), which cost between S$300-S$450/m2 in Singapore (DLS, 2009). This is roughly double the cost of a double glazing system with low-emissivity glass, which ranges from S$180/m2-S$200/m2 (DLS, 2009).
Similarly, building façades with large glazing areas of more sophisticated systems – such as double skin façades, triple glazing operable systems, photochromic glazing, and electrified glazing, require very high investment costs. The figure can be double or triple that of a building façade with a large wall to window ratio and low-emissivity glass.
Maintenance and cleaning costs of glazing systems are higher compared to that of solid walls. An upfront investment to apply a TiO2 coating on the external surface of façade systems can help reduce maintenance and cleaning costs, especially for glazing systems.
DLS. (2009). Green Building Products and Technologies Handbook. Singapore: Davis Langdon & Seah Singapore Pte Ltd.
LBNL. (2006). High-Performance Commercial Building Facades. California: Lawrence Berkeley National Laboratory. [Online]: www.energy.ca.gov/2006publications/CEC-500-2006-052/CEC-500-2006-052-AT1...
Liebard A. & Herde A. D. (2010). Bioclimatic Facades. Paris: Somfy.
Passarelli R. G. (2009). Sick building syndrome: An overview to raise awareness. In Journal of Building Appraisal 5, 55-66 (Summer 2009).
Poirazis H. (2006). Double Skin Façade: A Literature Review. A report of IEA SHC Task 34 ECBCS Annex 43. Lund, Sweden: Lund University. [Online]: www.ecbcs.org/docs/Annex_43_Task34-Double_Skin_Facades_A_Literature_Revi...