When it comes to green construction, the building envelope falls under heavy scrutiny. It is integral to meeting sustainability targets – a fact which is acknowledged by the majority of green building rating systems.
The scope of green façades involves a variety of aspects. These include improving environmental performance, allowing more natural daylight to minimise the need for artificial lighting, and minimising heat gain or loss.
It can also mean adopting systems or materials that have a reduced or even positive impact on the environment, such as energy-generating features, self-cleaning materials for maintenance reduction, recycled materials, the local sourcing of materials to minimise transportation-related energy use, or even the optimisation of panel modulations to reduce or eliminate wastage.
Some of these strategies need to work in combination either with their peers (glass selection involves multiple considerations), or with environmental strategies applied to other parts of the building (e.g. HVAC system, lighting systems), while other “green tactics” can actually work against each other.
Increasing window-to-wall ratio or glass visible light transmission, for example, will reduce the need for artificial lighting but increase the risk of glare and potentially increase heat penetration. Similarly, increasing the modulations will reduce the need for sub-framing (less materials), but this in turn means thicker panels (more materials). It’s all about balance.
Recyclability and use of recycled materials can have a tremendous impact. The most common materials for modern building envelopes - glass, steel and aluminium - are good performers in this respect. Broken glass pieces (cullets) are in fact used extensively in the production of fresh glass, which actually helps lower furnace temperatures, killing two birds with one stone. Recycled aluminium only consumes about five per cent of the energy used to produce new aluminium from raw bauxite ore.
Many façade materials which have traditionally been impossible to recycle, such as composite metal panels and laminated glass, have been the subject of major research initiatives. Meanwhile, a new system has been developed to delaminate bonded sheets of glass with minimal use of energy. Both the glass sheets and the interlayer can be reused or recycled.
Modern façades typically incorporate substantial glazed areas. This is considered wholly unnecessary in a tropical location, where the intensity of solar radiation is extremely high, and thus controlling the heat flux into buildings is of primary importance. However, there is more to it than this.
Double-glazed units (DGUs) with low-emissivity (low-e) coating offer a U-value (the measure of the thermal conductivity of a material) which is lower than solid concrete or brick walls. This can be further reduced by adopting argon- or krypton-filled insulated glazing units (IGUs). Scientists are also pursuing intensive research into “vacuum-filled” IGUs, which would yield a U-value close to zero (virtually no conducted heat).
Glazing also allows a certain amount of radiated heat (infra-red) to penetrate the building. This is measured by a quantity called the solar heat gain coefficient (SHGC). One option to control or reduce the amount of radiated heat entering the building is to use glass with a lower SHGC. To reduce this value without reducing visible light transmission, low-e glass has been developed with ever-increasing light-to-solar gain (LSG) ratios. However, the development of higher performance low-e coating seems to be plateauing. Next generation thin-film or nano-coatings appear promising, although commercially-available products currently fall short of expectations.
Another strategy is external shading devices. These consist of vertical or horizontal elements of varying density or opacity. While these can prove extremely useful at certain times of the day, they are useless at other times. A possible alternative would be to turn to traditional strategies.
In the Middle East, windows are typically placed high above floor level (lower-window-to-wall ratio), set deep into the surrounding walls, and orientated so the light coming in is channelled toward the ceiling. This has the double advantage of offering excellent protection against solar radiation while allowing sufficient light to enter and negating the need for artificial light for extended periods. This demonstrates how simple passive strategies can outperform active automated solutions. With the right strategy and a holistic approach, glazed façades can produce exceptional performance.
Framing systems for windows and curtain walls provide similar opportunities, since their aluminium or steel members excel at conducting heat. Adopting thermally-broken frames has a strong positive impact in most climates as they essentially act like a series of radiators or heat sinks. It is thus important to design façades with thermally-broken frames to avoid underestimating the amount of heat gained or lost through the building envelope.
Since facades takes energy to produce, it is logical to consider how this energy can be offset through energy generation by the façade.
Building-integrated photo-voltaic panels have been used for over 15 years. It is unclear, however, whether the amount of energy produced justifies the expense. In the tropics, the north and south elevation of buildings see little direct solar radiation. The east and west elevations are only exposed to the sun for a few hours and the rest of the time they only receive dimmer, indirect light. Additionally, the efficiency of the panels is reduced at elevated temperatures, and their exterior glass surface reflects much of the sun’s light at sharper angles, further reducing the range of times when panels can perform at peak efficiency. Roof-mounted panels would appear a more sensible option.
Non-silicon-based thin-film paints are more common, and have fewer of the issues mentioned above. Most are black in colour, however, which limits their potential use for aesthetic reasons.
With solar energy a hot research topic, there is nonetheless hope that the technology will become readily deployable.
Green façades may not be a reality yet in a holistic sense, but thanks to intense research combined with clever and practical design, they are coming much closer to becoming a reality.