Double skin façade systems are increasingly used in high profile buildings, designed by famous Architects across the world. It is a new technology that is more often found in high-end European and Pacific Rim architecture. The growing architectural trend is driven mostly by (Poirazis,2004)1:
• The aesthetic desire for a fully glazed façade that leads to increased transparency
• The practical need for improved indoor environment, lesser reliance on artificial plant
• The need for improving the acoustic performance of buildings located in noise polluted areas
• The reduction of energy use during the occupation stage of a building
The Double Skin Façade is based on the notion of an envelope construction covering one or multiple storey, which consists of two transparent surfaces separated by a cavity. The air cavity ventilation strategy may vary with time and ambient conditions, The highly dynamic system incorporates a range of integrated sun-shading, natural ventilation, and thermal insulation devices in order to improve the indoor climate with active or passive techniques.
The BBRI, (BBRI, 2002) includes in the Source book a satisfactory description
of the structure of a Double Skin Façade System.The layers of the façade are described below2:
• Exterior Glazing: Usually it is a hardened single glazing. This exterior façade can be fully glazed.
• Interior glazing: Insulating double glazing unit (clear, low E coating, solar control glazing, etc can be used). Almost always this layer is not completely glazed.
• The air cavity between the two panes. It can be totally natural, fan supported or mechanically ventilated. The width of the cavity can vary as a function of the applied concept between 200 mm to more than 2m. This width influence the way that the façade is maintained.
• The interior window can be opened by the user. This may allow natural ventilation of the offices.
• Automatically controlled solar shading is integrated inside the air cavity.
• As a function of the façade concept and of the glazing type, heating radiators can be installed next to the façade.
History of the Double Skin Façade
The history of Double Skin Facades can be trace back to end of 19th century.
Saelens, (2002) mentions that "in 1849, Jean-Baptiste Jobard,at that time director of the industrial Museum in Brussels, described an early version of a mechanically ventilated multiple skin façade. He mentions how in winter hot air should be circulated between two glazings, while in summer it should be cold air".
Crespo, claims that, the first instance of a Double Skin Curtain Wall appears in 1903 in the Steiff Factory in Giengen, Germany. According to her, "the priorities were to maximize daylighting while taking into account the cold weather and the strong winds of the region. The solution was a three storey structure with a ground floor for storage space and two upper floors used for work areas. The building was a success and two additions were built in 1904 and 1908 with the same Double Skin system, but using timber instead of steel in the structure for budget reasons. All buildings are still in
use.
Moving to the 20th century, with improved technology meant that the size and number of windows were no longer limited by structural considerations of the past. It was now possible to create large glazed facade but still limited by the constraints of building physics and thermal comfort. At that time, fully glazed buildings have problem of heat losses and problematic condensation and summer overheating, lots of energy is used by mechanical plant in the operation of the building
The outbreak of oil crises in 1973 & 1979 bring greater awareness on energy consumption. Innovative improvements on insulating glazing such as the addition of low-emissivity coatings and inert gas filled cavities were made. In the1990's a further breakthrough on insulation glazing was achieved with the development of spectrally selective glazing, electro-chromic and photo-chromic glazing. However, these products are unlikely available in short run as they are expensive.
In contrast, double skin facades offer us a cheaper and more flexible way of achieving internal thermal environment with reduced energy consumption.
moreover it offer flexible operation through a combined system of components, allowing the regulation of heat, cold, light & noise in such a way that comfort is achieved with low energy consumption.
Classification of Double Skin Facades
There are numerous way of classification on Double Skin Façade due to the variation in type of construction, the origin, destination and type of the air flow in the cavity, etc.
The Environmental Engineering firm of Battle McCarthy in Great Britain created a categorization of five primary types (plus sub-classifications) based on commonalities of façade configuration and the manner of operation. 3 These are:
• Category A: Sealed Inner Skin: subdivided into mechanically ventilated cavity with controlled flue intake versus a ventilated and serviced thermal flue.
• Category B: Openable Inner and Outer Skins: subdivided into single story cavity height versus full building cavity height.
• Category C: Openable Inner Skin with mechanically ventilated cavity with controlled flue intake
• Category D: Sealed Cavity, either zoned floor by floor or with a full height cavity.
• Category E: Acoustic Barrier with either a massive exterior envelope or a lightweight exterior envelope.
Belgian Building Research Institute Study (BBRI, 2002) investigate the potential of active facades, the related problems and risks in order to devise guidance in standardisation and technical approvals. They developed a classification system to describe different double-skin façade configurations based on a database of case-studies2.
One storey height facade
The air cavity is divided horizontally and vertically into small and independent facade modules. Naturally ventilated double facades with one storey height facade modules are also known as a 'Box window' type.
Corridor facade
Corridor facades are characterised by a wide air cavity partitioned at the level of each storey. The cavity partition are used for acoustical, fire security or ventilation reasons.
Multiple storey facade
Multiple storey facades are not partitioned vertically or horizontally. The air cavity extends a large height with metallic maintenance grids at the level of each storey allowing circulation. The air cavity ventilation is realised via large openings near the floor and the roof of the building.
Shaft-box facade
Shaft box facades are very similar in nature to the one-storey height
module. A set of box window modules are placed in the façade. These modules are connected via vertical shafts situated in the façade. Air are being drawn by stack effect from the box windows into the vertical shafts and emitted from top.
Advantages of the Double Skin Façade
The double skin façade incorporates the passive design strategies of natural ventilation, day lighting and solar heat gain into the fabric of the high-rise building. These key components of the double skin facade bring energy efficiency and comfort to the inhabitant.
Natural Ventilation:
One of the main advantages of the Double Skin Façade system is that it allows the inhabitant access to natural ventilation that is both burglar proof and protected against the weather. The cavity, a key component to the double skin façade, is typically the region accessible by the inhabitants for natural fresh air that can be used to cool and ventilate the space
"The reduction of wind pressure by the addition of the extra pane of glass means that the windows can be opened even in the uppermost floors of a high-rise building. Natural ventilation of offices by fresh air is much more acceptable to the building's users and it has the additional benefits of reducing investment in air handling systems and also reducing energy consumption." 4
During the hot summer nights, Double Skin Facades can also provide natural night ventilation, thus making the indoor temperatures lower during the early morning hours providing thermal comfort and improved air quality for the office occupants. In this case, lesser depending on mechanical means of air-conditioning reduces the energy consumption of the building and thus reduces the CO2 output of the building.
Day lighting:
Day lighting in the design of High rise building is important as it reduces the energy consumption of electrical lightings. Moreover, the quality of natural lighting is preferable than electrical lighting. The large area of glazing coverage in double skin façade allows more daylighting into the space.
"Good lighting of the workplace is one of the main factors of indoor comfort that can positively influence health and productivity of office personnel. Natural light, its variations and its spectral composition are of great importance for well-being and mental health. Natural light is a fundamental component of our life, helping our body to produce vitamin "D", an important anticancer element." 5
However, The increased coverage of glazed façade brings excessive glare and heat at certain times of the day. Solar shading devices are require to decrease solar heat gain and reduce the amount of glare caused by the increased access to daylighting.
Thermal Insulation and Solar Heat Gain:
Double Skin Façade System can provide greater thermal insulation due to the outer skin, both in winter and in summer. The control of solar heat gain with the double skin façade is obtained through the use of shading devices and the as well as the of the cavity's ability to absorb some of the incoming solar radiation.
• During the winter the external additional skin increase the external heat transfer resistance thus providing improved insulation. When the cavity is partially or completely closed, the increased temperature of the air inside the cavity also slow down the heat transfer rate on the surface of the glass which leads to reduction of heat losses. Thus, maintaining higher temperatures on the inside part of the interior pane.
• During the summer the warm air inside the cavity can be extracted by naturally or mechanically ventilation. The width and height of the cavity and the size of the openings can affect the intermediate temperatures and the airflow. A carefully selection on the combination of the type of the panes and the type of the shading devices is very important as it prevents overheat in the cavity and the interior space and also in achieving good ventilation.
External devices are the most efficient means of reducing solar heat gain in a highly glazed building as it prevents the heat initially getting into internal spaces. Shading devices are normally used in the outer half of Double Skin Façade, typically horizontal blinds. The horizontal blind allows day lighting penetration and maintains some of the view to the exterior. They can either be fixed elements or operable units that are either controlled by the occupant or by sensors within the building.
Acoustic insulation:
Double Skin Façade provides sound insulation to the building from external noise pollution. It reduced internal noise levels inside a building by reducing both the internal noise pollution and the external noise pollution transmits from outdoor sources i.e. heavy road traffic. A dampening of 20dB at mid frequencies was reported by Chiang et al. (2004) 6 The number of openings and the type of Double Skin Façade is also critical for the sound insulation concerning the internal and the external noise pollution.
Transparency:
Double skin facade's fully glazed façade bring more transparency to the building with more natural day lighting and views of exterior. For years, the Architects and the developer has always wanted a transparent building, but in the same time without compromising its energy performance and indoor climate. Double skin facades reducing solar gains in summer and providing thermal insulation in winter is an good example of a technology which aimed at creating fully glazed buildings with low energy consumption and high level of occupant comfort.
Energy savings and reduced environmental impacts:
Double Skin Façades can save energy when properly designed. By minimising solar loading at the perimeter of buildings, less electricity costs is used to cool the building. Natural ventilation also further reduces the energy used in mechanical air supply.
Disadvantages of the Double Skin Façade
There are always debates on the benefits of the double skin system The researchers and practitioners are clearly divided into two opposing camps.
The "Pro" camp claims the systems to be environmentally "responsible", netting overall energy savings. However the "Con" camp added that a wide range of quantities that must be accounted for in determining a final savings value of the building. These includes embodied energy, maintenance, life-cycle/durability of the system, mechanical costs (operating cost as well as physical plant), and additional floor area. The disadvantages concerning the Double Skin Façade concept are described below:
Higher construction costs:
Higher construction costs compared to a conventional cladding systems. The cost increases are due to the additional construction cost of the outer layer, mechanical and structural costs, the amount of special glass required, and the requirement of higher workman skill. With the additional layer of skin, Double Skin facade increases the weight of the building's structure loading ,so as the construction cost of the building.
Additional maintenance and operational costs:
Comparing the Double Skin facade and the Single Skin type of façade, Double Skin type has higher cost regarding construction, cleaning, operating, inspection, servicing, and maintenance. The air cavity must be cleaned because of the air movement within the space circulates dust particles more quickly. Life cycle costing of the high-tech mechanics must also be taken into account. They tend to have a higher failure rate and repair cost.
Reduction of rentable office space:
As the width of the intermediate cavity of Double Skin Façade can vary from 20
centimeters to several meters, this results to the loss of useful space and rental. It is quite important to find the optimum depth of the façade so as not to lose valuable space.
Overheating problems:
If the Double Skin Façade system is not properly designed. During the hot summer days, there is high chances the increase temperature of air in the cavity is going to overheating the interior space. To avoid overheating, the minimum width between the internal and external pane should not be less than 200 mm.
Double Skin Façades in High-Rise Buildings:
High-rise office tower is a major consumer of energy. The tower's fully glazed facade is not an effective design in terms of energy efficiency. However, Double skin façade's passive energy concepts and design strategies has increased the potential of high-rise building to come closer with sustainable architecture.
Double skin façades do offer a potential solution to the glazed office tower in terms of climatic control. The occupant are able to control over local environment with operable windows for natural ventilation and the shading device to modify the incoming solar radiation for either heating or lighting requirements. Both of these strategies bring the idea of passive design strategies into the modern office tower. The ability to engage and control these environmental aspects inevitably leads to increased energy efficiency.
In terms of sustainable design, the Double skin façades offers strategies for use and control of solar heat gain, increased daylight and moderation of temperature differences. The reduction of wind pressure in the cavity means that the windows can be opened even in the uppermost floors of a high-rise building. Natural ventilation of fresh air is much more acceptable to the building's users. It also the reduce investment in air handling systems and the energy consumption.Potential green house effect in the cavity can be used for heat production and exchange in the building.
Conclusions
Double Skin Façades for office buildings were build in order to achieving greater transparency with acceptable indoor environment, in the same time reducing energy consumption.
From economical point of view, Double Skin Façades does not score well sure to its high construction cost and additional maintenance and operational costs. However, Double Skin Façades fair better when we look at social and environmental aspect. They includes day lighting, solar control, access to and control of natural ventilation, and resultant employee satisfaction and productivity.
Double Skin Façades is the only system at present that offers a range of natural ventilation strategies to the occupants. No other system maximizes day lighting with integral solar heat gain control, blinds and buffer zone.
However Double Skin Facades are not suitable in every part of the world. They are highly depend on the outdoor conditions such as solar radiation, outdoor temperature, etc as the outside conditions has great influence on the indoor environment. Thus, each Double Skin Façade has to be designed according to different constraints such as: climate condition (solar radiation, outdoor temperature, etc), site conditions (latitude, local daylight availability, atmospheric conditions, exterior obstructions, ground reflectance, etc), building usage (operating hours, occupant's tasks, etc).
A thorough investigate and ability to control these environmental aspects inevitably leads to increased energy efficiency.
