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Abstract: Double skin façade systems are increasingly used in High rise buildings across the world. It is more commonly use in high-end architecture of European and Pacific Rim countries. The growing architectural trend is driven mostly by (Poirazis,2004)1:
â€¢ The reduction of energy use during the occupation stage of a building
â€¢ The aesthetic desire for a fully glazed façade that leads to increased transparency
â€¢ The practical need for improved indoor environment, lesser reliance on artificial plant
â€¢ The need for improving the acoustic performance of buildings located in noise polluted areas
1. Poirazis, Harris. (2004). Double Skin Facades for Office Buildings, Literature Review, Lund Institute of Technology
This essay attempts to study the Classification of Double Skin Facades, its advantages and disadvantages, and discuss on its application in high rise building and evaluate the benefits and limitations.
The Double Skin Façade is an envelope construction covering one or multiple storey, which consists of two pieces transparent glass layers separated by a intermediate cavity. Air ventilation strategy can be take place through the intake and air outlet on the external glazing. In the cavity, sun-shading louvers and thermal insulation devices are sometime installed to control the solar heat gain to improve the indoor environment.
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:
2. Belgian Building Research Institute (BBRI) (2002). Source book for a better understanding of conceptual and operational aspects of active facades. Department of Building Physics, Indoor Climate and Building Services, Belgian Building Research Institute. Version no 1.
Web address: http://www.bbri.be/activefacades/index2.html
â€¢ Exterior Glazing: Usually it is a hardened single glazing. This exterior façade can be fully glazed.
â€¢ 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.
â€¢ The air cavity between the two panes. It can be totally natural, fan supported or mechanically ventilated. The width of the cavity can vary between 200 mm to more than 2m according to the function of the applied concept. This width influence the way that the façade is maintained.
â€¢ The interior window can be opened by the user. This may allow natural ventilation of the offices.
â€¢ Automatically controlled solar shading is integrated inside the air cavity.
â€¢ 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.
According to Saelens (2002), the early version of a mechanically ventilated multiple skin façade was created around 1849. "Jean-Baptiste Jobard at that time director of the industrial Museum in Brussels", mentioned "how in winter hot air should be circulated between two glazing, while in summer it should be cold air".
Crespo claims that a double skin curtain wall appears in 1903 in the Steiff Factory in Giengen, Germany. It is possibly the prototype of, at least, is more close to the modern double skin façade system. She describes that:
"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 openings were no longer limited by structural considerations of the past. large glazed facade are able to construct but it is still very much limited by building physics and experiencing thermal comfort. At that time, fully glazed buildings have problem of summer overheating and heat losses and problematic condensation, lots of energy is used by mechanical plant in the operation state 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 comfortable internal environment through a combined system of components, allowing the regulation of heat, cold, light & noise 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.
Battle McCarthy, the Environmental Engineering firm 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:
3. Battle McCarthy website.
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.
2. Belgian Building Research Institute (BBRI) (2002). Source book for a
better understanding of conceptual and operational aspects of active
facades. Department of Building Physics, Indoor Climate and Building
Services, Belgian Building Research Institute. Version no 1.
Web address: http://www.bbri.be/activefacades/index2.html
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 is divided at every storey, the cavity is going horizontally along the building. Partition is used for acoustical, fire security or ventilation reasons.
Multiple storey facades
Multiple storey facades are not divided vertically or horizontally. The air cavity continues throughout the whole facade with maintenance grids at the every level to allowing circulation. The air cavity ventilation is realised via large openings near the floor and the roof of the building.
Shaft box facades are very similar in nature to the one-storey height module. A series of box type window modules in the facade which are connected via vertical shafts. Air are being drawn from the box windows by stack effect into the vertical shafts and emitted from top.
Advantages of the Double Skin Façade
The passive design strategies of double skin façade bring natural ventilation, day lighting and solar heat gain into the fabric of the high-rise building, thus bringing energy efficiency and comfort to the inhabitant.
One of the main advantages of the Double Skin Façade system is that it allows the inhabitant access to natural ventilation with protection against the weather and burglar. The cavity is a key component in the system as it allows natural fresh air to enter into the building to cool and ventilate the space. Natural ventilation is make possible even in the higher levels of a high-rise building due to the addition of extra layer of glass which helps to reduce the wind pressure. Natural ventilation of fresh air is much more preferable than air from mechanical sources as it will help to reduce the energy cost.
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 way lesser energy is use in air-conditioning thus reducing the CO2 output of the building.
Day lighting is important element in the design of High rise building as it reduces the energy consumption of electrical lightings. Moreover, the quality of natural lighting is preferable to electrical lighting. The large area of glazing coverage in double skin façade maximises the 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
5. http://www.buildingenvelopes.org Harvard University & Massachusetts Institution of Technology
However, The increased coverage of glazed façade brings excessive glare and heat during the hotter times. Solar shading devices are require to decrease solar heat gain and reduce the amount of glare.
Thermal Insulation and Solar Heat Gain:
Double Skin Façade System can provide greater thermal insulation to the high rise building over the year with its outer skin. Solar heat gain is controlled through the use of shading devices and the air in cavity also help to absorb some of the incoming solar radiation.
â€¢ During the winter the external layer of glass reduce the external heat transfer rate to provide improved insulation. Heat transfer rate is further reduced due to the increse in temperature when cavity is partially or completely closed.
â€¢ During the summer the warm air inside the cavity can be extracted by naturally or mechanically ventilation. The airflow of the air inside the cavity is very much depend on the size of opening and temperature of the cavity. A carefully selection on the combination of the type of the glass panes and the shading devices types is very important as it prevents overheat in the cavity and the interior space and also to achieve good ventilation.
In a highly glazed building, external Shading devices are use to reducing solar heat gain and heat entering into the interior. Shading devices can be fixed or operable.
Operable units allow the occupant to control environment to meet their desire. They are normally installed in the outer half of Double Skin Façade, typically horizontal blinds. The horizontal blind allows day lighting penetration and maintaining some of the view of outside.
Double skin façade provides acoustic insulation to the building from external sources. The cavity act as air barrier to reduce the external noise pollution. A reduction of 20dB by double skin façade at mid frequencies was reported by Chiang et al. (2004) 6 However, the number of openings and the type of Double Skin Façade will also determine the effectiveness of the sound insulation.
Chiang, W-H., Chao, Y-N. & Wu, C-J., 2004, Sound Insulation of Double Skin Façade
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 energy efficient.
Double Skin Façades can save energy significantly. The system minimises solar loading at the perimeter of buildings, thus 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 "Pro" camp claims the systems to be environmentally "responsible" as double skin system help to save energy cost. However the "Con" camp states that areas such as maintenance, life-cycle/durability of the system, mechanical costs and operation cost also need to take into account. The disadvantages of Double Skin Façade are described below:
Higher construction and additional maintenance costs:
Comparing the Double Skin facade and the conventional cladding systems. Double Skin facade has higher construction costs due to the additional construction cost of the outer layer, mechanical and supporting structure. It also require higher skill workman. With the additional layer of skin, the weight of the building's structure loading is also increase, so as the construction cost of the building. Double Skin type also has higher cost during the operation of the building. More costs will be added in maintenance such as cleaning, servicing.
Reduction of rentable office space:
As the width of the intermediate cavity of Double Skin Façade can vary from 20
cm to several meters (Uuttu, 2001), 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.
If the Double Skin Façade system is not properly designed. The temperature of air in the cavity will likely to be increased in the hot summer days and resulting in overheating of the interior space. To avoid overheating, the minimum width between the internal and external pane should not be less than 200 mm (Jager,2003)
Double Skin Façades in High-Rise Buildings:
High-rise office tower with fully glaze facade is a major consumer of energy. However, double skin façade's passive energy concepts and design strategies have increased the potential of high-rise building to come closer with sustainable architecture. In terms of climatic control, double skin façades allow the occupants to take better control of internal environment. The heating and lighting requirements could be adjusted according to occupants' needs by controlling the operable windows for natural ventilation and the shading device to modify the incoming solar radiation.
The GSW Headquarters building is a good example of architecture that uses double skin façade to achieve sustainability
(Note: Plan image north is down)
Image from Flickr: Uploaded byÂ runningforasthma
The plan demonstrates how the new 515,000 sf connects to the existingÂ building with the new core leaving a 36 foot clear lease span. This depth with the large glass area results in excellent daylighting characteristics.
Building Name GSW Headquarters
Architect Sauerbruch Hutton
GSW Headquarters consists of existing tower, three-story street-oriented bar, and new 22-storey, 11-m wide office tower building, which is the most important aspect to present the sustainable concept.
In response to urban and functional conditions, the major axis of the building faces east and west, a double skin is provided on the west façade: a single-glazed weather screen suspended 1m from the internal double pane windows, which acts a thermal buffer to protect heat loss and a thermal flue drawing air through the building. The second layer of automation is the colourful, perforated aluminium shading panels within the west double skin cavity manage solar heat gain and day lighting. The louvers also can be manually adjustable by the occupants but protects against the direct sunlight in the afternoon.
On the east façade, automatically and manually-operated triple-glazed windows with
between-pane blinds allows fresh air enter into the building to create cross ventilation.
Fresh air is also admitted independently from the window with the exterior louvered metal panels provided on the east façade. In hot summer season, the operable window of the double skin facade is closed resulting in increasing of temperature inside the cavity. warm air is being channel to central plant via riser for heat recovery.
ArupÂ who engineered the building claims the goal to achieve "energy savings of 30-40% in comparison to an ordinary building." And because of the cross ventilation created by the flue effect, mechanical ventilation is no longer required 70% the year. http://www.ecorussia.info/en/ecopedia/double-skin-glass-facades
Double Skin Façades are increasing incorporate into high rise building to achieving greater transparency, acceptable indoor environment as well as reducing energy consumption. Double Skin Façades is the by far the only system at present that offers a range of natural ventilation strategies to the occupants.
From economical point of view, Double Skin Façades does not score well due 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. Double Skin Façades provide better day lighting, solar control and access to natural ventilation, creating better working environment for the employee and increase the productivity.
However Double Skin Facades are not suitable in every part of the world. They are highly depending on the outdoor conditions such as solar radiation, outdoor temperature, etc as the outside conditions have 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 investigates and ability to control these environmental aspects inevitably leads to increased energy efficiency.