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Double skin facades is very popular apply in many European cities. This wall system is attractive due to its characteristic such as durability, ecology, greener technology, aesthetical viewing and etc. This double skin facade able to provide natural ventilation into a building space and also can reduce energy consumption. This double skin facades sometimes also referred to as a 'building in building'.
This wall system can be define as a traditional single facade doubled inside or outside by a second, essentially glazed facade. Each of these two facades is commonly called a skin. Each of the facade skin can be constructed by various different combination of materials, commonly by two skin of glazed. However, it is also popular to apply an outer layer of glass used together with a solid inner skin. The area between the two skins can called 'ventilated cavity' or 'air space'. It is purposely in such designed for vertical air circulation purposes. The ventilated cavity between the two skins can be in various width, normally range from as narrow as several centimetres to as wide as several metres (in order to formed a accessible cavities). The cavity width will influence the way that the facade is maintained. This air circulation space can be used in many different functions, but in the simplest analysis, the air will be drawn into the building by applying the circulation utilises stack effect, so that a natural ventilation effect will be created for the internal space of the building. However, this cavity space in some other design can be consists of fan supported or mechanically ventilated.
A) Natural Ventilation
The design of exterior part of the skin forming a protective shield for the building and through the circulation utilises stack effect the natural air will be drawn into the internal space of the building, so that a natural ventilation effect will be created. Therefore, the interior comfort would not be affected even though the windows maintain open throughout the whole day.
B) Promote Greener Technology
The natural ventilation for high-rise conditions relatively will reduces air-conditioning loads and thus will directly with reduce the CO2 output of the building in the building operational phase . According to the research, performed by 'Franklin Andrews, Professor Michael Wigginton of the University of Plymouth and Battle McCarthy', on behalf of the United Kingdom Department of Environment, Transport and Regions has shown that double skin buildings capable to reduce 50% of CO2 emissions in the cold temperate climate prevalent in the United Kingdom when compared to advanced single skin building.
C) Better acoustic insulation-improve noise protection
Besides improved the noise protection, this double skin facade are capable to gain an excellent acoustic insulation magnitude even though under the windows open condition. The magnitude of the acoustic insulation is equal to that obtained in classical glass facade with the windows closed condition.
D) Reducing heating energy requirement
The air stored in the cavity between the two skins would be heated by the sun rays especially in winter time. Thus improving both the heat-insulating functions of the facade and its thermal performance and subsequently will reducing the heating costs.
E) Reducing cooling energy requirement
In open windows condition, can allow for nigh-time cooling of the interior building and thereby lessening cooling loads of the building's HVAC system. Thus especially, apply during summer time, the natural ventilation or in another term so-called 'night cooling' can cut down the building energy consumption particularly the costs of air-conditioning in the summer.
F) Exploiting solar power
Both energy consumption and costs are possible to reduce by utilize the sun's energy particularly with the incorporation of photovoltaic glass. By this method, the air stored inside the cavity will be heat by the solar rays and reduce the energy consumption.
G) Increased Natural Daylighting
The double skin facade will improves the access of natural light transmission into the building space and thus will produce a better indoor comfort and give positive effect to occupants health and as well as increase the productivity of office personnel. Physically with this increased natural daylighting will cause a significant reduce in the amount of electrical lighting required because the quality of light from natural daylight is more preferential to electrical lighting.
H) Fire Escape
For some of the widest cavities ( approximately 2m of design width) located between the two skins will be able to provide an fire escape during fire occur time. Therefore, with this back up emergency escape will enable the fire brigade to save more life during there is fire to the building.
The above have indicate the benefit of double skin facade wall and make it gain a lot of popularity throughout the world.
- Construction Technology 5, Heriot-Watt University
- 'Franklin Andrews, Professor Michael Wigginton of the University of Plymouth and Battle McCarthy'
Coursework Unit 6 (b)
The technical challenges that would need to overcome to produce a double skin facade for this building are as below;
1) Overheating challenges
The overheating problem may happen especially on warm day where hot air will collects at the top of the air space. This overheating due to the accumulation of hot air in the cavity adjacent to the top floor, may cause suffer to those top floor offices . Therefore, technically approach to overcome this problem is to design the 'air space'. There are two type of air space,
- undivided air space and
- divided air space.
The undivided air space will receive a benefit from the stack effect. On warm days hot air collects at the top of the air space and with the appropriate openings at the top of the cavity, thus will siphon out warm air and at the same time the cooler replacement air is draw in from the outside.
The benefit of divided air space design is it can reduce over-heating particularly on upper floors. It is also can reduce noise, fire and smoke transmission within the division. Moreover, this floor-by-floor divisions add construction simplicity of a repeating unit and in turn can produce economic savings.
From the above, indicate that both air space using the natural physics principals (hot air rises) to draw air upward. We need to highlight that the second type; i.e. divided air space by floor is practically apply for fire protection and sound transmission purposes.
2) Maintenance challenges
Although some of the building which is use the fully glazed double skin facade to achieve an aesthetical pleasing view but its maintenance is critical in terms of cleaning process. The air space need to be cleaned more frequently because this area is tend to emerge of dust particles which is circulates quickly during the ventilation process. From an research carried out by 'Terri Meyer, Associate Professor, school of Architecture, University of Waterloo' indicate that glazed double skin facade need to carried out full cleaning regularly from 2 to 4 times a year.
Therefore, the air space's design criteria need to consider the imparts of cleaning especially for the continuous cavity. Similarly, others barrier elements such as louvers placed within the cavity must be removable in order to facilitate access during cleaning process. Normally a device called bosun's chair platform which is similar to the window washing rig is used to access the interior space of cavity for cleaning purposes.
In some double skin facade design an 'open grates' will be put at floor by floor or at particular place act as the cleaner standing platform and without affected the airflow design.
For divided air space or cavity, normally the interior windows will function as the access panels for In some instances, where the cavity is more divided, the interior windows, whether operable maintenance purposes. Therefore the design of air space need a consideration to provide an adequate space for maintenance purposes besides the ventilation functionality. The interior clear dimension for air space is usually range from 600 to 900 mm.
2.2) Replacement of deteriorate mechanical part
The high-tech mechanic which is incorporated for the functioning of double skin facade (particularly in ventilation process) tend to have a higher failure rate and repair cost. The same mechanics also necessitate higher replacement costs. For instances, the replacement of wiring after a certain number of years. Therefore, a preventive checking is need to carry out frequently in order to detect any mechanical problem in an earlier stage before its give a negative impact onto the ventilation system. Besides, a proper data record and operation manual on those relevant mechanic device also need to be keep properly. Through this data any cases of malfunction of mechanic device would be able to find the direct causes and the solution can be carry out as soon as possible as well as in finding the suitable spare part replacement within a shorter period of time.
3) Control Of Natural Ventilation
For high-rise building with double skin facade, normally will found a problem on how to control and maintain its natural ventilation (here mean the quality of air) to its occupant. The area between the double skin facade in principle is not affected by high velocity wind because this area have been protected by the exterior skin. Therefore, this region typically will access by the inhabitants for natural ventilation and this will cause some unexpected impact of sound, smoke, noise or heat transfer over this zone either from one section, level or room to the proximity area. In order to eliminates these impact efficiently normally this 'buffer zone' will be propose in compartmentalize design and separate into regions with air supplied by grilles or vents at the individual zone or each level. Then with the use of vents or grilles allows for the control of the incoming air by reducing air velocity, protecting from rain and reducing noise transmission from the exterior. Regular cleaning process also need to be carried out in order to make sure that the ventilated air is always in good quality such as out of dust particle. Hence, through this control will allows occupant access to natural ventilation in the high-rise constructions.
4) Fire Regulation
Although the air space between the skins can be use for fire escape but the fire regulation might cause difficulties if no extra attention is provided. Therefore, for safety measured a proper indication of fire escape route have to provide and place at suitable location at each floor.
5) Reduce of Space
As mentioned earlier the width of the cavity can be formed from few centimetres until few meters. So, this will cause some reduce in the building usable space. Therefore, it is significant to find the optimum cavity width in order to gain an optimum office space in this building.
In some European country the double skin facade will face the climate issue such as humidity. For those double skin facade associate with the passive design strategies, (i.e. thermal mass) and radiant (hydronic) system , the condensation control will become an issue. Therefore, this critical factor need to be solved in order it would not give a negative comfort effect inside the building. This problem can be technically solved by adopting condensation control through the appropriated mechanical ventilated system, i.e. 'extract air system'. This system normally apply for location where the natural ventilation is not possible (due to it locations inherent with high noise, wind and fume). Through this system the fresh air will be supply by HVAC and it is precludes the natural ventilation. These systems tend not to reduce energy requirements as fresh air changes must be supplied mechanically. Therefore, the occupants are prevented from adjusting the temperature of their individual spaces. However, the priority consideration will still be put on the potential use of natural ventilation. For instances, to come out a desirable hours of natural ventilation scheduled through the utilisation computerise control system can achieve this objective.
7) HVAC design
The HVAC will play a major role for the building where the natural ventilation is not suitable due to its negative climate conditions. Therefore, in cases the functionality of a HVAC system will become a significant point and it will give an impact onto the building ventilation system. So, in order to solve that problem, during the earlier design process need to fully integrates architectural and mechanical concerns is need to fully integrates during the earlier design process. By this earlier stage planning will able to achieve a smooth functioning of HVAC system. In economical point of view it will less costly and this can compensate for the cost of the second facade.
8) Solar heat gain
This point come to the issue of excessive of the incoming solar radiation above the comfort level especially in the summer time. So, in order to maintain solar heat gain under a standard design level is by preventing the heat from initially entering the space. Particularly for a highly full glazed building, normally an external shading devices are the most efficient means of reducing solar heat gain. However, this external shading devices need to be cleaned frequently in order its can function effectively.
In the other approach is to use the special glazing such as 'spectally selective glazing' and where this glazing materials is able to respond differently to various wavelengths of solar energy or in other means is to permit visible light while rejecting unnecessary invisible infrared heat. An ideal spectrally selective glazing permits only the art of the sun's energy which is useful for daylighting.
Another type of glass called 'electrochomic glass' also able to improve the solar performance. The type of glass able to change its colour from clear to dark using electrical current. The electrical current can be activated in two ways, either by manually activated or by sensor reaction to the light intensity. In physic principle dark colour glass will reduce solar transmission into the building. Adversely when it is little sunlight, the glass will perform brightens in order to permit more suns ray into the building and minimized usage of artificial light.
The above approaches is very practically apply for the full glazed double skin high rise building. Besides, the application of horizontal blind can permit use of daylighting and at the same time still can achieve the exterior view and it is a more economic approaches.
9) Unable to support self loading.
This double skin facade especially full glazed type, is unable to take its own load. Therefore, the dead load and imposed load of skins have to transferred to the adjacent structural wall and frames. Therefore, the design of structural wall and frames need to consider to carry the facade skins loading.
10) Installation challenges
His installation process is difficult especially for full glazed double skins facade because it is tough to work with the increase of the building height. Also encounter near misses and possibility of the hazard of falling. Besides the wind gust is always a question of safety to its installer. Moreover the are only limited movements in the working place. Therefore, the design of 'open grates' to put at floor by floor to act as the installer standing platform is advisable and can ease the installation process.
From the above, we have analysed the possible technical challenges of the double skin facade for this propose 10 storey of office. Therefore, the project design team need to consider that technical challenges during the earlier stage of design in order to come out a proper and efficient double skin facade building design in terms of aesthetical pleasing and maintenance capability.
- Construction Technology 5, Heriot-Watt University
- Terri Meyer, Associate Professor, school of Architecture, University of Waterloo