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Tuesday, June 21, 2016

DOUBLE SKIN FACADE SYSTEM

1. INTRODUCTION
With the emergence of energy consumption reduction as a major national concern, the search for better approaches and strategies in ameliorating energy efficiency of buildings is rapidly increase. Building construction sector are known as a major energy consumers. Their operational energy is commonly supplied in the form of electricity which is generated from fossil fuels. Overall, studies reported that buildings energy use constitutes about one third of the global final energy use. This was equivalent to almost a quarter of the global carbon dioxide emissions. It has become necessary to take immediate action to avoid dangerous consequences for future generations. Consequently, the level of interest in Double Skin Facades (DSF) has grown rapidly due to the benefits claimed in terms of energy conservation that contributes towards sustainable development, the underlying aim of which is to meet the needs of the present without adversely affecting the ability of future generations to meet their own needs.
Double-skin facades started to emerge across Europe, Asia and the United States beginning in the 1980’s. Many of the early constructions were implemented in low-rise buildings, shortly followed by numerous high-rise applications in the early 1990’s. It wasn’t until the early 2000’s that the use of double-skin systems began to increase in the United States. The Occidental Chemical Center (1981) building in Niagara Falls, New York is widely recognized as the first modern double-skin facade.
Many modern office buildings may have a lower energy use for heating, but on the other hand often have a higher use of electricity than older office buildings, due to a higher energy use for ventilation, cooling, lighting and office equipment. Since the nineties, there has been an increase in new office buildings with glazed facades. There has been and is a growing interest among clients to build and among architects to design office buildings with glazed double skin facades. The purpose of the double skin facades, has often been to reduce the high temperatures in the building behind during the summer and to lower the heat losses during winter compared with a glazed single skin facade. Other improvements, which can be achieved are aesthetics, preheating of ventilation air, sound protection, night cooling etc.

1.1 DEFINITION
The Double Skin Facade is a system consisting of two glass skins placed in such a way that air flows in the intermediate cavity. The ventilation of the cavity can be natural, fan supported or mechanical. The glass skins can be single or double glazing units with a distance from 20 cm up to 2 meters. Often, for protection and heat extraction reasons during the cooling period, solar shading devices are placed inside the cavity. The solar properties of the Double Skin Facade do not differ from the Single Skin Facade. However, due to the additional skin, a thermal buffer zone is formed which reduces the heat losses and enables passive solar gains. An example for Double Skin Facade is shown in figure 1.1.


Fig 1.1 Bayer Headquaters, Leverkusen, Germany


2. WHAT IS A GLAZED BUILDING?
The design and construction of the buildings depends on various factors such as aesthetical appearance, available land, thermal comfort etc. In the urban area we can found that most of the newly constructed buildings are highly glazed. The main reasons of using highly glazed facade are growing tendency on the part of architects to use large portions of glass that lead to higher transparency, users often also like the idea of increased glazing area, relating to better view and more pleasant indoor environment and companies who want to create a distinctive image of themselves (e.g. transparency and openness) often like the idea of being located in glazed office building. The increased glazing area may reduce the electrical internal lighting load of the building but may considerable increase the solar heat gain in hot and dry climates. This is one of the major drawbacks in using single skin glazed facade. The above stated problem can be considerably controlled by the introduction of double skin facade.

2.1 ARCHITECTURAL ASPECTS 
         
2.1.1 The concept of architecture
During the later half of the 20th century global, climatic changes together with diminishing resources were evident signs that we must change our view on buildings and our way of living. Architecture is now a complex concept that applies to the appearance, the function, the technology of a building, as well as the approach to environmental issues and the influence on users and viewers. Architectural aspects on the double skin glazed facade should rely on a complex evaluation of the whole of the building.

2.1.2 Glass architecture
No other building material has during the last two decades experienced such an innovative evolution as glass. It has evolved into a high-tech product that in its right use can create slender and bold constructions. In the beginning the increase of use was depending on the symbolic value of development and future. Glazed buildings have become an important part of modern architecture. Many modern glazed office buildings have been built. Architecturally an airy, transparent and light building is created, where the access to daylight is higher than in more traditionally built office buildings. The idea is often to create a building with openness. The complete transparency also showed a corporate will of communication and openness towards society outside. In many cases the glazed office building is meant to display the profile of a company.
The daylight and its positive effects on humans have always been a main ingredient in architecture. However, careful planning is necessary for a glazed facade with the amount of light that is allowed into the building. The glazed office building can be a vision of an energy efficient building, which however is not easy to realize. More recently the improved properties and its possibilities to be incorporated in a complex construction increased the use of this type of facades.

2.1.3 Light
The daylight and its positive effects on humans have always been a main ingredient in architecture. The treatment of the light affects the experience of space and the inner clock of humans: alert and awake, tired and drowsy. Surveys show that daylight has significant effect on performance as well as physical health. The glazed double skin facade has increased the interest for daylight issues. The large share of glass enables light to penetrate the building. A glazed building should be designed with an optimization of light intake in mind, in order to minimize the dark parts in the core. However, careful planning is necessary for a glazed facade with the amount of light that is allowed into the building. Glare may occur particularly when the sun is low.

2.1.4 The sound attenuating facade
 The site determines the orientation and shape of a building, as well as the choice of facade and technology. All sites are not suitable for single or double skin glaze facades, and all sites are not suited for a closed building typology. Close to a highway or a loud industry the double skin facade can act as an effective noise suppressor and create a comfortable indoor climate. Earlier the architect would choose a heavy facade with smaller windows, thereby creating a dim and dark interior. Now the transparent facade with its views and daylight can be used, yet protecting its inhabitants from the noisy exterior.
The acoustical facade insulation of double skin facades lies above 43dB. This corresponds to the acoustical performance of 14 cm of brick. The acoustical performance of a double skin facade of 54dB corresponds to the acoustical insulation of 19 cm of concrete blocks. This proves double skin facades perform very well in the field of acoustical facade insulation. The transmission of sound via the cavity of the double skin facade is an important parameter when considering the acoustical insulation between the rooms located on the facade side. This can be an issue especially when the inner glass can be opened.        

3. COMPONENTS
The following figure shows the components of a double skin facade.
1. Exterior glazing.
2. Interior glazing.
3. Structural frame.
4. Operable sun shade.
5. Sun shade canopy.
6. Upper operable ventilation.
7. Maintenance catwalks.

v Exterior and interior glazing:
The choice of the glass type for the interior and exterior panes depends on the typology of the facade. In case of a facade ventilated with outdoor air, an insulating pane (sealed double-glazed unit) is usually placed as a thermal break at the interior side and a single pane at the exterior side. In case of a facade ventilated with indoor air, the insulating pane is usually placed at the exterior side, the single pane at the interior side. For some specific types of facades, the interior window can be opened by the user to allow natural ventilation of the building.

v Openings: Openings in the external and internal skin allow the ventilation of the cavity. The choice of the proper pane type and shading device is crucial for the function of the double skin facade system. Different panes can influence the air temperature and thus the flow, in case of a naturally ventilated cavity. The geometry (mainly depth and height of the cavity) and the properties of the blinds such as absorbance, reflectance and transmittance may also affect the type of air flow in the cavity. When designing a double skin facade it is important to determine type, size and positioning of interior and exterior openings of the cavity since these parameters influence the type of air flow and the air velocity and thus the temperature in the cavity. The geometry of the facade, the choice of the panes and shading devices and the size and position of the interior and exterior openings determine the use of the double skin facade.

v Structural frame: Aluminium framing offers several advantages including long life span, fire resistance, corrosion resistance, strength, durability and the potential to be re-cycled when the building reaches the end of its useful life. Glazing infill consists of an aluminium frame attached directly to structure. The frame has rubber gaskets to hold the panes of glass in place. This type of system is simple to construct and is particularly prevalent in low to mid-rise office structures. Aluminium is more malleable and elastic than steel. And also it can be shaped into different shapes.


Fig 3.1 Components of Double Skin Facade

v Solar shading devices: Solar shading devices are placed between the inner and the outer skins. Typically this is an adjustable, horizontal blind that may be rotated or raised and lowered. Once the radiation passes into the building, it is absorbed by the building fabric and re-radiated as short wave, infrared energy that does not pass back through the glass. Instead it serves to heat the air. The role of the solar shading device is to absorb or reflect solar radiation, particularly during the cooling season, that would otherwise enter the occupied space.
The heat absorbed by the sun shading devices can then be removed, primarily through convection, if air flow is moved along the surface of the blinds and removed from the cavity. If in addition, the air passes through a cavity is cooler than the outside air, and then the difference in the temperature across the inner glazing will be reduced. It is recommended to place the sun shading system inside the double skin cavity so as to protect them from rain, wind etc. The position of the shading within this space therefore plays a major role in distribution of the heat gains in the intermediate spaces. The smaller space will heat up to a greater extent than larger.
If the sun shading is situated just in front of the inner facade and if the inner space between the two is not optimally ventilated, the air in the front of the window can heat up considerably an unsatisfactory phenomenon, regardless whether the windows are opened or closed. When they are closed, a secondary heat emission occurs; when they are open, the situation is even worse, since there will be direct inflow of the heated air. Thus, the sun shading should be positioned in the outer half of the intermediate space. It should not be too close to the outer pane of glass, either, so as to avoid excessive heating up and thermal loading of this layer. For the mentioned reason and for proper ventilation purpose it is recommended a minimum distance of 15cm between the sun shading and the external skin of the facade.
The absorbance of the shading device should not exceed 40%, and the proper shading device suggested is the venetian blind (fig 3.2). The cooling consumption decreases until 23.2% by paying attention to the location of the blinds, the blind color and the opening of the double skin.

Fig 3.2 Venetian Blind

v An air cavity between the exterior and interior glazing: The ventilation of the cavity may be totally natural, fan supported (hybrid) or totally mechanical. The depth of the cavity can vary between 20 cm to 2m. The depth influences the physical properties of the facade and also the way that the facade is maintained. The exterior cavity surface is made up by a cladding system. Usually, it is fully glazed (single glazing). The interior surface of a naturally ventilated facade is composed of an opaque wall and an operable window. The cavity is connected with the outside air so that the windows of the interior facade can be opened, even in the case of tall buildings subject to wind pressures; this enables natural ventilation and night time cooling of the building’s thermal mass.
In winter the cavity forms a thermal buffer zone which reduces heat losses and enables passive thermal gain from solar radiation. All types of double-skin facades offer a protected place within the air gap to mount shading and daylight enhancing devices such as venetian blinds and louvers. When solar radiation is high, the facade cavity has to be well ventilated, to prevent overheating. The key criteria here are the width of the cavity and the size of the ventilation openings in the outer skin. The air change between the environment and the cavity is dependent on the wind pressure conditions on the building’s skin, the stack effect and the discharge coefficient of the openings.

v Sunshade canopy: A sunshade canopy is provided to protect the solar sun shading device from sun and rain.

v Upper and lower operable ventilation: These ventilations can either be left open all the time (passive systems), or opened by hand or by machine (active system). Active systems are very complicated and therefore expensive in terms of construction and maintenance.

v Maintenance catwalks: Maintenance catwalks are generally provided for cleaning the windows. Sometimes maintenance walkways present in the cavity between the two skins can be, integrated into the emergency egress paths (fig 3.3).


Fig 3.3 Maintenance catwalk

4. CLASSIFICATION OF DOUBLE SKIN FACADES
Ventilated double skin facades can be classified according to two different criteria which are independent of one another and are based not only on the geometric characteristics of the facade but also on its mode of working. The criteria are:
• Type of ventilation
• Ventilation mode of the cavity

4.1 CLASSIFICATION OF DSF ACCORDING TO THE TYPE OF VENTILATION
1. Naturally Ventilated Wall
An extra skin is added to the outside of the building envelope. In periods with no solar radiation, the extra skin provides additional thermal insulation. In periods with solar irradiation, the skin is naturally ventilated from or to the outside by buoyancy (stack) effects - i.e. the air in the cavity rises when heated by the sun (the solar radiation must be absorbed by blinds in the cavity). Solar heat gains are reduced as the warm air is expelled to the outside. The temperature difference between the outside air and the heated air in the cavity must be significant for the system to work. Thus, this type of facade cannot be recommended for hot climates.

2. Active Wall
An extra skin is applied to the inside of the building envelope; inside return air is passing through the cavity of the facade and returning to the ventilation system. In periods with solar radiation the energy, which is absorbed by the blinds, is removed by ventilation. In periods with heating loads, solar energy can be recovered by means of heat exchangers. Both during cold periods with no or little solar irradiation and during periods with solar gains or cooling loads, the surface temperature of the inner glass is kept close to room temperature, leading to increased occupant comfort in the perimeter zone, near the facade. This type of facade is recommended for cold climates, because of the increased comfort during the cold season and the possible recovery of solar energy.

3. Interactive Wall
The principle of the interactive is much like that of the naturally ventilated wall with the significant difference that the ventilation is forced. This means that the system works in situations with high ambient temperatures, as it does not depend on the stack effect alone. The system is thus ideal for hot climates with high cooling loads. During cold periods with no solar irradiation (e.g. during night time) the ventilation can be minimized for increased thermal insulation. Apart from the advantages in terms of solar and thermal performance the system allows the use of operable windows for natural ventilation, even in highrise buildings.

4.2 CLASSIFICATION OF DSF ACCORDING TO VENTILATION MODE OF CAVITY
The ventilation mode refers to the origin and the destination of the air circulating in the ventilated cavity. The ventilation mode is independent of the type of ventilation applied. Not all of the facades are capable of adopting all of the ventilation modes described here. At a given moment, a facade is characterised by only a single ventilation mode. However, a facade can adopt several ventilation modes at different moments, depending on whether or not certain components integrated into the facade permit it (for example operable openings).The 4 main ventilation modes are the following: (fig 4.1)

1. Outdoor air curtain: In this ventilation mode, the air introduced into the cavity comes from the outside and is immediately returned to the outside. The ventilation of the cavity therefore forms an air curtain enveloping the outside facade.

2. Indoor air curtain: The air comes from the inside of the room and is returned to the inside of the room or via the ventilation system. The ventilation of the cavity therefore forms an air curtain enveloping the indoor facade.

3. Air supply: The ventilation of the facade is created with outdoor air. This air is then brought to the inside of the room or into the ventilation system. The ventilation of the facade thus makes it possible to supply the building with air.

4. Air exhaust: The air comes from the inside of the room and is evacuated towards the outside. The ventilation of the facade thus makes it possible to evacuate the air from the building.

5. Buffer zone: This ventilation mode is distinctive inasmuch as each of the skins of the double skin facade is made airtight. The cavity thus forms a buffer zone between the inside and the outside, with no ventilation of the cavity being possible.


Fig 4.1 Ventilation Modes for Double Skin Facades

5. MATERIAL CHOICE
While choosing the materials used for the construction of the Double Skin Facade, caution should be paid to the selection of glass, and the shading device.

5.1 GLASS
Glass is made from the raw materials such as sand, soda-ash, limestone, dolomite, feldspar and sodium sulphate. Glass plays an essential role in the facade. A facade is a special type of wall. It separates inside from outside. Glass is a uniform material, a solidified liquid. By its property of transparency it opens up our buildings to the outside world. In modern architecture there is tendency to open up our buildings by using very large facades that are as transparent as possible.
The most common pane types used for Double Skin Facades are toughened pane and sometimes it can be a laminated glass instead. Because such glasses have a high compressive strength of 1000 N/mm2. The most common exterior layer is a heat-strengthened safety glass or laminated safety glass. The second interior facade layer consists of fixed or operable windows. Low- emittance coatings on the interior glass facade reduce radioactive heat gains to the interior. For higher degree of transparency, flint glass can be used as the exterior layer. Since the number of the layers and the thickness of the panes are greater than in single skin construction, it is really important to maintain a clear facade. The main disadvantage in this case is the higher construction costs since the flint glass is more expensive than the normal one. For specific safety reasons the toughened, partially toughened or laminated safety glass can be used.
In hot climates, reliance on air conditioning, which would otherwise be increased by such larger glazed areas, is mitigated by the use of advanced solar control glass, allowing the sun’s light into buildings, while blocking much of its heat. In cold climates, low-emissivity glass reduces heat loss, while allowing high levels of valuable free solar gain to heat buildings without significant loss in natural light.

The following are the most commonly used glasses:

1. Low-Emissivity insulating glass
Low-E glass is a hi-tech product that is specially treated with a transparent coating on one surface which reflects heat back into the building, thereby reducing heat loss through the window. It also reduces the heat transfer from the warm (inner) pane of glass to the cooler (outer) pane, thus further lowering the amount of heat that escapes from the window. In addition, the coating also allows large amounts of free solar energy to enter the building, thereby heating it passively. The potential for Low-E insulating glazing to cut CO2 emissions from building has been analysed by the Dutch scientific laboratory TNO. According to this study, as much as 97 million tonnes of CO2 emissions could be avoided annually with the optimal use of Low-E insulating glass.

2. Solar control glass
Solar control glass is a product developed by the glass industry to allow sunlight to pass through a window or facade while radiating away a large degree of the sun's heat, thereby significantly reducing the need for air conditioning and sometimes even eliminating the need for it.

3. Toughened or Tempered glass
Toughened glass withstands a dead load more than four times that of ordinary glass. Additional compressive stresses in the surfaces make the glass stronger. The value for resistance to breakage is 120 -200MPa. Thus can safely accommodate high tensile forces due to the pre-stress. Tempered glass is an extremely strong glass which is heat treated to a uniform temperature of approximately 650ºC and rapidly cooled to induce compressive stresses of 770 kg/m2 to 1462 kg/m2 on the surfaces and of the order of 680 kg/m2 on the edges. Due to the inherent superior features of tempered glass like more strength, ability to withstand sudden impacts and breaking safely into small pieces, it is used as a safety glazing.

4. Laminated glass
Laminated glass limits the risk of injury by retaining the fractured glass on the plastic interlayer and thereby limiting fall-out of glass fragments. Laminated glass is composed of two or more layers of either sheet or polished plate glass with one or more layers of transparent or pigmented plastic sandwiched between the layers. When this type of glass breaks, the plastic holds the pieces of glass and prevents the sharp fragments from shattering. It absorbs impact, resists penetration and remains intact even if broken, holding glass fragments in place and lowering the risk of injury. It resists intrusion because the interlayer continues to safeguard the building even after the glass itself is broken. It tends to resist impact. It can even resist bullets, heavy objects or small explosions. Since it can prevent the fall-out of dangerous glass shards following fracture, it is often used as safety glazing. The plastic interlayer also provides protection from ultraviolet rays and attenuates vibration, which gives laminated glass good acoustical characteristics. Because laminated glass has good energy absorption characteristics, it is also a critical component of protective glazing, such as blast and bullet-resistant glazing assemblies.
It is an excellent barrier to noise. The shear damping performance of the plastic interlayer makes laminated glass an effective sound control product. It retains its colour and strength for the life of the building and is as easily cleaned as any conventional glass. When exposed to heat, laminated glass breaks but stays in place longer. The risk of thermal breakage is avoided only when heat strengthened / tempered laminated glass is used. Laminated glass is used in office buildings, hotels, restaurants, shopping malls, public walkways, hospitals, libraries, museums, churches, airport terminals, residences and apartment buildings, noise control applications, embassies, computer centers. High security places, for example, banks, teller and drive-through windows, ticket windows, gas stations, currency exchanges, armoured vehicles, jewellery shops and burglar resistant showcases. Other areas where laminated glass is used are: Curtain wall glazing, sloped glazing, skylights, glass roofs and floors, aquariums, safety glazing for partitions and security glazing for banks against bullets/ hand propelled objects.

5.2 SHADING DEVICE
The high daylight access for the building, combined with an intelligent lighting control system with daylight and presence detection, may lead to very important savings in the use of electricity for lighting up to 50 %. However this high daylight availability can cause glare problems and be responsible for visual discomfort. To avoid any glare problems, special attention has to be paid to the material of the indoor surfaces and the control of daylight. The indoor surface materials have to be non-specular and of light colour. Best is to have reflection coefficients of about 0.7 for the ceiling, 0.5 for the walls and 0.2 for the ground. The most effective way to assure visual comfort under daylight conditions is to control the daylight penetration with solar shading devices (Venetian blinds).
Solar shading devices have three different functions such as:         
• To protect against direct exposure of the sun 
• To protect against glare   
• To avoid overheating
Often the solar shading consists of Ventian blinds, which have the advantage when not needed they can be pulled up completely giving free view and access to daylight. The Venetian blinds are partially transparent for solar radiation, partially transparent for long wave (thermal) radiation, have an effect of scattering when transmitting the solar radiation and open for the air movements between the gaps. The shading device placed between the two glass skins is protected against weathering and soiling. The sun shading provides a complete screening of the area behind it. The absorbance of the shading device should not exceed 40%.

6. FACADE CONSTRUCTION AND WORKING

6.1 CONSTRUCTION
A curtain wall is a construction consisting of vertical profiles (mullions) and horizontal profiles (transoms), which is built in front of a building structure to form the outside facade of the building and to guarantee the wind and water tightness of the building.

The definition of a curtain wall is following: ‘Curtain walling usually consists of vertical and horizontal structural members, connected together and anchored to the supporting structure of the building and in filled, to form a lightweight, space enclosing continuous skin, which provides, by itself or in conjunction with the building construction, all the normal functions of an external wall, but does not take on any of the load bearing characteristics of the building structure’.
The main difference with windows is that curtain walls are built in front of the building structure, while windows are built into the building openings. Curtain walling panels are usually supported on four sides by the transoms and mullions which frame them. Transoms span side to side, supported by the mullions which span from floor to floor.
Loads are transferred by brackets, usually fixed at the edge of the floor slab. The mullions are usually provided with sleeved joints to achieve transfer of shear forces at the joints. Mullions are usually top-hung so that they act in bending and tension.


Fig 6.1 The Principles of a Curtain Wall
Two main systems of curtain walls exist:     
v The classic curtain wall: First the transom-mullion construction is installed, afterwards, the glazing of the facade is done, mostly from outside. Fig 6.2 and fig 6.3 shows the structural ladder erection and structural ladder frame respectively. The inner and outer glass skin installations are shown in fig 6.4 and in fig 6.5 respectively.
v Element facade: The facade is built up storey per storey
The different elements of the element facade are entirely pre-fabricated, including the glazing. The completely finished elements can be installed on-site, the elements are placed on the outside of the building and not in the building, as for windows. One of the main advantages of this kind of systems is the fact that the fabrication can be done completely in a factory. The curtain wall structures are usually made of extruded aluminium profiles because of the good ratio weight/stability and because with the extrusion process of aluminium profiles there is a very large flexibility in profile design.

Fig 6.2 Structural ladder frame erection               Fig 6.3 Structural ladder frame


Fig 6.4 Inner glass skin installed      
Fig 6.5 Outer glass skin installed
One of the main requirements of a glazed facade is to guarantee the air and watertightess of the building. In general two systems are used to achieve this:
• Siliconised glazing: The sealing between the frames and the glass is done by using silicones. When carried out correctly, this way of glazing gives very good air and water tightness, but depends strongly on the quality of the worker who applies the silicones.
• Use of pre-formed EPDM gaskets: One of the main advantages of this way of glazing is the fact that the quality of the sealant is guaranteed by the extrusion of the gaskets. The gaskets can be applied very easily and very rapidly on site during the installation of the glass. Replacement of damaged glass panels should be tightened in two steps, the inner skin and the outer skin.
For the design of double skin facade systems in India, ASTM / Euro Codes are normally followed, as Indian Codes do not address such information. Thus International Building Code(IBC) 2006 and Structural Eurocodes EN1990 and EN1991-1-X are used. This necessitates more study on the structural performance of glass panels for Indian conditions.

6.2 WORKING
An air inlet located at the bottom and an exhaust vent at the top are used to introduce airflow into the cavity. During winter months the bottom air inlet and top exhaust vent are closed to create a thermal buffer to insulate the interior spaces (Fig 6.2). In summer months, an outdoor air curtain is created by introducing cooler air through the air inlet into the cavity which rises as it warms and exits the top vent creating a stack effect (Fig 6.3).During the more temperate fall and spring seasons, an acceptable cavity temperature is maintained through operable windows, adjustment of air openings and variant shading configurations providing natural ventilation to the interior (Fig 6.4).
Shading within the cavity is achieved via perforated aluminium Venetian blinds at the upper half of each floor which are used to minimize glare and provide even light levels to the interior library space. The operable louvers vary in position throughout the day and year by computer controls tied to the building management system. The shading slats are closed during winter days to reduce glare resulting from low sun angles. During summer months the louvers are configured to provide shading against high-angled sunlight.


Fig 6.6 Winter Air Flow




Fig 6.7 Summer Air Flow




Fig 6.8 Spring/Fall Air Flow


7. DESIGN CONSIDERATIONS
As with all building systems and technologies there are a number of additional considerations that need to be addressed other than just benefits and system functioning. For the design of DSF the following factors are taken into account:

v Glare control: As DSF are usually highly glazed, the issue of glare within and around the building needs to be addressed. Daylight controls such as internal blinds and screens will be required as well as consideration given to the placement and orientation of work spaces to ensure that glare from the daylight
doesn’t adversely impact on the building occupants. Similarly, potential outward reflections need to be addressed by either the use of special coatings or films or careful orientation and positioning of glazing relative to sun angles.

v Additional facade cost: There is usually additional capital cost associated with DSF as an additional facades is required. The Double Skin Facade system may be mentioned as “Energy Saving Facade”. The construction and the maintenance cost of a Double Skin Facade is about 25% higher than a Single Skin one. However, if the facade is designed properly, it is possible to reduce the energy consumption mainly from heating, cooling and ventilating the building and thus reduce the “operational” cost.

v Maintenance requirements: DSF, especially wide cavity types, can have much higher maintenance requirements than single skin facade. This is because there are four glass surfaces that may require cleaning. Ventilation of the cavity also needs to be adequate to prevent condensation and the need for additional cleaning. While maintenance walkways can be provided within the cavity to clean the glass and maintain blinds, access to the outer face of the glass is required, with this normally being provided by another separate access system.

v The pane type and shading devices: The choice of proper pane type and shading device can be crucial for the function of the Double Skin Facade system. Different panes can influence the air temperature and thus the flow in case of a naturally ventilated cavity. A choice of panes which leads to preheating of the air inside the cavity during winter providing natural ventilation with lower energy use, can lead to overheating problems during the summer. The properties of the blinds (absorbance, reflection and transmission) and geometry may also affect the type of air flow in the cavity. It is possible to use Low-E coated, Solar Control, or other types of glazing units instead of clear glass.

8. ADVANTAGES AND DISADVANTAGES

8.1 ADVANTAGES
The primary benefits attributed to DSF are their ability to save energy and permit day lighting of the internal spaces of the building. In some cases DSF have been credited with eliminating the need for air conditioning altogether. The double skin buildings are able to reduce energy consumption by 65%, running costs by 65% and cut CO2 emissions by 50%, when compared to advanced single skin building. A reduction in energy consumption is achieved by minimizing solar gain through the facade and in turn reducing the cooling loads of the building. The benefits of DSF are the following:

v Acoustic Insulation: Due to the addition of an external skin, it is possible to achieve the same degree of acoustic insulation with the windows open as you can with the windows closed in conventional single skin facade construction. Reduced internal noise levels inside an office building can be achieved by reducing both the transmission from room to room and the transmission from outdoor sources i.e. heavy traffic. Increased space between glazing layers will result in increased noise reduction values, especially for low frequent noise, e.g. traffic from heavy vehicles. This is relevant for double skin facades with a certain air gap between the layers because it is often hard to reduce low frequent noise
26 with traditional window facades. Studies have proven that a double skin glazed facade can have an acoustical facade insulation that is far better up to 10 dB than that of traditional facades.

v Thermal insulation: During the winter the external additional skin provides improved insulation. The reduced speed of the air flow and the increased temperature of the air inside the cavity lower the heat transfer rate on the surface of the glass which leads to reduction of heat losses. During the summer the warm air inside the cavity can be extracted by mechanical, fan supported or natural ventilation. Certain facade types can cause overheating problems. However, a completely openable outer layer can solve the overheating problem during the summer months, but will certainly increase the construction cost.

v Natural ventilation: One of the main advantages of the Double Skin Facade systems is that they can allow natural ventilation. One of the specific benefits of daylight is its reduced artificial lighting requirement, as daylight can significantly reduce the requirement for artificial lighting within a building. And this results in reduced electricity demand and therefore saves energy. The another benefits of daylight is the improved occupant comfort. Access to daylight is seen as an important component of occupant comfort and believed to contribute to improved productivity, reduced eye strain and reduced stress levels. As occupants are able to control light penetration with louvers or shading devices and to regulate air movement and temperature with operable windows, the overall building comfort levels are increased.

v Night time ventilation: During the hot summer days, the interior spaces can easily be overheated. In this case, it may be energy saving to pre-cool the offices during the night using natural ventilation. The indoor temperatures will then be lower during the early morning hours providing thermal comfort and improved air quality.

v Energy saving: Due to the increased access to daylight, the use of artificial lighting required is less, which reduces the electricity demand. The double skin buildings are able to reduce energy consumption by 65%, running costs by 65% and cut CO2 emissions by 50%, when compared to advanced single skin building.

v Controlling solar gain: In warmer months and climates, the cooling demand can be very high due to solar gain through windows and the fabric of buildings. DSF can reduce the impact of this solar gain by allowing shading devices to be installed in the cavity between the two skins, preventing sunlight from reaching the inner skin. The shading devices are normally adjustable to ensure that views through the highly glazed façade are retained as much as possible. Warm air trapped within the cavity can be expelled by natural and/or mechanical ventilation to prevent it from heating up the interior of the building. The cavity protects the shading devices from rain and wind, especially on tall buildings, as well as providing access for maintenance of these devices.

v Aesthetic appearance: Since the outer and inner surfaces are fully glazed, it increases the aesthetic appearance of the building.

v Enhanced security: DSF are said to improve security due to the presence of an additional layer of building fabric that can impede illegal entry through the facade of the building. The outer skin can also allow internal windows to be opened for natural ventilation in high security buildings. The outer skin can also be reinforced or armoured to provide additional protection. Double-Skin Facades provide a relatively unobtrusive method of achieving building security due to a continuous glazing layer with small ventilation grilles as opposed to project opening with bars or vents.

v Pollution barrier: In much the same way as the acoustic and security protection, DSF are claimed to allow natural ventilation in polluted locations with the outer skin screening pollutants permitting windows in the inner skin to be opened.

8.2 DISADVANTAGES
The disadvantages of the Double Skin Facade concept are the following:

v Reduction of rentable office space: The width of the intermediate cavity of a Double Skin Facade can vary from 20 cm to several meters. This, results to the loss of useful space.

v Additional maintenance and operational costs: Comparing the Double Skin and the Single Skin type of facade, one can easily see that the Double Skin type has higher cost regarding construction, cleaning, operating, inspection, servicing, and maintenance. Thus it is said to be more expensive than the traditional single glass facade.

v Overheating problems: If the Double Skin Facade system is not properly designed it is possible that the temperature of the air in the cavity is going to increase overheating the interior space. In order to avoid overheating, the minimum distance between the internal and external pane should not be less than 200 mm. The key criteria are the width of the cavity and the size of the ventilation openings.

v Fire Protection: There is not yet very clear whether the Double Skin Facades can be positive or not, concerning the fire protection of a building. “Virtually, no information exists on the behaviour of this kind of facade in the case of fire”.

9. CONCLUSION
Double Skin Facade has been proven to be highly useful and significant in current building developments. The double skin buildings are able to reduce energy consumption by 65%, running costs by 65% and cut CO2 emissions by 50%, when compared to advanced single skin building. The only downside of double skin facade is that it is said to be more expensive than the traditional single glass facade. Double skin facades lead to improvement of daylight levels and view to the outside. It is obvious that double skin facades offers a better view to the outside and it reduces heat loss and external noise, when taking the extra layer of glazing into account, as compared to a traditional single skin facade. The Double skin facade incorporates the necessary features of energy responsible building design which is believed to be essential for further developments of sustainable buildings. Double skin facade is more cost-effective in the long run. This is because it is long lasting and more durable as compared to the single glass facade.

10. REFERENCES

1. Afsanehsadat Omidiani , Evaluation of the double skin facade in Warm and Humid climate, International Journal of Scientific and Research Publications, Volume 5, Issue 4, April 2015

2. Ahmed Hamza H. Ali, Ali K. Abel-Rahman and M. Suzuki Mostafa M. S. Ahmed, Journal of Clean Energy Technologies, Vol. 4, No. 1, January 2015

3. Aruna Malini and Premalatha , Facades of Tall Buildings – Modern Applied Science, Vol. 4, No. 12; December 2010

4. Halil Z. Alibaba and Mesut B. Ozdeniz , Thermal comfort of multiple-skin facades in warm- climate offices, Scientific Research and Essays Vol. 6(19), pp. 4065-4078, 8 September, 2011

5. Harris Poirazis, Double skin facades: A literature review, A report of IEA SHC Task 34 ECBCS Annex 43, 2006

6. Neveen Hamza, Double versus single skin facades in hot arid areas, Received 21 December 2006; received in revised form 25 January 2007; accepted 16 February 2007

7. Rajesh Sharma , Energy efficient facades for Hot and Dry climate in India, IJISET - International Journal of Innovative Science, Engineering & Technology, Vol. 1 Issue 6, August 2014

8. Ri Ryu,Janghoo Seo andYongseong Kim, A Study on Appropriate Temperature of Phase Change Material applicable to Double Skin Facade System for Heating Energy Load Reduction, International Journal of Smart Home Vol. 8, No. 6 (2014)

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