ADJUSTABLE SOLAR SHADING AND HEAT CONSERVATION LOUVRES FOR TRANSPARENT, MULTI-LAYERED GLASS FACADES

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1 ADJUSTABLE SOLAR SHADING AND HEAT CONSERVATION LOUVRES FOR TRANSPARENT, MULTI-LAYERED GLASS FACADES Werner Lang, Thomas Herzog Chair of Building Technology, Institute for Architectural Design and Engineering Faculty of Architecture, Technical University Munich, Arcisstr. 21, D-8333 Munich, Germany Phone: , Fax: , Gerhard Hausladen Institute for Building Services and Technologies, Faculty of Architecture, University of Kassel Gottschalkstraße 28, 3419 Kassel, Germany, Tel.: +49-(5) , Fax: +49-() Kaja Kippenberg, Phillip Jones Welsh School of Architecture, Cardiff University, Cathays Park Cardiff, CF1,3NB Tel.: , Fax: , Abstract - Transparent, multi-layered glass facades (double-skin-facades) are outer wall systems with an additional layer of glass, mounted either in front of or behind the actual external skin of a building. This 'second skin' enhances the function of the facade with regard to heat insulation, solar shading and sound insulation. Depending on the construction of this new type of facade, the second skin even allows for the natural of buildings, which are exposed to high wind speeds, such as high rise buildings. In order to specify the characteristics of the various types of double skin facades, a typology has been developed. The main purpose of this research project was the development of adjustable heat conservation and solar shading devices made of renewable materials. This was done to reduce the amount of energy for the heating and cooling of buildings with double skin facades and to minimize the amount of energy, embedded in the facade. The examinations and experiments have shown, that the use of timber for a new kind of heat conservation and shading louvres allows for the reduction of the amount of embedded energy to 25%, if compared to regular systems made of aluminium. The use of the louvre system reduces the U-value of double-skin-facades to almost,7 W/m 2 K, compared to the U-value of 1,33 W/m 2 K of the same system without a heat conservation device. With regard to solar shading, the system made of timber allows for the reduction of solar transmission down to 4-17%. In addition to these functional aspects, this new kind of louvres widens the possibilities of the architectural expression of facades to a large degree. 1st level Multi-layered, transparent facade system Double-skin facade is positioned within the outer wall Window systems in punctuated facades Double-skin is covering the outer wall in part Window systems projecting from the outer wall Double-skin is covering the outer wall completely Double-skin facades 2nd level openings in both skins openings in the inner skin no openings openings in both skins openings in both skins openings in the inner skin no openings Winter window Box window Air-extract window Compound window Attached conservatory, oriel Two-layer facade with natural ventil. Air-extract facade Buffer facade 3rd level Space within the twolayer facade not divided Space within the twolayer facade divided Unsegmented twolayer facade Integrated conservatory House-within-ahouse system Corridor facade Shaft type facade Box-window facade Fig. 1: Classification of the various types of multi-layered, transparent facade systems (double-skin-facades) [1]

2 User-related requirements Energy consumption in office buildings Weather influences Indoor air temperature Surface Temperatures Air change Indoor relative humidity Luminance, Lighting Building Skin determines: Consumption of heating energy Consumption of cooling energy Demand for artificial lighting External air temperature Air movement External relative Humidity Solar radiation Fig. 2: Manipulatory facade elements in Bozen, Italy Fig. 3: Factors influencing energy consumption in office buildings 1. INTRODUCTION One of the principal tasks of a building envelope is to ensure comfortable internal conditions for human occupation. The many functions of an enclosure include thermal insulation, solar shading, allowing the ingress of daylight and providing views out of the building, and facilitating. From early times, a wide range of facade components, such as blinds and shutters, have existed to perform these functions (figure 2). Today, manipulatory facade elements of this kind are assuming an ever greater role in building because of their ability to minimize the consumption of energy as parts of the building skin (figure 3) and in view of the many new materials and technologies that are available. As figure 4 shows, there is a great range of facade components for thermal and solar screening purposes available today. Among these, pivoting louvres have a number of advantages: they can be easily adjusted to specific conditions; they can function as daylight deflectors; and, depending on their position, they scarcely obstruct the view out of a building and do not impede the of the internal spaces. In recent years, many multi-layer glass facade systems have been developed (figure 1). Of these, the "two-layerfacade" is one of the most interesting. This system allows the requisite air change in buildings subject to great wind loads such as high-rise blocks to be achieved by natural means over a large part of the year. Ventilation openings in the outer and inner skin can allow a natural change of air, permitting installations and energy consumption for mechanical to be reduced to a minimum. The construction of a second skin also improves sound insulation and can incorporate the prewarming of the Thermal Insulation Heat protection Glare protection Adjustable systems External roller shutter good good good Folding shutter ext./int. good good good Sliding shutters ext./int. good good good External louvre blind unsuited good good Internal louvre blind unsuited limited good Awning/sunblind unsuited limited good External roller blind limited limited good Internal roller blind limited limited good Internal curtain limited limited good Pivoting louvres good good good Aluminium (sheet) Polythane (membrane) Timber (sawn timber) Density in t/m 3,96 2,7,6 Density in % 1% 281%, (vs. alum.) in relation to weight in relation to weight as % 62% (vs. alum.) 261 GJ/t 74 GJ/t 2,8 GJ/t 1% 28% 1% Fixed systems Cantilevered element unsuited limited unsuited Louvre screen unsuited good good Protective glazing limited good limited Protective film limited good limited in relation to volume in relation to volume as % 74 GJ/m3 9 GJ/m3 1,7 GJ/m3 1% 13%,2% Fig. 4: Selection of manipulatory facade elements and their functional properties Fig. 5: of various building materials [2]

3 intake air in the intermediate space between the two facade layers, thereby reducing thermal losses caused by. Furthermore, thermal and solar screening systems can be incorporated in the weather-protected intermediate space of these double-facades, thus extending the range of materials that may be used. Therefore, the aim of this research project was to examine the potential of the use of manipulatory facade components in two layer facades made of renewable materials, reducing the energy demand for heating and cooling as well as the amount of energy embedded in the facade. 2. THE DESIGN OF LOUVRES FOR DOUBLE SKIN-FAÇADES One of the aims of an investigation of manipulatory elements for double-skin-facades in administration buildings was the development of components with a minimum consumption of production and operating energy. A comparison of the primary energy content of various materials used in solar and thermal screening systems shows that wood has clear advantages over aluminium in terms of emboddied energy and it is lighter in both volume and weight (figure 5). Fig. 6: Section of the testing facility with two-layer-facade with adjustable louvres Preliminary investigations of different forms and types of wood led to the development of a louvre in birch plywood with a lenticular cross-section. In the investigation, the louvres spanned a distance of roughly 1.7 m and had a maximum thickness of 3 mm. During the development of the louvre design, various forms of construction were tested, including birch-plywood louvres with and without an insulating core; and aluminium louvres, also with a birch-plywood covering with and without an insulating core. 3. MEASUREMENT AND ANALYSIS Measurements and observations were taken under various weather conditions, including measurements of thermal transmission, solar shading, visual observations and the mechanical performance of the system. For measuring the thermal performance the double-skin system was installed into a test box, located on the roof-top of a university building in Munich. During cold periods, the louvres were kept closed at night to reduce heat transmission. In addition to the types described above, a standard form of aluminium louvre was tested to provide a comparison; and a special membrane-coated birch-plywood model with openings was also developed. This type of louvre has special qualities in terms of lighting technology, but it was not included in the thermal investigations (figures 15-18). A comparison between various louvre types shows that the wood louvres have a considerably lower primary energy content than the two alternatives (Fig. 8). 3.1 Visual Performance The reflective properties of these facade components are a major factor in preventing overheating within a Fig. 7: View of the test box with louvre system Fig. 8: Environmental balance of various louvre systems [3] building. In addition, materials with good reflective properties can be used to deflect daylight into the interior. The reflective qualities of pale types of wood were compared with aluminium with a matt silver finish.

4 Fig. 9-11: Interior view of the test box with louvres in various positions Depending on the surface treatment, the degree of reflection of unweathered maple and birch laminates lay between 55 and 71 per cent. After intense weathering in an artificial weathering chamber, the values measured for the reflectivity for solar radiation were between 33 and 63 per cent. In investigating the light diffusion and the spatial effects occasioned by the louvres, a further model was tested. It had openings in the surface and was covered with a recyclable translucent membrane skin. Depending on the position of the louvres, the four types influence the internal lighting quality in quite different Fig : Exterior view of the two-layer-facade with louvres in various positions ways. In a closed position, the wood louvres prevent the entry of light completely, whereas a certain amount of daylight penetrates the perforated membrane-coated model (figures 15-18). Light diffusion depends on the type of coating. In the case of the birch-plywood louvres, diffused or reflected daylight enters the internal space even when the facade is fully shaded (figures 9-11). The colour of the louvre surface also influences the colour of the light reflected into the interior. In contrast to the aluminium type, the birch-plywood louvres radiate a warm light. Multiple reflections result in a certain illumination of the rear face of the louvres, and the

5 Fig. 15, 16: Upper side of the plywood louvre with membrane coating Fig. 17, 18: Lower side of the plywood louvre with membrane coating characteristics of the materials become more pronounced (Fig. 9-11). The louvres may also assume a quite different appearance, depending on external and internal lighting conditions. By day, the appearance of the facade is determined by the coloration, the outward characteristics and the reflective behaviour of the surface materials (figures 12-14). At night, when the interior space is lighted, the degree of permeability to light is the dominant factor. The membrane-coated louvres allow internal light to filter through them, which creates a completely different effect externally from that with closed birch or aluminium louvres (figures 15-18). 3.2 Construction Performance After measurement trials lasting 2 weeks, it was possible to draw initial conclusions in respect of the deformation of the louvres in summer. The louvres were kept in a horizontal position to maximize the dead load. Despite the slender cross-section of the birch-plywood members, the deformation in the middle was only about 1.2 mm. In conclusion, one can say that wood louvres have significant advantages in minimizing the consumption of production and operating energy. In the present context, birch-plywood in particular is a highly efficient material that has a great potential for the future. 3.3 Thermal Performance During the design of the façade system its U-value was calculated for the range of different materials proposed. This included the consideration of blind elements filled and not-filled with thermal insulation. The predicted improvement of the U-value due to including thermal insulation was about 4%. The thermal performance of the insulated blind system with wooden elements was investigated using a combination of measurements and computer predictions. The test box was set up in the computer model, HTB2, with the double-skin-façade explicitly modelled in terms of its thermal transmission, and solar characteristics. Figure 19 and 2 present a comparison of the measured and predicted values of internal air temperature and average cavity air temperature for a summer day with the blinds in a horizontal open configuration, respectively. Figure 21 presents a comparison of the average cavity air temperature and figure 22 a comparison of the heating demand of the test box for a winter day with the blinds open in the day and closed at night. In both cases the comparison of measured and predicted results are good, indicating that the model is able to realistically simulate the façade system. [4]

6 temperature in C Fig. 19: Comparison average cavity air temperature for a summer day: 19/7/1999 grey line = outside air temperature temperature in C Fig. 2: Comparison internal air temperature for a summer day: 19/7/1999 grey line = outside air temperature temperature in C heater output in kw,4,35,3,25,2,15,1,5 Fig. 21: Comparison average cavity air temperature for a winter day: 7/3/1999 grey line = outside air temperature Fig. 22: Comparison heater output for a winter day: 7/3/ CONCLUSIONS The louvre system has shown to perform well in relation to its visible characteristics, providing good internal visual conditions. The system can easily be adjusted according to the needs of the user with regard to thermal insulation as well as heat and glare protection. Due to the reflective qualities of pale types of wood, such as birch and maple, daylight can be deflected into the interior. The specific visual qualities and the appearance of wood, such as birch, offer the chance to add a new kind of architectural expression to highly efficient facade systems. In addition to the examined systems (figures 9-14) a whole range of alternatives can be developed by combining the wooden louvre with various materials for the surface. The material for the coating of the louvres can be selected according to the desired functional qualities, such as transparency or higher grades of reflectivity. (figures 15-18) In this way, the amount of energy used for operating the building is reduced as well as the amount of energy used for the construction of the facade system. The construction of the louvre system made of plywood has shown to be highly efficient in terms of weight and stability. The louvres made of birch-plywood have less than 25% of the weight of comparable aluminium. An initial evaluation showed that all the louvre types tested (wood, wood and aluminium, and aluminium) possess good solar and thermal screening properties and are therefore suitable for use in double-skin-facades. Overheating of the internal spaces was largely avoided, even in cases of intense insolation and high external air temperatures. But intense insolation causes the air in the intermediate facade space to heat up, and this may reduce comfort levels internally if the windows are opened for purposes. To derive realistic results for the energy saving potentials, the model of the façade system is implemented

7 into a computer model of an office building. This investigation is currently being performed. REFERENCES [1] Lang, W. (1998). Zur Typologie mehrschaliger Gebäudehüllen aus Glas. Detail. 7/1998, p [2] Marmé, W.: Der Primärenergiegehalt von Baustoffen. Diploma thesis at the Institute for the Technologie of Building Materials, University of Karlsruhe, 198, p [3] The calculation of the environmental balance of various louvre systems was done by Dipl-Ing. Th. Ruehle of the engineering firm INTEP, Munich. Basic information on the calculation method can be found in the publication 'Hochbaukonstruktionen nach oekologischen Gesichtspunkten', Schweizerischer Ingenieur und Architekten-Verein (Ed.), SIA-Dokumentation D 123, Zuerich [4] Alexander, D.K. (1996, rev. Nov. 1997). A model for the thermal environment of buildings in operation, release 2.c, Welsh School of Architecture R&D