The Double Glass Skin Façade: a sustainable option for the Curtain Wall in Mediterranean Climate Countries?

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1 1 The Double Glass Skin Façade: a sustainable option for the Curtain Wall in Mediterranean Climate Countries? Pere Alavedra¹, Mauricio Torres², Amado Guzmán³, Samantha Gallowayº, Daniel Alavedra" Department of Construction Engineering, Catalonia Polytechnic University, Barcelona, Spain. Abstract: This paper compares the typologies of the Curtain Wall with the Double Glass Skin Façade in the Mediterranean climate countries. Although the curtain wall has not got a good performance in some latitudes and although the high cost it takes for the user s comfort maintenance. It is still considered a reference typology due to its transparency and the image or status it provides. The new themes that sustainability brought out like energetic efficiency and care for user s comfort have made designers and constructors search for others typologies which maintain the transparency image: can the Double Glass Skin Façade be a sustainable alternative to the Curtain Wall in the Mediterranean climate countries? Key words: sustainability; construction, double skin façade, curtain wall Introduction: The Curtain Wall become a modern construction reference which gives status to the enterprise or entity that uses the building. Important losses and heat gains were experimented by the Curtain Wall due to its use in so many different climates and under hostile conditions made indispensable the use of HVAC systems and converted it in a unsustainable typology from the energetic efficiency point of view. Generally the measures taken to improve the thermal comfort in these buildings are not enough to maintain the required comfort level without high use of energy. On the other hand the Double Glass Skin Façade has kept the transparency concept of modernity with a more reasonable energy use in the continental cold climate countries of Europe contributing with the environmental and economical sustainability of this façade s type of buildings although it requires investigation of its systems components besides the study of local conditions for its application in the Mediterranean climate countries for a sustainable construction. Authors Note: Pere Alavedra¹,Ph.D. Industrial Engineer, associate professor, (pere.alavedra@upc.es); Mauricio Torres²,architect, Ph.D. student on Construction Engineering (mauricio.torres@upc.es); Amado Guzmán³,architect, Ph.D. student on Construction Engineering, amado.guzman@upc.es); Samantha Gallowayº,architect (samantha.galloway@upc.es); Daniel Alavedra",lawyer, Ph. D. student on Law (dani.alavedra@upc.es).

2 2 1. General Agents implicated in the thermal comfort and energy conservation: The most important design parameters affecting indoor thermal comfort and energy conservation on the building scale are: orientation, building form and optical and thermo-physical properties of the building envelope. All these parameters are related to each other and the optimum value of each parameter should be determined in correlation with the other values (Oral 2002). Building orientation; Building Form; Optical and thermo-physical properties of the building envelope. As it happens in the continental cold climate zones, the orientation and form of the building and the optical and thermo-physical properties of the building envelope are extremely important factors for the envelope efficiency in the building studies.. While large surfaces and inconvenient orientation of a building can mean high heat losses in cold climate zones it may mean high heat gains in the Mediterranean climate zones. The materials used in the façade can increase or diminish the problems. 2. General Agents implicated in the Mediterranean climate: 2.1. Solar incidence Average A Mediterranean climate city like Barcelona will present a high average of solar radiation hours per day (table 1) than a cold climate city like Berlin (table 2). The solar angle incidence will also be different in both cities and will have a direct influence on the use of glass façades which will be subjected to high losses and heat gains contributing towards the users required levels of comfort and energy spends Annual Temperature Average The annual temperature average in mild climate cities transcend that of the cold climate cities ( table 1 and 2) and denote the predominant use of cooling system over the heating system in those cities. A large glass surface contributes with the increasing maintenance costs of the comfort due to its thermal properties Air Humidity the air humidity depends on many factors and can be higher or lower independently of the climate zone as we see in Barcelona and Berlin cities. The moisture can cause high condensation levels in large glass surfaces as it can be harmful to the finishes and equipments of the building as well as to the human health. All these factors are closely related and they depend on one another. Table 1. Climatologic Dates of Barcelona city (adapted table from the INM Spain) Month T TM Tm H I Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec Year

3 3 Table 1. Climatologic Dates of Berlin city (adapted table from the Deutscher Wetterdienst) Month T TM Tm H I Jan ,5 Feb ,8 Mar ,9 Apr May Jun Jul Aug ,8 Sep Oct ,6 Nov Dec Year Legend: T Monthly/yearly temperature average (ºC) TM Monthly/yearly maximum daily temperature average (ºC) Tm Monthly/yearly minimum daily temperature average (ºC) H Humidity Average (%) I Monthly/yearly hours of sunlight average 3. Mediterranean climate consequences on the Curtain Wall typology: 3.1. Important losses and heat gains the great heat gains are mainly due to the high solar radiation average verified in the Mediterranean climate zones. The most subjected element of a building to the heat exchanges may be the glazed surfaces. Generally the curtain wall buildings have not got an adequate insulation treatment to the heat gain as well as to the heat loss in the zone of Barcelona HVAC requirement systems extreme temperature are verified in summer and winter seasons in Mediterranean climate zones. The use of HVAC systems are necessary to maintain the user s required level comfort with predominance of the cooling system over the heating system Sick Building Syndrome Generally the Curtain Wall building in Mediterranean climate cities have no openings and stay completely closed while it s being used. It may bring some dangerous consequences as a variation of the Sick Building Syndrome Deterioration of the equipments and finishes the accumulated moisture in a building without an adequate ventilation system may cause the deterioration of its equipments and finishes with it s consequent higher costs on maintenance. 4. Possible solutions to be brought out by the Double Glass Skin Façade in Mediterranean climate countries: 4.1. Use of solar blinds the Double Glass Skin Façades let the location of solar blinds in its cavity. Due to the high solar radiation average in the Mediterranean climate its use is necessary. Its location in the cavity of the façade permits the heat gains process to occur outside the building and its cooling phase without heating the interior ambient by the emitted heat due to convection

4 4 process. They also can control the glare effects of solar radiation. The solar blinds could be compared as a thermal insulation and as its is located in the outer skin of the façade it could contribute to the increase of the time lag and the increase of the decrement factor similarly to the location of the thermal layers in a traditional brick wall according to studies done in the topic (Bikas, Cantoleon 2002) Use of Special Glazing nowadays we have many different types of glass with many characteristics that can improve the performance of the façade in winter and summer seasons in the Mediterranean zone. They can help control the losses and heat gains as well as the light entering the building. We could also decrease the demand of the HVAC systems if an adequate type of glass was chosen to compound the façade. Reflective glasses would work as the first thermal layer acting together with the solar blinds as the second thermal barrier contributing for a better performance of a passive solar building. Sustainable building with double skin facades should be designed to minimise energy consumption and thus reduce greenhouse emission. Glass design and glass choice have big a impact on energy consumption for lighting, heating and cooling (Avasoo 2002) Night Ventilation Due to the high thermal gains by the buildings during the day in Mediterranean climate zones the night ventilation process can be extremely helpful to diminish the starting point level of the HVAC systems by the morning of the next day. It could mean great energy saving. The outside skin of this type of façade allows the night ventilation despite of the climatic conditions with no security risk Natural Ventilation the possibility of natural ventilation for nearly all year in Mediterranean climate buildings could allow a smaller investment in a HVAC system for the building. Besides the possible saving of energy consumption. It could create a more healthy interior ambient preventing the proliferation of micro organisms harmful to the human health and put the users in contact with the outside environment. 5. Double Glass Skin Façades implicated in the possible solutions: The possible solutions presented above depend on the systems performance of the Double Glass Skin Façade that must be investigated in the Mediterranean Climate zones. Some factors of the system are: 5.1. Building s Aerodynamic the wind flows that reach the building will have a great influence in the basic working systems of the Double Glass Skin Façade in the Mediterranean latitudes such as the openings for the income and outcome of air flows due to the different pressures generated in both openings System Dimension The space in between the incoming and out coming openings will have pressure losses and heat gains that must be equalized for a good performance of the stack effect process. The width of the cavity, the distance between the openings and its dimension will affect its functioning due to the thermo-physical phenomena in this space like the thermal and pressure variation due to the air density changes in the cavity Solar Blinds in the cavity space the ventilation in the cavity of the Double Glass Skin Façade in the Mediterranean climate zones is extremely important due to the high solar radiation average in this latitude and the installation of solar blinds in the cavity space must be studied well not to become a barrier to the air flow impeding the stack effect performance Ventilation Opening system in the interior skin an efficient ventilation system is extremely important in the Mediterranean climate zones due to the cooling necessities for the day time and night time to get rid of the heat absorbed. Some types of openings in the interior skin of this façade

5 5 type can give a better or worse ventilation and the most adequate one must be chosen according to each case and necessity Glass types the glass type used in this kind of façade in Mediterranean latitudes will be greatly responsible of its performance. In this climate the type of glass is more important than the frame material choice. The amount of heat that can be blocked by the special treated glazed surface is much higher than the heat that can be transmitted by the most transmitting material chosen for the frame. Low emissive glazing will be adequate for the winter season avoiding a great heat losses. 6. Conclusions: Reach a transparent and sustainable building in all of its aspects is a challenge that must be faced in Mediterranean latitudes. Investigations taking into account the aspects of the Mediterranean climate zones on the design of the façade are necessary to demonstrate the convenience of the Double Glass Skin Façade in this zone or its inadequacy due to the available technology we have. The simple application of the Double Glass Skin Façade typology in the Mediterranean climate zones without taking into account the particular characteristics of this climate is not a prudent action for the sustainable construction. The Double Glass Skin Façade presents a higher construction cost than the Curtain Wall typology and for this reason the verification of the advantages it can give maintenance of the comfort levels by saving some of the energy is extremely important as well as its amortization time of the initial investment for its construction. The possibility of energy savings in the HVAC systems for buildings with Double Glass Skin Façades in Mediterranean climate zones during all over the year seasons is one of the key factor for its implementation. Generally the Curtain Wall buildings in Mediterranean climate zones have high maintenance costs with the HVAC systems and the use of special treated glasses, cover sheets and special films can represent a solution or a improvement in their function. Climatic conditions as well as the orientation and the building s shape are extremely important to choose the location of the glazed surface and it s treatment requirements, helping increase the level of sustainability of the building. References: Oral, G. K The limit u-value for building envelope related to building form in temperate and cold climatic zones. Building and Environment, Vol. 37 (2002) Avasoo, D Transparent and Sustainable Buildings. Sustainable Building Conference Oslo Bikas, D. and Kantoleon, K Environmental evaluation of thermal inertia factors of buildings envelopes as a function of Solar Absorbity. Sustainable Building Conference Oslo Chevalier J. 2002, Introducing practically the environmental concern when prescribing construction products, International conference Sustainable Building 2002, Oslo, Norway.

6 6 Web Links: Deutscher Wetterdienst Instituto Nacional de Meteorología