1 INTRODUCTION

By considering that more than the third part of the Co2 polluting gas responsible for the green-house effect is caused by building activities, one easily realizes the relevant role of architecture as regards the theme of the global sustainability. Nowadays, improving the quality of the exist-ing real estate is a widespread building activity that requires various and important interventions such as structural, technical, architectural, functional and energetic upgrading. In particular, poor materials and single paned deal window frames were used, without paying any attention to energy conservation especially owing to the lack of technical standards and building regula-tions.

The most important problems are connected with the façade decay, the lack of acoustic and thermal insulation and the presence of thermal bridges. In the last decade, an awaking policy fo-cused on the protection of the environment is in progression in Europe like as all over the world, with a special attention to energy waste in the building sector. A basic improvement in this field is the EPBD 2002/91/CE that, following the Kyoto Protocol, promotes the energy efficiency of buildings. As from January 2006 this set of rules has been acknowledged in Italy by enacting the Decree 192/2005, and his following upgrading Decree 311/2006, that states new standards for the energetic performance of buildings, considering also the existing stock. The experiences gained since now in other Countries seem to test wide chances of interventions on the post war building stock, even if the building techniques and the state of decay are various, by using tech-niques and materials free from cultural and historical meanings typical of cultural heritage. Ac-cording to these experiences it’s possible, therefore, not only to introduce basic functional im-provements but even proper architectural lifting. In the last decade the urban building envelope has come forth gradually into being a dynamic and active bounding surface. It means that the envelope is automatically able to gear its performance to the changes of the environmental con-ditions, as it integrates active functional devices.

Consequently, the refurbishment actions have especially to be made by considering the chance of the natural resources, following the sustainable architecture statements that outline the exploitation of alternative sources of energy. A sustainable upgrade of inadequate buildings should mainly provide active or passive energy from renewable sources in order to achieve the

Sustainable technologies in the refurbishment of existing building envelopes in Italy

Silvia Brunoro Architect, PhD University of Ferrara, Department of Architecture, Italy

ABSTRACT: With the new European Energy efficiency regulation building are forced to re-spect restricted standards for energy saving. In the last ten years building envelope has become more important in the overall energy balance, as a dynamic surface able to optimize its perform-ance in relation to different climatic inputs (passive or active). In this sense it will be desirable to manage the necessaries refurbishment actions in the direction of the passive solar gaining, natural cooling and other relevant sustainable technologies for building envelopes which are presented in this paper.

highest indoor comfort by restricting the use of air conditioning units and artificial lights. About the above mentioned issues, the most efficient technical solutions are based on fundamental principles of the sustainable architecture such as: heat gaining by collecting and storing solar energy in winter, use of passive cooling and natural ventilation in summer, maximum natural lighting, reduction of heat transmission through the walls, using systems with low environ-mental impact such as dry technologies. The strategies of intervention that can be usd in a sus-tainable refurbishment may be briefly summarized as follows: • Improving of technical and acoustic performance (External thermal insulating systems, Ven-

tilate façades, Double skin glass façades) • Solar shading (lamellas or panels, replacing of windows with Low –E high-performing glass) • Passive solar energy systems (Transparent Insulation materials, Solar glazing balconies) • Active solar energy systems (Solar collectors, Photovoltaic modules)

2 STRATEGIES OF INTERVENTIONS

A first evaluation of the above mentioned four basic strategies of intervention is based on the difficulty of realization and, consequently, the cost. The classification in the following tables can be useful to compare the efficiency of the different way of refurbishment, mainly related to the cost levels and the employment flexibility. Technical solutions that consist in the overlap-ping of a new façade, without modifying the existing façade, are more economical also because, in the most cases, the users can stay in the building during the works. Costs are higher when the refurbishment action requires the replacement of the existing envelope, where generally a multi-tude of variables has to be considered. Table 1: Strategies of intervention in relation to the existing envelope Type of intervention Techniques Overlapping

to the existing envelope Substitution of the envelope

ETICS X Ventilated façades X Double skin glass façades X X Solar shadings X Low E glass X TIMS X Glazing balconies X Photovoltaic modules X X Table 2: Strategies of intervention in relation to the typology of building Type of intervention Techniques Residential Office/tertiary ETICS X Ventilated façades X X Double skin glass façades X Solar shadings X X Low E glass X TIMS X X Glazing balconies X Photovoltaic modules X X

In this datasheet, the potentialities of ventilated façades, double skin glass façades and solar shadings as refurbishing solutions, mainly for post war buildings, will be assessed.

2.1 Ventilated façades The use of a ventilated façade in the refurbishment and upgrading of existing buildings is indi-cated for a multitude of cases, for example: - Lack of thermal and acoustic performances;

- Façades with detachment of cladding, attack by weather or moulds; - Façades which presence of damages in the cladding, or cracks in correspondence of the

concrete frame and the infill walls; - Walls with strong construction dimensional tolerances, compensated by plaster thick-

nesses, which is very variable depending on zones. In these cases the integration of the plaster is technically problematic due to the big thick of the layer and it’s quite impossible the gluing of other claddings;

- Walls with high hygrometric dilatation coefficients, usually interested by diffused cracks; - Façades which are not more coherent to the user changes (typical of commercial, offices

and industrial buildings).

The system is composed of external cladding, dry mounted, fixed to the existing wall by means of stainless steel devices that also supports insulating panels. Between the cladding and the panels an air cavity, 5-10 cm of thickness, allows natural air circulation that is helpful to re-duce heat gains during the summer, to increase thermal insulation and to reduce heat losses and condensation in winter.

The use of a ventilated façade is becoming increasingly common in the refurbishment and upgrading of any building envelope. This technical solution, overlapped on the existing façade, can solve a multitude of technical problems by improving thermal and acoustic insulation and protecting from moisture and atmospheric agents. Furthermore, the recent development of high-performance claddings offers a wide range of opportunities for upgrading the image of the building to meet new architectural requirements. The use of a ventilated façade is not limited to any type of building. The multitude of claddings available on the market allows to find a pecu-liar solution for each problem, and the design optimization for the different building envelopes. The fundamental factors to evaluate for the addiction of a ventilated façade are: - The planarity defects and other issues related to the condition of the existing wall, which

can influence the modality of anchorage; - The mechanical resistance of the support, which can be heterogeneous an insufficient in

some parts; - The adequate protection of the insulation layer, which has to be permeable to the vapour

coming from the inside but protected from external water; - The adequate protection of the air cavity to avoid the entrance of insects and other ani-

mals; - The realization of the façade joints, to avoid mechanical tensions in and on the cladding

elements. A specific design, particularly related to the peculiarity of the situation can be profitable in

the improvement the technical and architectural quality of the building with reasonable costs and environmental positive effects. Figure 1-2: Ventilated façade in the refurbishment of a residential building in Modena (before-after)

2.2 Double layer glass façades A particular case of ventilated façade is represented by the so – called “double layer glass fa-çade”. The most useful field of application of this solution is the tertiary sector (mainly offices and productive buildings) in which is easier to find financers who believe in the power of a new high-tech image of the building. Recent studies are investigating the possibility of application of this technology even for residential buildings. A typical double glass envelope system com-prises a layer of single glass and a layer of double-glazing, separated by an air space. An oper-able shading device or heat absorbing glass can be installed in the air space to minimize the so-lar heat gain. In addition to the energy savings, the double envelope system has other potential benefits such as acoustic control, water penetration resistance, and improved office atmosphere because of the view and utilization of daylight. The double envelope system also offers a choice for renovation of existing building facades to transform into more energy efficiency buildings.

Double glass façades in the refurbishment are generally hybrid systems, formed by the exis-tent wall and a new glass envelope. The external envelope is a glass façade: this helps in the so-lar gaining during winter season, in the heating of the air cavity and in the consequent im-provement of the thermal capacity of the whole system. The use of high – performing glasses is fundamental to obtain solar reflection and to avoid overheating. If the refurbishment is oriented to the complete substitution of the existing envelope, the internal wall is either a glass façade, forming the conventional double-layer skin. In the most cases it’s more convenient to leave the existing wall, replacing the windows and adding insulating panels on the structure and on the parapets.

In winter, during the warmest hours, the heating of the air cavity is the fundamental factor that helps in the reduction of thermal losses and of air permeability through the wall. During the night, the air vents are closed to avoid the entrance of cold air. In summer, thanks to the solar shadings, good levels of internal comfort can be obtained. During the day the cavity is closed, to avoid the entrance of warm air, while during the night the natural cooling can refresh the walls and the rooms. The cavity in double glass façades is either naturally or mechanically ventilated. Natural ventilation can provide an environmental friendly atmosphere and reduce the require-ment for mechanical ventilation. On the other hand, natural ventilation is not without risk. It may create a door-opening problem due to pressurization. Besides, if the air path is not appro-priately designed, the solar heat gain within the façade cavity will not be removed efficiently and will increase the cavity temperature. In urban environments, natural ventilation systems may also experience significant problems of noise transmission and pollution and may result in uncomfortable indoor environments in extreme weather conditions.

Therefore, a natural ventilation system is more suitable in suburban areas with temperate weather where the airflow in the cavity will be close to the indoor air condition. The thermal performance of double façade systems depends on many factors, such as: - The composition and performance of the layers (glass/glass or glass/wall and the type of

glass); - The height of the air cavity (which can be continuous or divided in horizontal or floor-

height cells, in this case the ventilation is related to each cell); - The thickness of the air cavity (which can range between 20 cm to 90 if contains mainte-

nance routes); - The type of ventilation in the cavity (natural or forced) that is strictly related to the height

of the façade and to the climatic conditions; - The relationships between air cavity and HVAC systems (possibility to utilize warmed air

from the cavity and expel internal air. This is rare in the refurbishment).

Because the interactions among these variables are complex, current computer simulations approaches may be inaccurate. Therefore the energy savings and cost/benefit of these systems are not well established.

The relationships between typologies of façade, type of ventilation and applicability in the re-furbishment are synthesized in the following tables.

Table 2: Main typologies of a double-layer glass façade and applicability in the refurbishment

Typology of façade Scheme Description Air cavity

(cm) Applicability Cost

Full height

The external fa-çade is inde-pendent from the internal, the air cavity is con-tinuous

40/90 X X

Pipes

The external fa-çade is fixed to the internal by

means of a common frame or punctual an-chorages. The cavity is verti-cally sectioned.

20/50 - -

Cells

The façade is formed by the aggregation of modular cells,

independent each other. Each cell has his inlet airs.

20/30 - 0

X= High - = Medium 0 =Low Table 3: Main types of ventilation in a double-layer glass façade and applicability in the refurbishment

Typology of façade

Scheme Description Applicability Cost

Natural ventilated façade

The air in the cavity flows naturally (chim-ney effect). Internal layer=insulating and performing layer External layer= single glass

Relationships be-tween in-side/outside

Internal windows can be opened. This allows the natural ventilation of the rooms, even it’s necessary to carefully evaluate the right period of opening (winter day- summer night), other-wise the positive effect of the natural air are neutralized.

X -

No Relationship between in-side/outside

Internal windows can not be opened. This helps in the air cavity control, but decreases the user requirements (necessity of fresh air).

X 0

Forced ventilated façade

The air in the cavity flows by means of me-chanical devices. Inter-nal façade is closed. In some cases warm air in winter and cold air in summer are utilized by HVAC systems. External layer= insulat-ing and performing Internal layer=single glass

0 X

X = High - = Medium 0 =Low

The most useful typology in the refurbishment is the full height façade: generally the juxtapo-sition of a new glass layer doesn’t requests big modifies to the whole system. In these cases original windows are substituted with double-glass windows and an insulating coating is applied on the opaque parts. The natural ventilation of the cavity doesn’t involves plants modifies. The careful design of the air cavity (thickness, position and dimensions of air vents) is fundamental as the phenomenon of natural draught works properly. The use of a pipes or cells façade is more convenient where the height of the building is such that natural flow is impossible.

Since the Italian regulation imposes the natural ventilation of the rooms in residential build-ings, windows can not open only on the air cavity, as it happens in tertiary buildings. By the way, there are two main solutions which can be evaluated: - The realization of a partial glass façade, cells or pipes formed, only in correspondence of

the opaque walls; - The realization of a moveable external glass envelope.

The realization of the intervention has to be carefully planned and designed, to evaluate

which will be the more coherent and resolute solution. This could be done with the support of the producers, which put at designer’s disposal their technical competences for the realization of the façades, supporting more than the diffusion of specific programmes, the executive designing and the performances verify. Figure 3-4: Double skin glass façade in the refurbishment of an office building in Milan (before-after)

Fields of application of a double layer glass façade are, at the moment, mainly related on of-fices buildings. Many example of refurbishment can show that, with this high-energy perform-ance technical solution, high levels of thermal comfort and energy saving are obtained (ranging from 20 to 40% of reduction of the overall building consumption). The high investment costs for a refurbishment are, in the long term, remunerated by the reduction in HVAC use. The chal-lenge is represented by the application on residential buildings.

2.3 Solar shadings A solar shading can be used to control the diffusion of the light through a glazing surface and to reduce the heat gains through an opaque wall. Solar shading systems allows the incidence of the rays of sun during the winter which consists a benefit for the overall thermal balance of the building. On the other hand, during the summer, the overheating of the rooms due to the inci-dence of the sun on the windows has to be avoided. The use of solar shadings is becoming in-creasingly common in the refurbishment of building envelopes. The recent development of high-performing envelopes and the growing importance assumed by glass, has influenced the development of techniques and materials oriented towards the control of the solar factor. The choice of the solar shading depends on the type of surface: adjustable solar shadings are more suitable for glass windows while fixed elements can be used for opaque surfaces. A lot of ty-pologies are available, which can be classified according to: shape, material, movement. A first classification can be done in relation to the envelope in: external solar shadings, internal solar shadings and solar shadings between two glass panes in the windows or double layer glass fa-çades. In the field of the external solar shadings, windows elements (e.g. venetian blinds) and solar shadings for façade (e.g. lamellas or panels) are included. Comparing to an internal solar shading, the overlapping of an external solar shading is more effective for the overall energy balance in terms of energy efficiency, for the following reasons: • The interception of the rays of sunlight before they heat the surface and the optimal control

of the diffusion of light and heat; • The reflection of a part of solar radiation and its dissipation, which depends on the material

and on the color of the surface; • The reduction of thermal losses, in the case of adjustable elements, which can be closed dur-

ing the night.

For these reasons the efficacy of an external solar shading is about the 30% above to internal shadings. In south facades, where the sun is high on the horizon, horizontal shadings are more suitable. Particularly indicated in summer season, an horizontal shading can hamper the rays of light in winter, therefore is more suitable the use of adjustable or mobile systems.

Vertical solar shadings are more adapt for East and West oriented façades, where the sun is low. Comparing to a fixed system, the use of an adjustable system allows to change the position of the elements in relation to the direction and the intensity of the rays of sun. Therefore, with this solution it’s possible to control each element during the day and also during the different sea-sons, to achieve the optimal daylight factor and to avoid undesired solar gains. Figure 5-6: Overlapping of lamellas and panels solar shadings in the refurbishment of an office building in Lecce

3 CONCLUSIONS

In the last two decades, good examples of new sustainable architectures provided realistic inspi-ration and practical experience of how architects, and their clients, have achieved sustainability.

They demonstrated a wide variety of approaches to design and technical issues – including passive solar design, good daylighting, natural ventilation, night cooling, combined heat and power, photovoltaics, grey-water recycling, and the integration of landscaping. The today’s challenge is focused on the improvement of the existing buildings, in order to reduce the overall energy balance which is mainly related to the envelope. Renovating façades often coincides with changes in the architectural appearance of a building, and large investments to improve the quality of the building envelope are justified if the building block as a whole can meet future re-quirements.

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