The concept of comfort, in the sense of the well-being of the occupants of a building, needs to be completely reassessed in conjunction with the appropriateness of the means; in other words, seen against the background of finite resources and environmental pollution in the world. The various criteria that influence this sense of well-being – thermal, hygrometric, acoustic and visual – together with the relevant technical parameters, are defined in standards and codes or practice. Architects are, therefore, legally obliged to implement them. What latitude remains for interpretation?
One is aware of the problems of assessing comfort. The individual, subjective sense of warmth or coolness, for example, is related to the physical activity a person undertakes, so that desirable temperature levels will be determined in part by the nature of the work performed in a given space. Economically and in terms of energy consumption, it is probably more sensible to allow individual control of a person’s immediate spatial climate than to try to achieve “ideal” conditions for everyone. At all events, the refurbishment of the building skin usually results in a marked gain in comfort in terms of the indoor climate.
The second important aspect to be considered in refurbishing the outer skin of a building is the choice of materials – not only in relation to technical standards and the design, but in the environmental context as well. No comprehensive system exists for evaluating materials according to predefined ecological criteria, although the Wuppertal Institute for Climate, Environment and Energy has made a promising start in this direction with its “material input per service unit” (MIPS) concept. This allows the flow of biotic and abiotic materials and of water, land, air and energy to be calculated in units per service unit – usually in relation to a square metre of facade. The aim is to minimize the intensity of material use, also known as the “ecological backpack”. In selecting materials and forms of construction, the order of priority is to “reduce, reuse and recycle”. In other words, the primary aim is to reduce the use of materials altogether, and following this, to ensure that they have a subsequent application. The latter implies a form of construction based on a unit system, the individual components of which can be reapplied elsewhere. The final option is the reuse of materials, possibly after they have been reprocessed. Unfortunately, recycling usually implies “downcycling”, with a corresponding reduction in the value of the materials.
In the use of energy, two lines of development exist that treat the building envelope as an interface between energy considerations and technical installations. Low-energy forms of construction are leading increasingly to the implementation of passive-energy building concepts, particularly in the field of housing. With thermal insulation up to 40 cm thick and a solar optimization of the fenestration, heat losses through transmission can be reduced to such an extent that it is possible to do without conventional systems for the necessary back-up heating. This requires the installation of a ventilation system with a heat-recovery unit, however, since the thermal optimization of the building skin leads to a proportional increase in the heat losses caused by ventilation (ill. 2). As a result, there will be a shift in the technical installation from conventional heating to controlled ventilation.
With the need to provide mechanical ventilation, there is a danger of sick building syndrome (SBS). By heating the air intake, the natural content of ozone is lost, and it is this ozone content that provides a sense of freshness. Since SBS is not recognized medically as an illness, it is often disregarded in the development of passive-energy buildings.
The second line of development is in the design of facades in which natural ventilation is used to reduce the service technology installed in a building (climate-responsive facades). The leading example of this today is the double-skin facade. By reducing the level of thermal transmission, improving the sound insulation and achieving balanced airflow conditions, this type of facade greatly increases the scope for natural ventilation. It also reduces heating-energy needs to such an extent that low-temperature systems can be used.
The constructional costs of active facade concepts that respond to weather conditions and influence the indoor climate are considerably greater than for conventional forms, so that even if all aspects are taken into consideration, this kind of facade is often not justifiable economically (ill. 3).
The refurbishment of a facade has a direct influence on the energy balance of a building and thus on its economic viability. For that reason, the costs of the work have to be seen in conjunction with the improved energy balance and the reduced operating costs.
The following strategies illustrate the various conceptional approaches available in rehabilitation work to building envelopes today.
One of the most striking illustrations of the double-skin facade principle, where it has an active influence on the indoor climate, is the SUVA building in Basle, refurbished and extended in 1993 by Herzog & de Meuron (ill. 1). The heterogeneous fabric, consisting of a 1950s structure with a stone facade and the new extension, was visually united by a glass skin. This new envelope is in a post-and-rail construction, divided storey by storey into three horizontal strips. The upper strip is in double glazing with integral prismatic sheets; it serves to deflect light and provide shading and can be computer controlled. The middle strip contains manually operated double-glazed top-hung lights. Sunshading louvres are housed in the cavity, where they are protected against the weather. The bottom strip consists of glass apron panels with a printed surface. In winter, they create a thermal buffer between the facade layers. In summer, the top-hung flaps are opened to prevent overheating of the intermediate space and to support the process of night-time cooling. The advantages of these varied facade responses to weather conditions are available only with a very elaborate form of construction, which makes the economic viability of this kind of refurbishment questionable.
The bank building erected in Berlin in 1997 by Benedict Tonon is an example of a double-skin facade in a complex conservational situation (ill. 15). A slenderly dimensioned single-glazed outer skin is drawn over part of the existing historic sandstone facade, over the new storeys added to the top of the building and over the courtyard. The space between the stone facade and the layer of glass is roughly a metre in depth. The minimal form of the glazed construction does not allow an optimization of the indoor climate for users, since there is no provision for openings. The main benefits are the improved sound protection and better thermal insulation in winter as a result of the buffer zone.
If one were to draw a skin of this kind over the entire building, a protective enclosure would be created akin to the climate domes of Richard Buckminster Fuller. The recently completed covering to the British Museum courtyard (see p. 1042) by Foster and Partners shows the potential of combining conservation needs with the innovative construction of an enclosing envelope. On the whole, though, it is easier to realize innovative solutions where listed buildings are not involved.
Hybrid forms of construction, using steel, glass, mineral fibre and plaster products, are an ongoing development of classical dry, lightweight structures (ills. 4, 5). A material such as plasterboard, with its fire-protecting properties, usually forms the space-defining surface layer, with the load-bearing structure consisting of cold-formed sheet steel, and a layer of single glazing as the external finishing to the element. A minimum ventilated space should be created between this layer and the insulation. On sunny winter days when the external temperature is 0 °C, the temperature in the facade intermediate space can be as high as 40 °C, so that the flow of heat is no longer from inside to outside, but vice versa. In this way, thermal losses through transmission can be reduced – depending on the time of day, the facade aspect and the weather conditions. One speaks of a dynamic U-value. In a hybrid element, the typical U-value of less than 0.2 W/m2K – equivalent to at least 20 cm of thermal insulation – can be reduced by roughly 40 per cent even on the north side of a building. On the south face, negative U-values are possible (ill. 6). A form of wall constuction suitable for a passive-energy house can be achieved with an overall thickness of 25 cm instead of the 40 cm normally required. This is of interest especially in the refurbishment of buildings. The use of hybrid elements also leads to interesting synergetic effects. If elements of this kind are applied without an additional layer of insulation (which may be unavoidable because of lack of space for thicker forms of construction), the heat generated in the intermediate cavity can be stored in the solid walls and given off internally at a later time.
When refurbishing reinforced concrete skeleton-frame structures, it is important to minimize any additional weight imposed. Buildings planned in the 1950s and early 60s in particular possess few load-bearing reserves, and inadequate concrete cover to the reinforcement often makes their refurbishment a costly process. The complete replacement of an existing strip facade with hybrid elements was carried out for the first time in Düsseldorf in 1997 in a prototype scheme (ills. 4, 14). In this project, the overheating of the cavity that may occur in summer does not lead to a noticeable heating of the interior, but it does result in an exceptional strain on the materials.
Cardboard honeycomb elements made from recycled paper provide an economic means of placing the protective layers in front of the insulation. In pilot schemes, absorber mats with water flowing through them have been used to exploit excess thermal energy for the hot-water supply or to modify the wall temperature. A similar principle was used for a facade-air collector with integrated heat recovery, which was developed at the Centre for Applied Energy Research (ZAE) in Bavaria. The fresh-air supply for the rooms is preheated by means of solar energy, and part of the heat in the exhaust air is later extracted. In summer, the system serves to provide noise-free ventilation.
A completely different problem arose in the refurbishment of the Karstadt department store in Gütersloh. The bulk of the operating costs are caused by the great cooling capacity required to reduce the internal heat loads. The architects Gatermann and Schossig developed a facade system in which special, gill-like openings are used for night-time cooling (ill. 16). These flaps form part of a rear-ventilated, glazed, curtain-wall facade in which recycled toughened safety glass was used. The glass elements are fixed with an elastic adhesive to a thin concrete supporting layer and held in position by visible aluminium point fixings. The frameless opening flaps are incorporated into the facade grid and, in a closed position, cannot be distinguished from the fixed glazing. They open automatically at night.
When one speaks of changes to the facade core layer, this usually implies the replacement of the insulation or the addition of further layers to comply with modern building standards. One is familiar with the outcome of such measures from the rehabilitation of many buildings dating from the 1950s: deeper window reveals, smaller roof projections, a jump in plane at plinth level, and changes in the building line caused by the greater thickness of the insulation. For these reasons, the use of systems of this kind is rarely possible in listed buildings. New developments in vacuum insulation could alleviate this problem (ill. 8). The first rehabilitation of a listed facade with this type of insulation was completed at the end of 2000 in Nuremberg with the collaboration of the ZAE in Würzburg. (A fuller description will follow in Detail 7/2001.) In this project, 15 mm vacuum insulation panels (VIPs) were fixed with adhesive to the existing wall and protected with an additional 35 mm layer of expanded polystyrene with conventional studs. To prevent condensation in the joints, an aluminium composite foil was glued to the surface of the wall. The thermal transmission value after completion of the work was 0.2 W/m2K with an overall insulation thickness of only 50 mm, compared with the 200 mm that would be necessary using mineral wool.
VIPs have a compressed powder core, consisting of extremely fine, nano-structured silicic acid enclosed in special metallized polythene and aluminium foils. The foil wrapping allows the sheets to be vacuum packed in much the same way as freshly ground coffee. Although VIPs are already on the market, there is no long-term experience of their use.
Improving the thermal insulation and the airtight seal of older buildings is inseparably linked with questions of ventilation and the balance of the building physics. Energy-saving regulations in Germany require a n50 value of 3.0 per hour. This means that, with a mechanically generated pressure below that of the atmosphere, the entire volume of air within the building may be exchanged only three times per hour. Where modern airtight windows are built into older buildings in the course of rehabilitation work, the loss of this basic source of ventilation can often lead to the formation of mildew. Users will have to ventilate the rooms regularly by opening the windows.
Special developments in the field of window casings and opening lights and self-regulating openings are a promising low-tech step towards achieving natural ventilation. Some window frames create a link between indoor and outdoor space via a series of small compartments that facilitate a regulable form of basic ventilation (ill. 9). Concealed sound-absorption surfaces prevent the ingress of condensation and also filter the air intake. With this system, external air enters through the lower part of the frame and flows into the internal space via the upper part. One condition for the proper functioning of this system is an adequate pressure difference created by the action of the wind, and an aerodynamically effective opening area. In other words, all the windows should be fitted with this system. It helps to prevent unpleasant draughts, and the windows can be easily cleaned. As well as low costs, a further advantage of this system is that the operation of the ventilation lies entirely in the hands of the users.
Self-regulating openings in windows and in the facade can be used to support manual controls and provide a low-tech form of natural ventilation. They are operated via sensors that increase or decrease the air-intake area according to the humidity in the rooms (ill. Self-regulating openings are economical and virtually maintenance-free. They, too, depend on adequate pressure differences between the inside and outside air for their efficient functioning.
If conservation requirements or the existing construction do not permit a change to the appearance of a building externally, it is possible to modify the inside of the facade. This will cause the dew point to shift inwards, but the use of a moisture-adaptive polyamide membrane as a vapour barrier can prevent water vapour diffusing into the insulation and air flowing through the joints. The University of Technology in Dresden, in collaboration with the Fraunhofer Institute for Building Physics, has also developed an innovative system to cope with condensation. It consists of a layer of fibrous calcium-silicate insulation slabs applied internally. Condensation is absorbed by capillary action and stored for a short period, before being released again when the moisture loading abates. No vapour barrier is required for this form of construction.
The architects Abelmann and Vielain are pursuing an original design approach in their rehabilitation of the Institute for Social Sciences, built in 1904 as part of the Humboldt University, Berlin (ill. 8). In compliance with the requirements of the conservation authorities, the arcading to the ground floor was reinstated. The inner facade layer, however, has its own independent geometry and accommodates a number of service installations.
With their conversion of a former industrial hall, erected in Munich in 1937, into a building with offices and studios for the Siemens design department, the architects Thomas Herzog and José-Luis Moro developed a new type of facade. In view of the limited lease for the building and the responsibility of the tenant for the entire construction costs, the budget was necessarily tight. The building was single glazed and had inadequate thermal insulation. An optimization of the building physics was necessary that would not affect the outer appearance of the hall. The architects designed an internal cocoon in the form of a biaxially tensioned membrane that is completely separate from the outer facade (ill. 11). A roof light ensures adequate daylighting in the offices. The membrane is stretched from a height of 2.50 m up to the ridge. Below the line of the membrane, the wood-and-glass facade provides natural ventilation and affords visual links with the outside world. There is a closed buffer zone between the existing building envelope and the two-layer thermally insulating Teflon membrane (U-value: 2.5–3.0 W/m2K). On hot summer days when condensation occurs, this space is ventilated via an extract fan in the roof. When temperatures are extremely low, the buffer space has to be heated by ceiling-level air-heating units. A supporting mechanical ventilation system was also installed in the double floor. The ventilation systems for the buffer space and for the working areas below are independent of each other. The sound-absorbing effect of the membrane creates pleasant acoustic conditions. The construction costs for membrane systems of this kind will have to be reduced, however, to make them economically viable for wider application in rehabilitation work.
With the design for their house in Bergün, the Swiss architects Daniele Marques and Bruno Zurkirchen sought a separation of inner and outer structures that went even further (ill. 17). An old stable building was converted into two dwellings. Initially, the solid rubble stone walls, the visible timber structure and the saddleback roof were secured and restored. The architects then inserted an independent, timber volume within the old building. The lines of the old and new structures, set at a slight angle to each other, the projecting form of the timber container and the arrangement of the new window openings establishes a subtle interplay between old and new.
A further revolutionary potential for facade construction lies in the research work being carried out in other spheres of technology. In the distant future, so-called “smart materials” will be available that have been used hitherto only in areas such as aircraft construction or robotics. These intelligent materials will be able to respond to changing conditions, such as temperature, pressure or electrical fields, by varying their own mechanical properties: form, stiffness, position or stress behaviour. Technologies adopted from robotics, for example, include alloys with a memory for form, and piezoelectric materials that can be made to expand or contract by applying an electrical charge. In trials, the latter are being used to absorb sound and other waves.
The new materials, or “actors”, are also being combined with sensors and glass-fibre data carriers or other piezoelectric elements for processing signals and implementing controls. These systems function like a network of artificial nerves that can be programmed (ill. 13). From these developments, one may expect a tremendous saving of materials, and possibly responsive building envelopes that can gather a store of experience on which to act.
In the realm of biotechnology, too, new materials with innovative properties are being developed. The biomolecular machines that are being created from elastic polymers will be capable of transforming one form of energy into another: light, heat, pressure, movement, chemical and electrical energy.
In the refurbishment of facades, new technologies are increasingly becoming available alongside the conventional systems with which one is familiar. The new technologies include double-skin forms of construction, multilayer membranes and vacuum panels. By carefully considering at which level an intervention should be made in the existing facade substance, even listed buildings can be made to comply with modern technical standards. All this calls for a high level of knowledge on the part of architects in the fields of material technology, building physics and energy use.
