LINKING TRADITIONAL COMPETENCE TO CONTEMPORARY KNOWLEDGE FOR THE CONSERVATION OF HISTORIC WOODEN WINDOWS

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1 LINKING TRADITIONAL COMPETENCE TO CONTEMPORARY KNOWLEDGE FOR THE CONSERVATION OF HISTORIC WOODEN WINDOWS Susanne Gampfer 1 ABSTRACT: In order to save historic building parts from demolition, reliable strategies for a substantial improvement of their physical properties have to be found. Using the example of a listed monastery in Bavaria, this paper deals with strategies to preserve, repair and upgrade historic windows. An outline of the techniques available for repair as well as an overview over different methods of improving the physical properties of windows is given, evaluating the advantages and disadvantages of each given example. KEYWORDS: windows, conservation, historic buildings, retrofitting 1 INTRODUCTION 1 In many cases, old buildings cannot be sustained by the individual or community who owns them without converting them to a contemporary use. Only rarely will the historical value of buildings justify their maintenance for conservational purposes only. When historic buildings are put to a new use, in many cases windows are among the first parts to be replaced. In order to save the diminishing stock of traditional windows from demolition, it is necessary to find competitive solutions for their repair and reliable strategies for a substantial improvement of their physical properties. of the outer appearance prevailed, the alignment of the rooms and passages relative to the light from windows nevertheless gained importance as windows became larger and allowed more interconnection between indoor and outdoor spaces. The importance of windows for the holistic appearance of a building can be shown best where original building materials and genuine manufacturing techniques have been preserved (Figure 1). Reconstruction or even the substitution of original parts rarely lead to satisfactory solutions. 2 WOODEN WINDOWS IN HISTORIC BUILDINGS 2.1 INFLUENCE ON THE APPEARANCE Remaining contemporary wooden windows constitute an essential part of the integrity and appearance of historic buildings. While in some cases windows are protected by laws and regulations for the conservation of cultural heritage, even windows of listed buildings are often in danger of replacement or disfiguring alterations for economic reasons. More than other building parts, windows influence the outer appearance of the façade: the number, shape and proportion of openings are a main characteristic of architectural style. The relation and alignment of windows relative to the interior of a house is also subject to change throughout architectural history: while in most cases the importance 1 Susanne Gampfer, Faculty of Architecture, Department of Timber Construction, Technische Universität München, Arcisstr.21, München, Germany. s.gampfer@lrz.tum.de Figure 1: original late Baroque window with brick vault visible in surrounding stone wall

2 2.2 SIGNIFICANCE OF WINDOWS AS A TECH- NICAL BUILDING COMPONENT Windows are highly sophisticated components of a building. In order to make rooms habitable and comfortable, windows have to fulfil the essential functions of letting in light and air, while at the same allowing outside views and protecting the interior from the weather. By no means have the openings in buildings been able to satisfy these needs at all times. Windows with moveable casements and clear glazing mark a high level of technical development in the history of craftsmanship. From this point of view, the conservation not only of rare or outstanding examples but also of the great diversity of historic window constructions should be encouraged. 3 STRATEGIES FOR CONSERVATION 3.1 REPAIR OF WOODEN WINDOWS Windows are complicated building components, always made up of a number of different materials. While only windows with wooden frames will be considered here, other materials like glass, metal fittings, putty, as well as oils and paints have to be looked at. All components interact in a way that may once have been well matched and expertly made, or that may have born the risk of damage and decay from the beginning. It is part of a careful investigation prior to every restoration, to find out about the original condition Repair and substitution of wood Historically, the way wooden windows were constructed took into account the need for repair of broken glass and the replacement of damaged parts of the frame. Later these reversible connections were given up in favour of glues for joints and putty for glazing. Nevertheless, casement frames of windows from the 19 th century or earlier can usually be taken apart without destroying the components. Repair of the wooden frames and casements can be achieved by substituting damaged or decayed parts. This technique is well documented and can also be used for the in-situ repair of window frames, minimising the damage to adjoining surfaces Glazing The original glazing is an integral part of the appearance of historic windows. It has to be considered, however, that even in earlier times, glazing may have been changed from an original, subdivided lead glazing to a later substitution with larger panes, making it difficult to determine, what the original situation looked like. Only after the invention of modern float glass in the 20 th century was there a clear and undisturbed view through window glass. All former sorts of glass clearly showed a surface structure, caused by the respective manufacturing process. Up to the recent past, there has always been a difference between a view through the glazing of the closed window and the unobstructed view through the window opening (Figure 2). Therefore it is of high interest to try and safe the original glazing, even though its preservation is among the greatest challenges in window restoration. Figure 2: structure of historic, hand made glass 3.2 RETROFITTING AND IMPROVEMENT OF PHYSICAL QUALITIES Historic windows in most cases do not fulfil modern requirements as far as thermal insulation and noise protection are concerned, whereof the improvement of the thermal qualities of windows are by the far the more urgent issue. Aside for a general need to improve airtightness and overall operability, there are basically three strategies for the improvement of the physical qualities of single glazed windows: an improvement of the thermal quality by replacement of the original glazing, the addition of a new interior layer of glazing by installing supplementary casements or the addition of additional exterior casements Additional casement on the inside Adding casements on the inside of existing single glazed windows offers the possibility of introducing an airtight seal on the warm side of the window, reducing heat loss and the danger of condensation on the exterior glazing. In most cases it will be necessary to add an extra profile to the window frame on the inside, which will result in some damage to the reveal and a significant change of the internal appearance of the window (Figure 3). If no alteration of the original, now exterior, casements is possible, the new inner frame may reduce their opening angle. A position of the new inner window close to the inner wall surface reduces cold air within the window recess as shown in Figure Casement added on the outside In certain cases it may be advisable to add a casement as a new outer layer of an existing single glazed window construction. Reasons for this approach may be a priority given to the inner appearance of the window over the outside, avoiding damage of valuable inner wall surfaces, or a wish to protect unusual or valuable windows from the weather after their restoration. With this strategy, the possibility to improve the physical qualities of the original windows are limited if they are to be left unchanged. There are basically three options for the addition of an external window: Single glazed casements without a seal may be added, on the outside, to the unchanged original windows (Figure 5). These must have an air gap of sufficient crosssectional- area to ensure ventilation of the space between

3 the two layers of glazing. This construction will only lead to a minor improvement of the thermal qualities of the window. However, the air filled space between the panes will act as a thermal insulation and the original window will be protected from direct wind. The system results in minimal changes of the original physical system of the room (Figure 6). In addition to the construction described above, a sealing may be integrated in the frames of the original (inner) window. The groove required will lead to loss of material. While the thermal qualities of the window can be further improved, the construction will change the original air exchange of the room and has to be matched with other measures, like additional insulation of surrounding surfaces. The third alternative is the extension of the original situation by adding a new window with insulation glass on the outside. With this measure, an adequate thermal standard can be reached. However, the outer appearance of the building will be altered considerably. In most cases, the original window loses all its functions and will only be kept as an exhibit. The natural ventilation of the room will be altered considerably. Additional insulation measures and sufficient natural ventilation have to be provided for in order to avoid mould growth. This alternative may be the only way to ensure continuous use of a historic building if room heating is a prerequisite for the new dedication. Figure 5: window in a house from the mid-19th century with outside casements added Figure 6: thermal image of window shown in Figure 5 Figure 3: window in a house from the mid-19th century with casements added inside 3.3 SUBSTITUTION OR RECONSTRUCTION OF ORIGINAL WINDOWS There are several reasons for a replacement of original windows in historic buildings. Even after a careful restoration, some windows may not reach the standard required for the new use of the building, while in other cases the original substance may be damaged to a state beyond the possibility of repair. It seems advisable to state criteria for or against the preservation of each of the windows in question, as well as for other building components related to the windows and affected by the refurbishment. These criteria can also be a helpful instrument for the classification of the historic, architectural or technical quality and value of listed buildings and their components. Reconstruction should be limited to those cases, where reliable information about the original shape and construction of the windows of a building is available. Figure 4: thermal image of window shown in Figure 3

4 4 THE WINDOWS OF THE MONASTERY IN RAITENHASLACH 4.1 SITUATION AND BACKGROUND The Cistercian monastery in Raitenhaslach was first founded in The presently remaining appearance of the complex was shaped during a time of excessive building activity around the middle of the 18 th century. A large part of the baroque monastery buildings were demolished around 1803 after the secularisation in Bavaria, but several buildings from the first half of the 18 th century have been preserved almost unchanged form the time of their creation (Figure 7). Figure 7: view of the main façade of the central monastery building from the middle of the 18 th century After almost 200 years of private ownership, the surviving monastery buildings where bought by the local community of Burghausen. Many parts of the building, like the roof construction, stucco and representative wall paintings in the rooms, but also most of the original windows, are an example of Baroque building techniques in a rural area of Southern Germany. In order to finance the conservation of the monastery buildings, it will be necessary to convert them to a modern use, which in this case is a study and residence centre for a University. Among the most difficult problems of such a conversion, next to the implementation of modern building services will be to repair and amend the existing windows to a standard suitable for the future use of the rooms. 4.2 AN INVENTORY OF WINDOWS AND TYPICAL DAMAGES Almost all the windows found in the monastery buildings date back to the time of construction around While some alterations to the windows on the first floor could be found, most of the other windows were preserved including the original glazing. The unusual number and variety of surviving window types in the main building is a consequence of long neglect, which on the other hand led to an extreme dilapidation and severe damage of many of the most valuable windows. An inventory of all the windows in the main building brought forward four main types of windows with casements. A modification of window type 4 classified as a fifth variety, while all other varieties were seen as subtypes to the same construction. In addition, there is a number of windows of different sizes and shapes made for special architectural situations within the building. Some of the windows belonging to the same type showed slight differences of construction, which could be correlated with different stages of the building progress verified through dendrochronological probing of the roof and floor beams. An overall tendency to use extremely thin profiles for frames and glazing bars was found. This led to a deformation of some of the larger windows, resulting in an additional challenge for the conservation of these parts. Detailed drawings of the profiles of outer frames, casement frames and glazing bars also showed that the same profiles were used throughout the building, with variations in partitioning, glazing and coating. All windows described in the following sub-chapters have single glazing and only one layer of casements Window type 1 Window with transom and mullion, with four casements: Most of the original leaded glazing has survived. The frames and casements are made from oak with fine wrought iron cross bars and fittings. The wood was either untreated or oiled. All of these windows are along the hallway on the first and second floor, facing towards the back of the building where the courtyards of the monastery were, not toward the representative side of the building. These windows show best the different construction phases of the building, as the wood joints of the frames were altered slightly with each new section of the wall completed. All profiles are extremely thin, the windows fragile and difficult to operate. No putty was use for glazing, the glass is held in a glazing groove. Lower corners and bottom profiles of frames and casements are severely damaged from constant exposure to the weather and have to be replaced. Figure 8: window type 1, photograph and drawing Window type 2 Window with transom and mullion, with four casements and clear glazing: The same profiles as in window 1 were used for frames and casements. These windows are on the second floor of the building in the main façade and consequently treated with white paint. In most cases the glazing is fixed with putty although the profiles are

5 not wide enough for this construction: the glazing appears to have been in a groove originally. The white paint may also be a later addition, used in order to match these windows with those on the first floor (see 4.2.3) after a redecoration of the facade. Some of the windows also show traces of alterations to the frame Window type 4 and 5 Along the sides of the large ceremonial hall on the first floor, high windows of a daringly delicate transparency have survived for almost two centuries. The window opening is divided into four parts with a mullion and transom. The two casements on the bottom make up two thirds of the total height, the top casement one third. The two casements on top can be opened with the help of a complicated device. Figure 9: window type 2, photograph and drawing The windows are in the West façade and severely weathered on the outside. The Window sill of natural stone which was possibly an addition to the original detail, was set in almost flush with the upper edge of the bottom frame. Therefore, water collects in the gap between stone and wood. An additional frame for large shutters has been added later, which retains snow and wind blown dust on the window sill, worsening the moisture damage to the lower parts of the window Window type 3 Figure 10: window type 3, photograph and drawing Large windows in the stately rooms on the first floor within the main façade and facing towards the West: The windows have the same size and construction as window type 2, but the transom and mullion have been removed. The original partitioning into four casements was thus changed to two large casements subdivided into three sections with two horizontal glazing bars. Large domed panes of clear glass were used. The expressive appearance of these windows dominates the main façade of the building. As the casements are a later addition, they are in a much better state than most other windows parts in the building. However, the original, refitted frames show the same damages as window 2, due to neglect and an unsuitable detail of the windowsill and outer shutter frame. Figure 11: window type 5 with the two lower casements opened. In the end wall of the room, an especially interesting variety of these windows was found and classified as window type 5. It proved to have been put in after a major alteration to this part of the building: Where originally the door of an external staircase leading to the cloister had been, a window was put in after the staircase had been removed. This is the only specimen remaining of a possible number of these windows, which may have been in the staircase or another part of the building no longer in existence. This window does not have a transom, but four casements with a central mullion. The subdivision of window 4 is reversed: The lower casements make up a third of the height and can be opened without much effort, while the top casements with two thirds of the total height would usually have been kept closed and form the rebate for the lower casements. The profiles of the casements and the glazing bars of the large windows of the hall have proved too weak for the Figure 12: window type 5, damage of frame

6 wind load over the years. The windows are therefore severely deformed, some of the wooden glazing bars failed due to overstress. Beside these structural damages, the windows are severely damaged by weathering and rot. Window sills made of natural stone, which are probably a later addition, were put in flush with the bottom parts of the frame. Rain and melt water caught in the joint between wood and stone has caused an almost complete deterioration of the frames (Figure 12). 4.3 STRATEGIES FOR CONVERSION As has been shown in the previous chapter, most of the windows are already severely damaged not only because of their old age and lack of repair and maintenance, but also by faulty details caused by parts added for decorative reasons. At the same time the whole building is a precious piece of cultural heritage not only for the region. Both its outside appearance and the interior surfaces, especially decoration of the rooms on the first floor and the ceremonial hall, deserve to be protected. Any additions to the windows necessary to improve the quality of the building skin will require intrusions and damage to the original substance of the building Repair of damaged windows It will need an especially sensitive restoration concept in order to preserve as much of the original material, construction and even traces of aging found in the many windows of the building. A large number of the windows are damaged to an extend where even a technically faultless restoration with traditional means of joinery would lead to a replacement of most of the original material. For some of the windows, especially types 4 and 5 in the large hall, this would mean a loss of much of their history and originality (Figure 13). Figure 13: window type 5, historic repair of glazing bar Improvement of thermal properties Fortunately, the issue of noise protection can be neglected for the future use of the rooms because of the quiet and solitary situation. As far as the improvement of thermal qualities is concerned, the necessary measures depend on the standard demanded by the future user. As will be explained in the subsequent chapter, in this case the measures will be determined both by the technical possibilities and by the constrictions of the energetic standard accepted by the user. In any case, the condition of all of the original windows, as well as the fact that they were built very fragile from the beginning, will make it necessary to protect them from further exposure to the weather if they are to be saved. The necessary interference with adjacent building parts and surfaces must be kept to a minimum. The only practical solution will be to add outside casements, allowing for the limitations discussed in chapter However, outside casements are not acceptable as a permanent alteration to window type 3 on the first floor, as they would change the building s appearance decisively. A reversible construction will have to be found here Influence of parameters set by the user The quality of the restoration and refurbishment of the valuable windows of the monastery will depend to a great extend on the restrictions the future user will accept as far as comfort and the characteristics of use are concerned. Lower than usual room temperatures in some of the rooms, as well as a limitation of occupancy to the summer months will reduce the necessity of alterations to the windows and wall surfaces substantially. 5 CONCLUSIONS Conservation of a historic building will in most cases depend on whether or not it can be adjusted to a contemporary use. Climate change and the related discussion about energy conservation in buildings have brought new challenges for the refurbishment of such buildings, especially of their outer skin, of which the windows are often the most vulnerable part. Even the highest technical effort will not suffice to achieve a modern energetic standard with very old and delicate window constructions. In these cases, the careful adjustment of the prerequisites for a future use can relieve the pressure on the restoration and refurbishment. On the whole, a satisfactory result of the rehabilitation of historic windows can only be achieved by linking the traditional restoration techniques to a highly sophisticated knowledge of the building skin. ACKNOWLEDGEMENT The investigations at the monastery in Raitenhaslach were made in connection with a joint project at the Technische Universität München. The inventory and description of the windows were compiled in cooperation with the Institute of Building History, Building Archaeology and Heritage Conservation, Prof. Dr.-Ing. M. Schuller and Dr. A. Kutnyi. REFERENCES [1] Gerner, M., Gärtner, D.: Historische Fenster: Entwicklung Technik Denkmalpflege. Deutsche Verlags-Anstalt Stuttgart, JuliusHoffmannVerlag [2] Nickl, P., editor: Kleine Fenster. Springer-Verlag München [3] Tutton, M., Hirst, E., editors: Windows, history, repair and conservation, Donhead Publishing Ltd., Shaftesbury 2007