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50 years of quality and safety Part 1 The technical and social changes in windows and facades
Data dodania: 13.01.17

50 years are longer than an entire working life. In 50 years not only vehicles, entertainment technology and communications have changed, but buildings as well. Windows, façades and glass in particular have developed into high-tech products. The Institut für Fenstertechnik (also known as the ift Rosenheim), established in 1966, has made a crucial contribution to this development through its many research projects in cooperation with manufacturers and government funding agencies. Not only have materials and designs been invented or optimised, but often new measurement and test methods developed which were not previously available. Our focus has always been and remains the improvement in quality, safety/security and serviceability plus a sense of aesthetics and practicality.



Experience shows that megatrends such as safety and energy efficiency change very little and are still applicable today. Legal provisions such as the energy conservation regulations can significantly promote and accelerate this development. In the case of the windows industry, this has led to German products being global technical leaders. Now we need to look ahead and identify and influence future trends in good time. These include further digitalisation and automation, new materials based on nanotechnology and biotechnology, the Internet of things, and sustainability in relation to health and scarce resources.


This publication aims to present the basic technical and structural relationships which can be applied to the designs of today and tomorrow – we hope you find it interesting!



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Figure 1. Windows from the past – present – future (2nd Image right FH Dortmund, Image right LC window, photo: Merck, photographer: Eva Speith)





Many developments have occurred or been initiated over these 50 years which have influenced and advanced our window industry. Experience shows that megatrends such as safety and energy efficiency change very little and are still applicable today. But new trends in society and developments in technology also emerge. These must be recognised and their opportunities grasped. Join us in reviewing the topics of the past and the challenges for the future.



A look back to the era of the Institute’s founding 


The rebuilding phase in the German Federal Republic was just reaching completion when the Institut für Fenstertechnik e.V. was established in 1966. This period was essentially characterised by the meeting the quantitative demand for residential construction. 


The foundation of the Institut für Fenstertechnik e.V. (ift Rosenheim) in 1966 was therefore no coincidence. The existing window technology was characterised by traditional knowledge and experience and by a considerable emphasis on efficient mass production.



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Figure 2. The beginnings – at the entrance to the first research hut: chief building officer Erich Seifert and Josef Schmid with a group of students – a lively lecture outdoors



At the time, however, technical regulations were already in place which defined the characteristic values of different window designs. For example, DIN 4701 from January 1959 already contains tables giving functional values for calculating thermal properties, such as Table 3 „Thermal transmittance k for windows and doors” or Table 4a „Joint permeability per m joint length of windows and doors of flawless execution and normal joint dimensions”. However, what does „flawless execution” mean? These gaps were the first challenge for the ift Rosenheim. Requirements had to be specified for the expected use. Documentary evidence that the classifications were achieved had to be made possible via tables or tests using reasonable effort. Statements about the properties of series products had to be ensured using statistical quality controls.


These original tasks for the ift Rosenheim are more relevant than ever. Today we are in another growth phase in the building industry. A large quantity of housing is required and brisk construction activity leads to a high capacity utilisation of the manufacturing plants. This dynamic period is defined by debates on cost savings and less stringent requirements, plus the creation of new efficient construction methods. However, experience also shows that the upturn in demand usually leads to a downturn in component quality. Discussions on quality, damage claims etc. show up with several years’ delay. In order to avoid a rude awakening it is important, especially nowadays, to maintain a high quality level in window technology.


Further, urbanisation is progressing in many parts of the world and new concepts in urban development and therefore indirectly in building components are becoming more important. In the process, environmental questions, sustainability issues, etc. are constantly being rethought, and in future cannot be disregarded when it comes to windows and façades. The challenges will certainly not diminish.




In the beginning was a wooden window

Poor quality, especially in wooden windows, was therefore an important starting point for the ift Rosenheim. „The service life of the window as a component is generally defined by changes in architecture. Natural ageing barely plays a part.” When defining the problems in the first ift research report, service lives of up to 100 years were claimed based on experience. However, in the years following, premature ageing due to attack by wood-destroying fungi was identified to an ever increasing extent. The planned studies were tasked with finding the causesof the frequently noted moisture penetration and the resulting wood decay. The aim was to establish new processing guidelines adapted to the current manufacturing methods. This process covered the topics of

  • Determination of load-bearing cross-sections, 
  • Watertightness,
  • Specification of groups for bonding,
  • Glazing (bonding of glass to frame),
  • Thermal load due to coloured coatings.



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Figure 3. Tables for determining the stress groups for window glazing, the stress groups for watertightness, and recommended maximum casement dimensions depending on the stress group



The reasons for the high accumulations of moisture were clearly found to lie in the areas of structural and technical workmanship (entry of moisture e.g. via the glazing, the connection to the wall and via exposed adhesive bonds), and also in excessive requirements for the materials and in faults in building physics. Additional tests also pointed to the effects of the execution of the construction work, type of installation and installation direction. Further, clear links were established to the paint system used. What is interesting is the resolutely expressed wish even then: „The research institute sets itself the aim of developing a comprehensive DIN standard for windows. …“.


As practical results from the research projects, the ift Rosenheim was able to provide the industry with fundamental basic knowledge in the form of various tables and further information, even back in 1969:

  • Table for determining the necessary moments of inertia for wood,
  • Table for determining cross sections for mullions and transoms,
  • Table for determining the stress groups for watertightness,
  • Table „Recommended maximum casement dimensions for watertightness, depending on the stress group”
  • Table for determining the stress groups for window glazing,
  • Recommendations for invitations to tender for wooden windows.


Most of these documents which were useful for building practice were developed further, and count as essential requirements for the design and manufacture of serviceable windows right up to the present day. The German standards DIN 68121-1 and -2 published in 1973, were produced almost entirely from the previously described research work. Many of the figures, tables and diagrams were used exactly as they were. Further guidelines on lamination, surface coatings, glazing details, etc., paved the way for the principles of window design still applicable today and which can be applied beyond wooden window to all other frame materials and designs.


Window manufacturers were happy to use these design manuals: from that point onwards there was an improvement in manufacturing quality and the technical characteristics achieved.


It was only in the 1990s that the picture changed. In 1993 a revised version of DIN 68121 was published. Due to a combination of

  • Increasing building technology requirements for thermal insulation, airtightness, short construction times and high moisture loads during the construction stage,
  • Discontinuation of robust tropical hardwoods and replacement by less suitable native wood species,
  • Unwillingness of the owner to increase painting maintenance following this,
  • With pure wooden windows, tricky architectural designs with exposed oriels, conservatories, etc., 
  • Individual changes and further developments of the standardised details but also
  • Loss of wooden window specialists from the early days due to age 


there was an increase in damage which permanently lost wooden windows their market share (Figure 4). In addition, all wooden window designs looked practically identical: the rigid standardisation had developed into a disadvantage compared to other frame materials.



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Figure 4. Conflicts when using pure wooden windows



In 2016 tenders still state „IV 68 acc. to DIN 68121” for describing wooden windows. Of the now over 40 year-old principles, only generally applicable details such as drainage slope, edge roundings, geometry of water drips etc. are relevant for the present day.


The European classes and performance characteristics in accordance with EN 14351-1 have replaced the stress groups, type-size diagrams, etc. Nowadays DIN 68800 is also applicable to modern wooden windows and, besides design and structural protection, also demands reasonable chemical timber protection. Strong chemicals on their own have not proved effective. Particular attention must be paid to protection from blue stain and to frame corners which are particularly at risk.


The top priority over the last 20 years has been thermal optimisation of wooden windows. Parallel to this, the areas of the design particularly at risk from condensation have been adapted. The market demands modern wooden windows whose features have developed to the state-of-the-art shown in Figure 5.



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Figure 5. Current state-of-the-art for wooden windows



Development trends seen over recent years are a specified basic ventilation, minimum heat losses, thermal insulation in summer, safety and security features (burglar resistance but also safety in use) and also operational convenience. All current development trends (Figure 6) show that the flexible use of the material wood is the way ahead for wooden windows. Its durability combined with low maintenance and servicing is always one of the most important arguments for its market success.



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Figure 6. Coupled configurations in wooden windows with arrows, displaying the tendency in implementation/spread in practice over the last few years



The trend is away from pure wooden windows to wood/metal windows, new kinds of glazing with integral sashes (Figure 7) and integrated solar shading systems for coupled windows. Issues connected to condensation prevention in multi-layered configurations must also be dealt with.



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Figure 7. Concept of an integral window (highly simplified schematic without any relevant detail solutions)



After many years of decline, the wooden window has therefore claimed a strong market share: Consumers who go for an elegant appearance, natural products and (higher-priced) quality. Other materials must be careful not to walk into the same trap. Durability and suitability for use are also of the utmost importance here. The durability of coatings, materials, risk of deformation, etc. apply to all windows.



Thermal insulation as a driving force

The development of the ift Rosenheim was linked to events on the world stage. The 1970s were marked by the oil embargo by the OPEC states which, in the autumn of 1973, led to the most far-reaching oil crisis up until then. On 9 November 1973 the German Federal Government passed an energy security law which provided for emergency measures to save energy. Development of the field of thermal insulation was stepped up.


The proportion of window surface in the whole façade gradually increased. From the 1970s onwards, ratios of over 50% were not uncommon. These had a significant effect on the heat balance of the building, along with other structural causes. As heating oil had been relatively cheap up to that point, there were hardly any suggestions for energy saving worth mentioning. The sudden price rise for heating oil of 300% and more due to the oil crisis brought a drastic change to the framework conditions and forced rapid action.



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Figure 8. Hans Hartmann, long-term head of the thermal insulation team at the ift Rosenheim, at the k value test rig  developed by him (1970s)



As part of the research contract „Reducing energy consumption in residential buildings”, the ift Rosenheim was commissioned to study the section „windows” and the sub-section „walls – windows” (connection of the window to the wall). The overall project was tasked with analysing the factors which determined energy consumption in apartments. After establishing the theoretical principles, research on buildings from 1951 to 1974 was aimed at identifying measures for reducing energy consumption.


The research report published in 1974 distinguished two areas which influenced energy consumption: 

  • Factors which could be directly influenced by design and calculation;
  • Factors which could principally be influenced by workmanship/manufacturing quality.



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Figure 9. Sticker issued in 1980 by the Federal Ministry of Economic Affairs



The focus lay on comprehensive tests of the joint permeability of windows. The measured values showed large unconnected fluctuations between year of manufacture and level of joint permeability. The main causes were to be found in the design and quality of the windows tested. In addition, considerable quantities of air were entering via the wall connections. The necessity of measures which could be put in place quickly was clear, including for old buildings.


The knowledge gained from the study enabled practical improvement measures for windows and the definition of quality characteristics.


Only a short time later, in the research project „Windows for the refurbishment of old buildings”, the ift Rosenheim presented a list of measures (Figure 10) which provided assistance with deciding which measures were possible, technically correct and therefore economically appropriate.



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Figure 10. Measures for windows when refurbishing old buildings



In 1977 the first Thermal Insulation Ordinance gave a legal form to all the information on energy savings in buildings. Double glazing and a seal in the functional rebate between the casement and the frame member were required for the first time. This was the start of a rapid further development in window technology in relation to design and manufacture – the details being:

  • Rebate design with seals, design of seals/gaskets and drainage,
  • Further development of associated hardware, 
  • Use of insulating glass units,
  • Glazing systems for insulating glass units (blocking, sealing, rebate design, etc.),
  • Thermal break in metal profiles,
  • Ventilation devices for use in windows and façades etc.


PVC windows quickly became established due to efficient manufacture and good material properties. Renovating residential buildings largely with windows of the same type was very convenient with this frame material. With the increase in and complexity of design details, correct manufacturing and fabrication became an ever more important factor. A corresponding quality control became essential.



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Figure 11. Design of connection between window and wall



Water leaking through the connection into the wall was identified as an important cause of damage. As the building envelope was not yet sealed, the subject of internal sealing due to internal exposure to moisture was not relevant. Recommendations for sealing were therefore only related to external weathertightness. 


Nowadays most windows in Germany are used for refurbishing old buildings. Improvements in energy efficiency, safety/security and convenience are crucial factors in deciding to fit new windows.


There are a series of regulations for the technical implementation which have been introduced into building law. These include DIN 4108 (thermal insulation), DIN 4109 (sound insulation), the German Energy Savings Act (EnEV) and DIN 1946- 6 (ventilation). This means that almost all requirements for installation, planned ventilation concepts etc. are mandatory. In practice it is shown that these requirements for old buildings can almost never be met in full without additional construction measures. Not all of the necessary measures to avoid thermal bridges, to implement a ventilation concept etc. are frequently assigned. The renovation measure is therefore incomplete and susceptible to condensation and mould problems (for interrelationships, see Figure 12).



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Figure 12. Effect of new tight windows on otherwise unrenovated buildings



The only significant argument for window refurbishment is ostensibly energy saving. However, the actual decision in favour of refurbishment tends to be based on removing defects and deficiencies in the old windows:

  • Visually obvious defects in the windows, 
  • Usage defects,
  • Low level of safety/security and convenience,
  • Frequent maintenance, care and servicing measures. 


If the new windows do not satisfy the actual reasons for undertaking the refurbishment, then this can quickly result in complaints and dissatisfied customers. Features such as ease of use, sound insulation or similar can of course be directly perceived by the user themselves. The same applies to defects from unprofessional workmanship. On the other hand, energy saving can only be detected indirectly and after a delay. Unfortunately, many recommendations for refurbishment still focus on energy savings alone. In addition it can be established that, in practice, the actual windows often do not comply with the promised classes. Non-compliance in subdivisions and dimensions, materials and fitting technology no longer permit an assignment of the original classes e.g. in relation to burglar resistance – the entire basis of the refurbishment decision therefore hangs in the balance, with justifiably upset customers.



Prof. Ulrich Sieberath, Ingo Leuschner, Karin Lieb,
Gabriele Tengler, Jürgen Benitz-Wildenburg,
Jens Pickelmann
ift Rosenheim



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