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Fig. 37: The newly developed frame is suitable for both vacuum glazing and conventional triple glazing.
© SKZ

Fig. 38: Overview of thermal characteristic values for frames and windows
© BINE Informationsdienst
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A suitable frame

Parallel to vacuum glazing, suitable frame structures are also being developed: an optimal frame for vacuum glazing should cover the thermally bridging edge seal, be slim and have good thermal insulation properties. With profiles made entirely of polyurethane, the researchers are relying on a completely new technology. This innovative system is also suited for conventional triple glazing.

The thermal insulation values of conventional window frames are considerably worse than those achieved with the best glazing systems, whereby even passive house frames rarely achieve Uf values less than 0.7 W/(m²K). The downside here is that these are inelegant systems with larger profiles and fittings and with increased installation depths. In developing the frame, the researchers adopted new approaches in regard to the materials and manufacture. Instead of pressing the profiles from aluminium or extruding them from PVC as has been common up to now – methods that provide little scope for optimisation – the core of the new frame ismade by filling foam into a mould and then covering this foam core with a plastic layer.

In detail

The new frame entirely consists of just one material: polyurethane (PU), whereby a thin layer of form-stable and weatherproof solid polyurethane material encases the insulating core made of foamed PU. Whereas the core ensures excellent thermal properties, the casing provides good mechanical and structural stability. Moreover, it provides freedom of design: there are virtually no limitations to how the surfaces can be glued and painted. Since the frame is manufactured from just one single material, there are not any problems with its later disposal; besides, PU can be recycled. This frame achieves the necessary stability without steel or aluminium reinforcement. This also results in a lower weight and lower thermal insulation values. With tripleglazed insulated glass in the standard size of 1.23 m x 1.48 m, such a window achieves a Uw value of 0.8 W/(m²K), while VIG even achieves 0.7 W/(m²K). The new profile is therefore comparable to passive house window frames, but weighs less and is considerably thinner with a frame depth of just 90 mm. The newly developed frame also provides a solution for the thermal weak point of vacuuminsulated glass – namely the metallic edge seal – by providing a large insertion depth. The profile can be fitted with glass ranging from 9 to 50 mm in depth and is suitable for both new-build and refurbishment schemes. As regards the installation of these windows, there are no differences compared with conventional frame systems. For example, the distances between the screw connections are similar to those for conventional windows.

For facade systems that are up to several storeys in height, such a pure plastic profile cannot be used since it does not provide any load-bearing function. For this reason a thermally optimised mullion-and-transom structural system has been developed based on aluminium supports.

New approaches in manufacturing

The new system has consequences for the production and further processing. Specialmachines are necessary for foaming, casting and coating. Window manufacturers buy the material in six-metre lengths and then cut these down according to their needs. In contrast to conventional frames, profiles made entirely from polyurethane cannot be welded; instead the corners, frames and glazing are bonded together with a newly developed adhesive. The manufacture of the new profile can be integrated into an existing production process, providing an additional profile option. Pilot production and initial discussions with system manufacturers are currently underway. The production technology for manufacturing polyurethane profiles will already be available in 2011.

Highly insulating frames

The highly insulating frames currently available have heat transfer coefficients of between 1.1 and 0.7 W/(m²K). There are also cases where slightly lower values are achieved. Uw values of between 0.7 and 0.8 W/(m²K) are possible using improved spacers. However, this also leads to increased installation depths. Values of around 120 to 130 mm apply for passive house window frames.

The following categories of frame can be identified:

  • Extruder frame profiles made of PVC with interior steel reinforcement and several air chambers. Additional PU foam elements can further improve the thermal insulation performance of the frame.
  • Wooden frame with core insulation or as a sandwich structure with an insulating middle or exterior layer. The insulating material is either PU integral foam, PU recycled material (Purenit), Styrodur or soft-fibre insulating material. The arrangement of the insulating layers differs among the various manufacturers. There are also wood-aluminium windows available with a PU insulation core.
  • Aluminium frames where the frame shell is filled with a PU insulation core.
  • Other developments: foam-filled plastic profiles where the steel reinforcement is replaced by profiles strengthened with glass fibre; wooden frames which are combined with externally fitted profiles that are strengthened with wood fibres.

 

Vacuum Insulation in the Construction Industry (ViBau) research area

The development of highly efficient thermal insulation elements for building envelopes has been promoted for many years with funding from the German Federal Ministry of Economics and Technology as part of ViBau, a research area forming part of the energy-optimised construction (EnOB) research initiative. Whereas the focus was initially on developing and conducting basic tests on the evacuated elements, their feasibility in practice was later tested and demonstrated. Parallel to this, individual components were optimised and systems for practice-oriented system integration were developed and tested. In addition, specialmethods for the quality control and assurance were also developed. Now that the first VIP products have been granted national technical approvals and have become commercially available, their functionality in general construction practice is being monitored as part of scientific accompanying research. The intention now is to foster greater publicawareness of the potential and special features offered by this innovative thermal insulation technology and to anchor this knowledge in the training and continuing education of specialist designers and practitioners. With the vacuum-insulated glazing that does not require any considerable rethinking among the designers, the next step is now being taken to progress from the prototypes to mass production. It is not expected that this will lead to any special issues in terms of the planning and application.

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