Lightweight, energy efficient glazing units for modern architecture

Improving the energy performance of windows and façades continues to be one of the central challenges in modern architecture. As well as optimising frame sections, a major role is played by the efficiency of the thermal glass that is used for this purpose. Thermal double glazing has been proving its worth for several decades now. By applying highly efficient functional coatings and using thermally optimised warm edge spacer systems, the relevant Ug value of a glazing unit – so crucial for thermal insulation performance – has been reduced to 1.0W/m2K with argon-filled glazing. Moreover, a low-emissivity coat is added on the indoor side of a unit, so that the latest products now have values as low as 0.9W/m2K.

Thermal triple glazing is now state-of-the-art
Thermal double glazing still cannot achieve the same excellent insulating effect as thermal triple glazing, where the relevant values are now as low as 0.5W/m2K.
This type of glass has the best level of performance and is currently state-of-the-art in traditional thermal glass applications. Its excellent insulating effect has led to an increased use of triple glazing in buildings, as it can best satisfy today’s rising requirements on energy performance. Germany, in particular, has seen substantial growth in this segment. Within just four years, the market share has increased from about 10% to over 60, and the trend is rising. At the same time, there is a tendency towards bigger glazing units. This trend is supported by ongoing developments in manufacturing technology which now permit the serial manufacturing of very large, high-precision double and triple glazing.

Heavy-weight glass can cause problems

There is a snag in this trend towards large panels and triple glazing. If the glass is too heavy, glaziers can easily come to the limits of their strength. Thermal glass with a triple-glazing structure and the usual thickness of 4mm is about a third heavier than double glazing. As a result, the weight of a glass panel 1m2 in size increases from 20 to 30kg, and a panel that weighs several hundred kilograms can often only be installed with the help of technical equipment. This leads to a substantial rise in installation costs.

Moreover, heavy glass panels make it necessary to develop new handling solutions for the manufacturing process, while also driving up the cost of transport. Another area that is coming to its limits, despite intensive development, concerns the required heavy-duty hinges. For several years now, research institutes and glazing companies have been working with glass machine manufacturers on possible remedies in the form of alternative products with similar or better energetic properties, but less weight. The focus is, in particular, on thin units, transparent film and plastic panels as well as on vacuum-insulated glass for maximum insulation.

Weight reduction through thin glass
Thin glass technology is, in fact, ready for series production. As regards to the relevant values of heat insulation (Ug value), total energy transmittance (g value) and light transmission, thin thermal glass has the same values as conventional triple glazing and even exceeds those values.   

Sensible alternative but not suitable everywhere
Triple glazing with thin panels is a sensible alternative to conventional structures but not a definitive answer to all weight issues, as it is not suitable for all thermal glass applications. This applies both to ultra-large panels and for applications where the law specifies greater minimal thicknesses, e.g. for fall protection purposes.
  
Nonetheless, thermal glass with thin panels can be used in a large number of cases. As long as climatic stress has been calculated with sufficient accuracy in a given location, so that the structural stability of the glass meets the relevant requirements, then lightweight thermal panels are a very good way to complement traditional triple glazing.

Vacuum insulated glass as a complementary product
One alternative to thin glass and plastic film and panels is to use vacuum-insulated glass (VIG) which has a double-glazing structure and is barely 10mm thick and therefore very light. Both convection and thermal conductivity are virtually eliminated through the evacuation of air in the space between the two panels. This space is less than 1mm wide. In combination with low-emission coating, this ensures excellent thermal insulation. To ensure that the high atmospheric pressure of 10 tonnes per m2 does not cause the panels to collapse, barely visible spacers are placed between the two panels.
  
Industrially manufactured vacuum-insulated glass is already being made by two companies. However, due to the high thermal stress on the edge sealing systems that are used (i.e. the glass solder), these product solutions cannot be used as single panels but only as hybrid solutions in an insulated glass structure, involving a counter-panel. Moreover, both systems have visible evacuation valves.
  
Some European companies are working intensively on VIG solutions that do not need such valves and which can be used as single elements (i.e. without counter-panels), allowing variable use in conjunction with float glass, single-sheet safety glass and laminated safety glass, achieving a Ug value of 0.5W/m2K.