WEB EXCLUSIVE: The burden of weight

Glass is an ideal material for application in building shells. The transparent material can be used in a variety of applications and – irrespective of the type of finishing – fulfills individual functions. Modern insulation glass offers reliable heat insulation and solar protection, prevents high noise pollution and also, if required, corresponds to the highest safety levels. In addition, individual design highlights can also be achieved through the use of glass. The elementary functions of glass products in the building shell also include heat insulation. In view of the increasingly tighter legal requirements placed on architectural heat insulation and the rising energy prices, in recent decades the glass industry has been continually further developing its products and has achieved considerable improvements in efficiency.

Increasing requirements

The limits of physics can, however, not be overcome even with the use of perfected glass formats and highly functional coatings. In the case of double (insulation) glazing, which has been used over decades, the limit has already been reached with a heat transfer co-efficient (Ug-value) of 1.0 W/m2K. To satisfy the current heat insulation requirements by law, this value is absolutely sufficient, but not for future requirements. According to the European Union Energy Performance of Buildings Directive (EPBD), which came into force in 2010, as early as January 2021, only “nearly zero-energy-buildings” are to be erected in the private construction sector, in other words buildings, which require almost no external energy supply. For new buildings, which are used by the authorities as the owners on an owner-occupier basis, this requirement will already apply two years earlier. As poorly insulated existing buildings account for a large part of high building energy consumption, here too the minimum requirements for heat insulation applying to new buildings have to be satisfied for larger restoration projects and new extensions.

Dramatic increase in the use of triple insulation glazing

In order to fulfill the expected high requirements placed on energy efficiency in buildings, insulation glazing will also have to achieve even better results in future. The glass industry is therefore increasingly relying on triple insulation glazing. With U values of up to 0.5 W/m2K, in the past this functional glazing was previously mostly fitted in passive housing. In the last five years sales of high-insulation glazing have, however, risen dramatically, because when it comes to glazing, an increasing number of building owners are relying on high energy efficiency – even if their buildings have not reached the level of passive housing.

According to the German Flat Glass Manufacturers’ Association (Bundesverband Flachglas e.V.), in the period from 2008 to 2011 alone, the share accounted for by triple insulation glazing of total glazing sales in Germany rose by around 10 per cent to over 50 per cent. For the current year the association already expects a share of around 60 per cent. And the upward trend will continue further. “We are convinced that the market share will grow to over 90 per cent. This trend is being accelerated by the forthcoming Energy Savings Ordinance (Energieeinsparverordnung – EnEV), which will probably prescribe the use of triple insulation glazing”, explains Dipl.-Oec. Jochen Grönegräs, Executive Director of the German Flat Glass Manufacturers’ Association (Bundesverband Flachglas), and Managing Director of the Multi-Pane Insulating Glass Quality Association (Gütegemeinschaft Mehrscheiben-Isolierglas).

Triple glazing in the Scandinavian countries Sweden and Finland along with Austria and Switzerland has a very high market share and similar trend perspectives as those in Germany. In view of the current trend, the specialist world is in agreement: at least in central and northern Europe the triple structure of insulation glazing is set to become a successive standard.

In parallel with the increase in energy efficiency, in the residential as well as commercial construction sector, the trend towards ever larger glass units is continuing. Architects and building owners want to have an open room ambience for their buildings with the maximum amount of daylight incidence and highest degree of external views. In winter the glazing should also ensure solar energy generation.

Higher glass pane weight

Both the current trends basically present no problem for the insulation glazing manufacturers. The know-how and technology involved in the manufacture of corresponding products are available. The problematic nature of climatic burdens (suction and pressure effect on glass panes and the edge seal), which is increasingly a feature of triple insulation glazing due to the larger inter-pane volume, can be overcome. What is problematic, however, is the increasing weight of the glass panes. By comparison: a glass pane measuring one square meter, designed as double insulation glazing with 2 x 4 mm and 16 mm inter-pane gap, weighs 20 kilograms. The same format as triple insulation glazing in the 4/12/4/12/4 format already weighs 30 kilos. The increase in weight by 50 per cent has far-reaching consequences. The insulation glazing manufacturers must gear their in-house processes to the heavy glass panes, while transportation becomes more expensive because the maximum loading capacity for trucks is already reached with fewer units and the fittings manufacturers have to deliver extremely durable solutions. In the window sector this presents a special challenge, because in this area the aim is to develop high-load bearing fittings which can reliably hold the heavy elements in place over decades, whilst at the same time conveying a filigree impression. Even at this stage, fittings systems are already reaching their limits in this balancing act between the requirements. On top of this the frame profiles for facades and windows also have to be adjusted to take the high weights.

The installation and fitting teams are particularly affected by the increased weight of the construction elements. For them the trend towards triple insulation glazing means a considerably increased burden. “The weight is enormous. Particularly in old buildings when no technical aids can be used, the burden is striking and personnel are clearly called upon much more than before”, reports Martin Gutmann, Master of the Federal Glazers’ Guild, who goes on to say: “If you are unlucky, you also have to do with triple sound-proof or burglary-proof glazing, then the weight of the glass panes is even higher.” The situation is also made more difficult by the increased weight of the thermally optimized frame profiles.

Thin glass as a solution concept

In view of this extensive problem area, the glass industry, insulation glazing manufacturers and research institutes are working intensively on solutions. Their aim is to reduce glass pane weight without cutting down on its energetic functionality. The development of light, high-insulation vacuum glass, which has been on-going for years now, has still not really moved forward sufficiently for it to be used in classic window and facade construction. However, use of the very thin glass panes is already practical in flat roof-top windows in standard sizes. A further possible solution is light, transparent synthetic film and plates, which are aimed at replacing the central pane in insulation glazing. Whether this technology will assert itself on a broad front depends on the practicality and durability of the products as well as on their acceptance by the end- customers.

The application of thin glass appears to be the most potentially successful. As early as 2004, architect Prof. Stefan Behling, who for some years now has been presenting the latest trends and developments from the glass industry in the “glass tec
hnology live” special show as part of the leading international glass trade fair glasstec, declared: In the flat-screen sector thin glass is becoming part of a revolution. At some stage perhaps whole walls, ceilings and floors will be able to change.” In the area of entertainment and communication electronics, his forecast relating to modern smartphones and also products such as Apple‘s iPad, which are almost exclusively operated using thin glass touch-screens, has long since become reality. In addition, the current development tendencies on the insulation glass market confirm that Stefan Behling is right with his expectation for the architecture sector. Today, windows incorporating triple insulation glazing are already available on the market, and are no longer produced using triple 4 mm, but triple 3 mm-thick heat-treated glass panes. This means a weight reduction of one quarter. And even thinner glass is possible. In the insulation glass sector, industry tests are already underway using hardened glass in the 3/2/3 mm format. Machine manufacturer Lisec recently received the 2012 Austrian State Prize in the “Research and Innovation” category in recognition of its special pre-stress technology. It enables the manufacture of flexible and robust glass in a thickness of only 2 mm without optical distortions. According to the company, the light thin glass is outstandingly suited for use in modern architecture.

When it comes to the thin glass used in insulation glass theme, critics point the increased breakage risk of the thinner glass panes. Their objection: although the glass is lighter through the reduced thickness of the individual planes, at the same time edge breakage risk is increased. The supporters of thin glass in contrast point to the higher durability of the thermally hardened glass.

Joint research project

In order to explore the possibilities of weight reduction in multi-pane insulation glazing on a well-founded, secure basis, the renowned Rosenheim Institute of Window Technology (Institut für Fenstertechnik – ift Rosenheim), in cooperation with the German Flat Glass Manufacturers’ Association (Bundesverband Flachglas), has launched the “Energy-efficient multi-pane insulation glass – Investigations into technical measures aimed at the reduction of glass pane weight” project. The aim of the project is to investigate which measures can be used to reduce the surface weight of multi-pane (insulation) glass and the effects resulting from this. Dipl.-Phys. Norbert Sack, Head of Research and Development at the ift and project director, explains in this connection: a reduction in the surface weight of triple insulation glazing is desirable and would in principle be possible through the use of thinner glass or transparent plastics. Thinner glass could be used in all three levels, i.e. on the outside, the room side as well as the central pane of triple insulation glazing.” He said that within the framework of the project, however, no general investigation of all the principle factors was possible. On the contrary, decision-making bases for an assessment and future implementation should be developed. BF Executive Director Jochen Grönegräs adds: “We have included an assessment programme especially for 3 x 3 mm format insulation glass in the project in order to increase awareness for this theme. If the calculation bases are taken into account, it is definitely possible to manufacture insulation glass in the 3 x 3 format.”

In view of the relevant heat insulation (Ug-value), total energy transmission (g-value) and translucence (tV) values, insulation glass using thin glass comes closest to matching the values of conventional triple insulation glass. And in the area of sound-proofing too, the integration of sound-proof film or varying glass thicknesses ensures a high level of protection.

Even at this stage, on the Internet you can find references to quadruple insulation glazing, which with 3 mm-thick glass, is designed to deliver an Ug-value of 0.3 W/m2K. A questionable development, because this format once again brings us back to the starting problem. The quadruple pane is just as heavy as today’s conventional triple glass with standard format.

Thin glass in photovoltaics

In the solar energy sector too pre-stressed thin glass is already being used to reduce the weight of glass-glass modules. According to the manufacturers, the life cycle of these modules is clearly higher than those modules based on film laminates. In addition, due to the higher mechanical rigidness, we can forego the enclosing aluminium frame. As a result, the thin glass modules are also suitable for the increasingly significant building-integrated installation of photovoltaic elements.

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