The combination of the glazing, the formation of the glazing rebate and the seal within the frame is referred to as the glass system. The possible combinations available give rise to various properties for the window construction, whereby of course the material and the type of frame construction must also be considered. To evaluate a glass system, the thermal insulation, sound insulation, fire protection and the potential energy gain from the glazing must be taken into consideration. Important key figures here are the thermal transmission coefficient (Uw value), the energy transmission (g-value) and the sound reduction index (R).
When toughened safety glass is made, the glass is heated to what is referred to as the transformation temperature and then abruptly cooled. This creates compression stresses in the surface; these pretension the glass and, above all, increase the bending strength. It is impact-resistant and can, for example, withstand a ball being thrown at it. If the glass breaks, it breaks into blunt glass particles. In laminated safety glass, two or more glass panes are joined together with synthetic films and the glass splinters are bound together if the glass breaks.
When two or more panes of glass are joined together and made into one unit using air-tight edges, this composite construction (insulating glass) satisfies greater requirements for heat and sound protection than single-pane glazing. The composite edge is a decisive factor for the seal and is made from metal (aluminium, stainless steel) or fibre glass-reinforced UPVC. Filling the gap between the panes with noble gas (argon, krypton, xenon) improves the thermal insulation further. For heat protection glass, a thermal insulation coating of tin oxide, gold, or silver is applied to the outside of the pane facing the room. This allows the majority of the thermal radiation to be reflected back to the inside.
The penetration of sunlight and thus thermal radiation into the inside of a building can be prevented by the use of sun protection glasses. Use of such glasses may mean that further shading is not necessary. With reflective glass, a metal oxide coating on the inside of the outer pane reflects the radiation energy to a large extent but still generally allows visible light to pass through. If the coating is applied to the outside of the outer glass pane, the degree of light reflection is higher and a mirrored effect is created. With absorption glass, a colouring of the glass has the effect that the radiation energy is partially absorbed and converted into heat energy. This is mainly released to the outside — only a small part of the heat energy reaches the inside and does so with a delay.
The evaluated sound reduction index Rw indicates the sound-insulating properties of an insulating glass unit. To improve the value, thicker panes are used. This reduces the sound transmission. However, the inside and outside panes should have different thicknesses to prevent resonance. The use of composite panes also improves the sound reduction index, as does an increased gap between the panes. However, the value can only be improved to a limited extent as otherwise the thermal insulation is impaired.
For fire protection glass, a distinction is made between fire resistance classes G and F. G-class glasses prevent flames and gas from passing through; a G30 glass fulfils this requirement for 30 minutes. However, they do not provide any protection against the heat of the fire, which can mean that materials on the other side of the glass ignite. In the event of a fire, F-class glasses also prevent the heat of the fire from penetrating. For example, escape routes or stairwells behind a fire protection section can be protected against the fire spreading. The way this works is that in the event of a fire, a gel filling inserted in the gap between the panes turns into a tough, solid foam mass, or it evaporates. Approval from a supervisory authority is required for F-class glasses. Fire protection glasses must always be considered in conjunction with the frame and its attachment to the building shell.
A wall created in a solid construction, with individual, clearly delineated window and door openings is referred to as a perforated façade. This most original form of opening is a result of the technical design conditions that have specified the position and size of the openings within a specific framework since people have been building houses: The size of the openings was determined above all by the materials used to close the openings. Static aspects, the bearing of loads and the formation of lintels defined the outside walls as perforated façades with overlapping openings for centuries.
In the 1920s, Le Corbusier propagated his "five points for new architecture": In addition to the "pilotis", which placed the house on reinforced concrete stilts, the roof garden, the open floor plan and the free façade, the long window (bank of windows) in particular was a feature that immediately defined modern buildings. The semi-detached house in the Weissenhof Estate in Stuttgart, Germany, and the Villa Savoye near Paris, France are examples that show how Le Corbusier replaced the high windows common up to that point with extreme openings. The use of reinforced concrete and a post system allowed the manufacture of this type of opening in the façade, providing significantly better and more even illumination inside the building.
A large-scale opening in the façade that is closed off with multiple window and/or door elements is referred to as a window wall. It can often extend over multiple storeys, for example, in stairwells or for the glazing for lobby areas, and is usually designed as a transom/mullion construction. The increased wind forces affecting the static load of the entire construction must not be overlooked. The increased span of the frame profiles also influences the dimensions of the profiles and the type and size of the division of the window wall. In the design, you can work with a deliberate placement of opening sashes in an otherwise large-scale fixed glazing window wall. When planning window walls and the surrounding façade, you must take into account that the glass areas must be accessible for cleaning, and the frame, fittings and any sun protection must be accessible for maintenance. These activities can take place via cradles, ladders, or movable cleaning devices.
The scope for balconies, conservatories and recessed or protruding glazed entrance or habitation areas within the depth of the façade offers further options for façade design. These interfaces can designate spatial crossovers and transitions or form particularly private or particularly public spaces. For example, during the Renaissance, death sentences were announced from the balcony of the Doge's Palace in Venice. However, private openings can also be designed like display windows in residential constructions — mobile shading or privacy protection elements allow the degree of opening to the outside to be determined individually.