Material
AluminiumProduct level Unitized system
EPDM
Insulated
glass unit Isolator
Inner glazing gasket
Outer glazing gasket Outer frame
Polythermid
slot slot
slot slot
slot
Unitized façade
component Unitized façade
component sectional
Figure 73
Product levels of a unitised system. Even though similar to curtain walls, it shows a sectional interface on the component level.
§ 5.3.3 Frameless systems
With the curtain walling system being complex and, in terms of thermal insulation, the weakest spot in façades, frameless structures made from insulated glass units seem a logical step. These systems have been in existence for quite some time. In fact, this development is architecturally motivated by the strive for more transparency. But the same functions have to be fulfilled.
Glass panes come in limited sizes; which is a disadvantage but is also useful in terms of handling the parts. They need to be joined with water, wind and moisture tight connections. These connections should also have appropriate thermal qualities. The glass panes have to be connected to a load-bearing structure. This connection has to allow tolerances and movement.
One type of frameless systems is the so called ´Frog hand´ or ´Spider´ façade. Several systems exist on the market, which include the construction of the structural spider and the point fixing.
The connection between spider and point fixing has to handle tolerances and movement, and the connection is a typical slot interface. After being bolted to the inner or inner and outer glass pane, the point fixings are integrally glued to the glass.
Depending on the thickness of the glass, the joint is sealed by an EPDM backfill profile and than closed with a silicone joint.
Figure 74
Frameless facade system. The Glass panes are held by a steel ´spider´
(Prinsenhof, Delft, Octatube).
Stainless steel spider Commercial material
Element
Sub component
Component
Building part Facade
Standard material Material
Product level Frameless facade
Rubber Stainless steel
Insulated glass unit
Point fixing system
Joint sealing
Silcone
integral integral slot
Insulated glass unit integral
Filler compound
EPDM
Figure 75
Product levels of a frameless structural glazing system.
Frog hand façades have a rather integral product architecture compared to standard curtain walling systems. The result is a high degree of transparency and flush finishes.
Frameless systems have the disadvantage as well; one of which is that they do not allow the attachment of dividing walls. This, in combination with high costs is the reason why its application is limited to entrance halls or buildings with representative functions.
§ 5.3.4 The insulated glass unit
An insulated glass unit uses a strategy of layering to reach a better thermal
performance. It consists of layers of glass which can be enhanced with certain coatings, depending on the requirement. A typical edge of the unit is made out of several subcomponents: spacer, desiccant to absorb moisture within the glass space to prevent condensation, interior sealant (butyl to connect glass and spacer) and exterior sealant to fully seal the unit. The spacer can be filled with different gases such as argon or krypton to improve the insulation value.
Float glass in different thicknesses is processed into security glass. It is cut to size, coated, receives different edge treatments and is finally assembled into an insulated unit. Depending on the requirements, it is composed of different glass types, sizes and coatings. The final product therefore only differs in size and weight; and any façade system is designed to accommodate it.
If a glass breaks, the entire unit is replaced. Three layered units with a high insulation value are becoming the current standard. A fourth layer would theoretical further improve the U-value, but glass manufacturers hesitate to take this step. The glass thickness will have an impact on the visual quality, and unit sizes and weight can no longer be integrated into today’s framing systems. On top of that, the gain in thermal improvement does not relate to the additional costs.
Figure 76
Edge detail of insulated glass unit
Commercial material
Element Standard material
Material Aluminiu
m
Product level Insulated glass unit (IGU) Glass
The product levels of the insulated glass unit show an integral architecture.
The construction is integral (it cannot be taken apart after assembly), but the glass unit is highly standardised. Different IGU products can be placed in all standard curtain wall systems. It owes its variability to a highly flexible production process and is customised for each project, each different window.
§ 5.3.5 Double façades
The traditional arrangement of functions within the physical façade space is either side-by-side, layered or a combination of both. The insulated glass unit, as described before, uses a strategy of layering on the ´element´ level - double façades on the level of ´building parts´.
With double façade, the construction is extended by a second glass layer that acts like a rain and wind shield to protect the sun-shading system. This can be very important for high rise buildings. Outside sun-shading systems are very efficient and will still be in operation under high wind loads. Double façades are also used to block traffic noise or to allow natural ventilation of buildings. In winter, they can provide an additional thermal layer.
However, the concept brings with it several downfalls: The cavity works like a
greenhouse and needs controlled ventilation to prevent overheating. Condensation on the inner side of the outer glass pane can occur, if windows are opened in the inner, thermally insulated layer during cold outside conditions. Cleaning costs are high. This is why double façades only make sense in noisy locations or for high rise buildings where the investment is justified.
Figure 78
Classical arrangement of façade functions: side by side or layered.
There are different climatic strategies for double façades: The cavity can be separated horizontally in extension of the floor slabs (See figures80 - corridor façade). Each façade unit of the RWE Tower (Figure 85) is individually ventilated (box façade). The second skin of the Ramboll building (Figure 87) forms one cavity (chimney façade) and is not unitised.
But alternating concepts are conceivable as well. They have boxed windows in combination with single layer areas that have direct operable parts (Figure 91). New double façade concepts show sealed cavities. The reason is a reduction of cleaning costs, particularly in desert regions. Overheating is an issue of concern.
Figure 79
Stadttor Düsseldorf, Petzinka, Overdiek und Partner, Düsseldorf 1998 Figure 80
Detailed view
Figure 81
The cavity is horizontally separated. It is ventilated via vertical adjustable ventilation grilles.
Figure 82
RWE Tower, Ingenhoven Overdiek Kahlen & Partner, Essen, 1997 Figure 83
Detailed view Figure 84
Each façade unit forms a separate cavity (box façade) and is individually ventilated through vertical and horizontal gaps.
Figure 85
Perspective drawing of facade unit.
Figure 86
Head Of fice, Dissing+Weitling Architecture, Copenhagen 2010 Figure 87
Corner situation Figure 88
Rotating flaps in form of glass louvers allow controlled ventilation of the cavity (chimney façade).
Figure 89
Debitel Stuttgart, RKW Architektur + Städtebau, Stuttgart Figure 90
Corner situation Figure 91
Alternating façade: Boxed windows with internal sun-shading alternate with vertical operable windows that are protected by fixed louvers. These cannot be opened in strong wind conditions.
The functional extension of double façades requires a large constructive effort. Double façades are usually custom made. Most constructions belong to the family of curtain wall or unitised systems. The outer layer is specially designed.
Interesting for this research is the fact that the design of double façades is interwoven with the building services concept and that, in most cases, the façades are combined with decentralised or central HVAC systems. Therefore they require a design team of architect, building physics consultant, climate designer and façade consultant.