Curtain walling

Stick system

Stick construction is the traditional form of curtain walling, comprising a grid of mullions and transoms into which various types of glass and/or insulated panels can be fitted. The systems are usually fairly low cost, using extruded components supplied by the system manufacturer. Mullions are usually assembled first, with transoms then fitted in-situ.

A variety of jointing systems are employed but most stick systems have some form of stub connection between individual lengths or transoms. Mullions are usually fixed back to the structure at floor levels, although longer spans are possible according to the section properties of the component.

Aluminium is the usual material for the framework, although some older buildings experimented with steel. Most of the grid assembly work is done on site. This permits some measure of dimensional adjustment and tolerance, but it is highly workmanship-sensitive and relies on accurate cutting of components and the correct use of sealants at joints.

Assembly on site is fraught with risk as many systems are superficially similar so that a contractor who is familiar with one system may omit crucial components such as seals at junctions when assembling components provided by another manufacturer. (Weatherproofing problems in modern glazing systems, CIOB, 1992) Similarly, if shop drawings are hard to read (perhaps in a different language) or do not detail items such as drainage or ventilation slots, there is a risk that these items are neglected or installed incorrectly.

Unitised system

Unitised systems comprise narrow-width, storey-height units of aluminium framework containing glazed and/or opaque insulated panels. The entire system is pre-assembled under factory-controlled conditions, thus avoiding some of the problems of assembly encountered on site.

The unitised panels are usually craned into position, with pre-positioned brackets attached to the floor slab or the structural frame. Modern installation techniques increase the speed of erection and often minimise the requirement for scaffolding.

Unitised systems do have higher direct costs but nowadays the curtain walling to most prestige buildings is of this type.

Panellised systems

Like unitised systems, panellised curtain walling is constructed in the factory to improve quality control and speed of erection. However, the systems comprise large prefabricated panels of bay width and storey height, which are connected back to the primary structural columns or to the floor slabs. Panels may be of precast concrete or comprise a structural steel framework and are often clad with a variety of stone, metal and masonry cladding materials.

The advantage of these systems is improved adherence to specification as a consequence of factory prefabrication, allowing improved control of quality and rapid installation with the minimum number of site sealed joints. Panellised systems are less common and more expensive than unitised construction.

Structural silicone

Structural sealant glazing is a form of glazing that can be applied to stick or unitised curtain walling systems. With structural sealant glazing the insulating glass units (IGUs) are attached to the grid framework with factory applied structural silicone sealant rather than by pressure plates and gaskets in a more traditional system.

Structural glazing

Structural glazing typically comprises large thick single panes of toughened glass assembled with special bolts and brackets that are supported by a secondary steel structure. This form of glazing is often referred to as 'Planar' glazing and is commonly used to form the enclosure to atriums and entrances. Planar is a trade name rather than a generic type of glazing system.

Rainscreen systems

Rainscreens are not curtain walls as such, as they provide a screening function rather than an enclosure in its own right. A rainscreen is used to shield a wall, whether this be of masonry, metal studwork or in some cases glass. Usually a rainscreen is designed to permit some controlled leakage with the main functions of resistance to air and water being provided by the shielded wall behind.