Curtain wall glazing

[edit] Introduction

Curtain wall glazing is a non-loadbearing external wall system in which glazed and, where required, opaque infill panels are supported by a lightweight framing system attached to the primary structure of a building. It is widely used in multi-storey commercial, institutional, retail and mixed-use buildings, where large areas of glazing can provide daylight, views and a particular architectural appearance.

Curtain walling must resist environmental actions such as wind and rain, accommodate movements arising from thermal expansion and contraction and building deflection, and provide appropriate levels of thermal, acoustic and fire performance. Its performance depends on the design of the complete façade system, including the framing, glazing, seals, fixings, drainage and interfaces with the surrounding construction.

[edit] Construction and components

A typical glazed curtain wall consists of vertical mullions and horizontal transoms forming a framed grid. These are commonly manufactured from aluminium, although other materials can be incorporated. Glazing units or opaque infill panels are installed within the framing and retained using pressure plates, caps, gaskets, sealants or other proprietary components, depending on the system.

The curtain wall is connected to the primary structure using brackets and anchors, generally at each floor or at other engineered support locations. The connections transfer the loads imposed on the curtain wall to the building structure while allowing for movements between the façade and the structure.

Curtain wall glazing is not intended to carry the loads of floors, roofs or other parts of the primary structure. However, it is not entirely non-structural: the framing, fixings and glazing must be designed to resist wind loads, the self-weight of the façade and other relevant imposed actions. The system must also accommodate movements without compromising its performance.

Common components include:

• Mullions: vertical framing members that provide the principal structural support for the curtain wall.
• Transoms: horizontal framing members that divide the façade into individual panels and support glazing or other infill.
• Glazing units: commonly double or triple insulating glass units, although single glazing and specialist glazing may be used in particular applications.
• Opaque infill panels: panels that may incorporate insulation, metal, glass or other materials where transparent glazing is not required.
• Gaskets and sealants: components used to retain glazing and provide weather, air and movement control.
• Anchors and brackets: components that connect the curtain wall to the primary structure.
• Drainage components: systems that collect and discharge water that enters the curtain wall's drainage cavities.
• Thermal breaks: insulating components within the framing that reduce thermal bridging and improve thermal performance.

[edit] Curtain walling systems

Two principal methods of constructing curtain walling are stick systems and unitised systems.

A stick curtain wall is assembled largely on site from individual mullions, transoms, glazing components and associated fittings. This approach can provide flexibility where dimensions vary or where the building geometry requires significant adjustment during construction. However, it generally involves more site-based assembly and installation than a unitised system.

A unitised curtain wall consists of factory-assembled panels that are transported to the site and connected to the building structure. The panels can incorporate glazing, framing, seals, insulation and other components before delivery. Unitised systems can reduce the amount of work undertaken at height and can provide greater factory control over assembly, making them particularly suitable for large or repetitive façades. Their use requires careful coordination of panel dimensions, structural tolerances, interfaces and installation sequencing.

[edit] Performance and design considerations

The principal advantages of curtain wall glazing include the ability to provide large areas of daylight and views while creating relatively lightweight external wall construction. The appearance can be varied through the selection of glass, framing profiles, colours, panel proportions and opaque infill materials.

Glazed curtain walls can also contribute to the thermal and environmental performance of a building, but extensive glazing can increase heat loss in cold conditions and solar heat gains in warm or sunny conditions. The specification of glass, framing, shading and ventilation therefore needs to be considered as part of the overall building design. Double or triple glazing, low-emissivity coatings, solar-control glass and thermally broken framing can be used where appropriate.

Acoustic performance can be important where buildings are exposed to road, rail, aircraft or other sources of external noise. The performance of the complete façade depends on the glazing, framing, seals and junctions rather than the glass alone.

Curtain walling should also be designed to resist water penetration and uncontrolled air leakage. Many systems incorporate drained and pressure-equalised cavities so that water entering the outer part of the system can be collected and discharged rather than penetrating into the building.

Other considerations include maintenance access, cleaning, replacement of glazing units, durability of finishes and seals, compatibility between materials and the ability of the system to accommodate structural and thermal movement.

[edit] Fire safety and compliance

Fire performance is an important consideration in the design of curtain wall façades, particularly in multi-storey buildings. The requirements depend on the building's use, height, location and applicable building regulations. Design should consider the spread of fire and smoke through the façade and at interfaces between the curtain wall and floor or wall construction.

Where the curtain wall passes floor slabs or compartment walls, appropriately designed cavity barriers, fire-stopping and perimeter fire containment may be required. The details must accommodate the expected movement of the curtain wall without compromising their fire performance.

The materials and system should be assessed against the requirements applicable to the particular building and jurisdiction. Fire performance should not be inferred solely from the fact that a curtain wall uses glass and aluminium, as other components, including seals, gaskets, insulation, spandrel panels and cavity barriers, can affect the overall performance.

In the UK, curtain wall systems are generally assessed against the requirements of the relevant Building Regulations and applicable standards. BS EN 13830 specifies requirements for curtain walling relating to characteristics including resistance to wind load, watertightness and air permeability, and provides a framework for assessing the performance of curtain wall systems.

[edit] Design and installation

Curtain walling should be considered early in the design process because its dimensions, interfaces and structural connections affect the primary structure, floor edges, internal finishes and building services. The design should establish performance requirements for structural resistance, thermal performance, air permeability, watertightness, acoustics, solar control, fire safety, durability and maintenance.

Particular attention is required at interfaces with roofs, floors, parapets, windows, doors and other façade systems. Poorly coordinated interfaces can result in water penetration, thermal bridging, air leakage or inadequate fire stopping.

Installation tolerances are also important. The curtain wall must be accurately positioned and securely connected to the primary structure while retaining sufficient capacity to accommodate movements. Glazing and seals should be installed in accordance with the requirements of the specified system so that the designed weather and thermal performance is maintained.

[edit] Related articles on Designing Buildings

• Curtain wall systems
• Cladding for buildings
• Types of cladding
• Glass mullion system
• Glazing
• Triple glazing
• Single glazing v double glazing
• Exterior wall
• Building envelope
• Elements of buildings