Curtain Wall

Definition and Mechanical Function#

A curtain wall is defined as a non-structural outer covering of a building designed to keep out weather. Unlike load-bearing masonry or concrete walls, a curtain wall system does not support the roof or floor loads of the building; instead, it supports its own weight and resists environmental forces—such as wind and seismic activity—which it transfers back to the primary building structure via connections at the floor slabs or columns.

In the UK, these systems are typically classified into two main categories: stick systems and unitised systems. Stick systems are assembled on-site, where mullions (vertical members) and transoms (horizontal members) are installed before the infill panels—usually glass, metal, or stone—are fitted. Unitised systems consist of large, pre-assembled factory units that are craned into position. Because the system is "hung" from the structure like a curtain, it can be designed using lightweight materials, allowing for extensive use of glazing and expansive elevations.

Relationship to Building Fabric and External Walls#

Within the context of a building’s external wall system (EWS), the curtain wall acts as the primary thermal and weather barrier. It is a multi-component assembly that must manage air infiltration, water penetration, and thermal transfer. The relationship between the curtain wall and the internal building fabric is critical, particularly at the interface where the wall meets the floor slab.

The void between the edge of the floor slab and the back of the curtain wall is a significant element of the building fabric. This gap must be appropriately addressed to maintain the fire-separating function of the floor. In technical terms, this involves the installation of perimeter fire stopping or "linear gap seals." These components are designed to restrict the passage of smoke and flame between floors within the cavity, ensuring the building’s compartmentalisation strategy remains intact.

Regulatory and Process Context in England and Wales#

The regulatory framework for curtain walling in England and Wales is primarily governed by the Building Regulations, specifically Approved Document B (Fire Safety) and Approved Document L (Conservation of fuel and power). Following the introduction of the Building Safety Act 2022, the scrutiny regarding how these systems are designed, installed, and maintained has increased, particularly for "Higher-Risk Buildings" (HRBs) which are at least 18 metres tall or have seven storeys.

The "Golden Thread" of information is a central concept in this regulatory environment. It requires that detailed records of the curtain wall’s components—including the specification of the framing, the type of infill panels, the insulation used within spandrel areas, and the details of the fire stopping—are maintained throughout the building’s lifecycle. During the planning and construction phases, the Gateway process managed by the Building Safety Regulator (BSR) ensures that the design of the external wall system is reviewed before work commences and before the building is occupied.

Cladding Testing, Repair, and Replacement#

In the UK, curtain walls frequently undergo specific testing and assessment regimes, particularly when concerns arise regarding their performance or compliance with contemporary standards. Testing typically falls into two categories: destructive and non-destructive.

Repair and replacement work often focuses on the "spandrel panels"—the opaque sections of the curtain wall located between the glazed areas of different floors. In older systems, these panels may contain combustible insulation or cores that do not meet current guidance. Replacing these components requires a detailed understanding of the original system’s structural capacity, as new, non-combustible materials may have different weights or thicknesses, potentially affecting the wind-load calculations of the original framing.

Guidance for Building Owners and Managers#

Building owners, Managing Agents, and Residents' Management Company (RMC) directors must view the curtain wall as a complex assembly rather than a single product. It is often the case that the framing is produced by one manufacturer, the glass by another, and the spandrel insulation by a third. Understanding this "system-based" nature is vital when commissioning fire risk appraisals of external walls (FRAEWs) under PAS 9980.

A key responsibility for those managing buildings is ensuring that cyclical maintenance is carried out. This includes checking the condition of gaskets, sealants, and pressure plates. If a curtain wall system begins to leak or show signs of condensation, it may indicate a failure of the internal drainage strategy (often involving "weep holes") or a breakdown in the thermal break. Professional assessment is required to determine whether the issue is a localised maintenance failure or a systemic design flaw.

Common Misunderstandings#

A frequent misunderstanding is the conflation of "rainscreen cladding" with "curtain walling." While both are parts of the external wall, a rainscreen is typically an outer skin attached to a secondary structure with a ventilated cavity behind it. A curtain wall is a self-supporting, fully sealed system that provides the primary weather barrier. Treating a curtain wall as a rainscreen during an inspection can lead to incorrect assumptions about how fire or moisture behaves within the system.

Another misconception relates to the glass itself. It is often assumed that all glass in a curtain wall is identical; however, different panes may have different coatings (Low-E), solar control properties, or safety ratings (toughened vs. laminated). During replacement projects, using the incorrect glass type can result in visual inconsistencies ("checkerboarding") or, more importantly, a failure to meet thermal or structural safety requirements.

Closely Related Terminology#

To navigate technical reports effectively, it is necessary to be familiar with terms that describe the constituent parts of the system:

• Mullions and Transoms: The vertical and horizontal aluminium or steel members that form the grid.
• Spandrel Panel: The opaque panel used to hide floor slabs, ducts, and ceilings.
• Pressure Plate: The component that screws into the mullion to hold the glass or infill panel in place.
• Thermal Break: A material of low thermal conductivity placed between the inner and outer halves of the aluminium frame to reduce heat loss and condensation.
• EPDM (Ethylene Propylene Diene Monomer): A synthetic rubber used for gaskets and weather membranes to ensure air and water tightness at junctions.
• Bracketry: The heavy-duty steel components that fix the curtain wall to the building’s concrete or steel frame.

The complexity of these systems necessitates that any intrusive inspection or remedial design is led by a qualified facade engineer or a competent professional with specific experience in curtain wall technology.