What Is a Curtain Wall System?

A non-load-bearing building skin

A curtain wall encloses a building without carrying the gravity loads of the floors or roof. It supports its own weight and transfers wind pressure and suction through its framing and anchors to the primary structure. That distinction is important: non-load-bearing does not mean non-structural.

Most systems combine vertical mullions, horizontal transoms, insulating or monolithic glass, opaque spandrel panels, gaskets, pressure plates or structural-glazing components, brackets and perimeter seals. Together they must resist environmental loads while accommodating normal dimensional tolerances and limited building movement.

How loads travel through the facade

Glass receives wind pressure across its surface and transfers it to supported edges. Transoms and mullions carry those reactions to brackets, which connect the facade to slab edges, beams or another approved part of the structure. Glass dead load is normally supported on setting blocks rather than allowed to bear directly on metal.

Each component is sized for its actual span and load. A pane that becomes taller or wider may require a different glass thickness even when the visual grid is unchanged. Mullion depth, aluminum reinforcement and anchor arrangement likewise depend on free span, support spacing, facade geometry and design wind pressure.

Because wind effects can be higher at corners, roof zones and exposed elevations, a single rule of thumb should not be applied to every panel. Glass selection should also account for human-impact locations, overhead glazing, fall hazards and the required post-breakage behavior.

Water, air and drainage

A durable curtain wall does more than place sealant around glass. Many drained systems use an exterior rain screen, pressure-equalisation cavities and controlled drainage to the outside. Gaskets, baffles, end dams and weep paths have to remain continuous across mullion and transom intersections.

Water that passes the first line of defence should encounter a second line and a clear route back outdoors. If fabrication swarf, sealant or site modifications block drainage paths, water can migrate to the interior even when the facade looks complete.

The perimeter joint is equally important. The interface between curtain wall, roof, floor edge, adjacent cladding and air barrier must be detailed as one enclosure. A high-performing center-of-panel assembly cannot compensate for an unsealed perimeter.

Thermal and solar performance

Large glazed areas can deliver daylight and an open architectural character, but performance depends on the complete assembly. Glass U-factor, solar heat-gain coefficient, visible transmittance, edge spacers and thermal breaks all influence interior comfort and energy use.

In a hot, sunny climate, glass area, orientation and external shading should be considered alongside the coating. A low solar heat-gain coefficient can reduce unwanted solar gain, while a thermally improved aluminum system limits conduction through the framing. The correct balance depends on facade direction, room use and daylight targets.

Condensation risk is also an assembly issue. Cold aluminum, glass edges or poorly insulated spandrel zones can create low interior surface temperatures. Thermal modelling and careful continuity at anchors and slab edges may be needed on demanding projects.

Stick-built and unitised systems

In a stick curtain wall, mullions and transoms are assembled progressively on site, followed by glass and pressure components. It is flexible for many low- and mid-rise projects, but installation quality, alignment and field sealing need close control.

Unitised curtain wall is assembled into larger factory-built panels that are installed floor by floor. Factory conditions can improve repeatability and accelerate enclosure of large buildings, while transport, tolerances, stack joints and installation sequencing require more intensive planning.

Face-capped, semi-capped and structurally glazed appearances can be developed within these broader construction approaches. The visible style should not be confused with the fabrication and drainage strategy behind it.

From survey to handover

Work should begin with an as-built survey and confirmation of structural attachment zones. Shop drawings then coordinate grid lines, glass sizes, anchors, movement joints, perimeter interfaces, drainage and opening vents. Approved samples—and, where warranted, a full-size performance mock-up—make aesthetic and technical expectations measurable before mass production.

Fabrication control covers cut lengths, machining, weep preparation and component identification. On site, brackets and primary framing are installed and aligned before glass, gaskets and caps or structural seals. Final inspection checks joint continuity, drainage, glass clearances, vent operation and damage.

Curtain wall is therefore not simply a glass facade. It is an integrated envelope whose architecture, load path, water control, thermal design, manufacture and installation must agree before the first production piece is cut.