A curtain wall is a non-load-bearing outer skin attached to a building’s structural frame. Unlike traditional walls that support the weight of floors and roofs above them, a curtain wall carries only its own weight and transfers environmental forces like wind back to the main structure. It literally hangs from the building like a curtain, which is where the name comes from. Most curtain walls are made from aluminum framing filled with glass panels, though they can incorporate stone, metal, terracotta, and other materials.
How Curtain Walls Differ From Regular Walls
The defining feature of a curtain wall is that it does no structural work. A load-bearing wall holds up the building. A curtain wall simply encloses it, keeping weather out and conditioned air in. Because it doesn’t carry structural loads, a curtain wall can be made from virtually any combination of lightweight materials that meet insulation, wind resistance, and aesthetic requirements.
This distinction matters in practical terms. Since the curtain wall isn’t part of the building’s skeleton, it can be designed with far more glass than a traditional wall. That’s why modern office towers and commercial buildings can have fully glazed facades stretching dozens of stories. The steel or concrete frame does the heavy lifting, and the curtain wall wraps around it.
Curtain Wall vs. Window Wall
These two systems look similar from the street but attach to the building differently. A curtain wall’s vertical framing members run past intermediate floors, meaning panels can span more than one story. The system hangs from mounting brackets secured to the structure, bypassing the floor slabs entirely.
A window wall, by contrast, sits between concrete floor slabs on each level. It does not span more than one floor, and there’s a visible or concealed break in the glass at every floor line. If you look closely at a glass-clad residential tower and see a band of opaque material at each floor level, you’re likely looking at a window wall rather than a curtain wall.
Key Components
Every curtain wall breaks down into a few core elements, regardless of how complex the finished facade looks:
- Mullions: Vertical aluminum box-section members that form the primary framework. These carry the weight of the glass and panels down to the anchor points on the building structure.
- Transoms: Horizontal members that connect between mullions, dividing the wall into individual openings for glass or panels.
- Glazing: The glass units themselves, often double or triple-paned insulated glass with coatings that control heat and light transmission.
- Spandrel panels: Opaque sections, usually made of coated glass, aluminum, or glass fiber reinforced concrete, that hide floor slab edges and ceiling zones behind the facade.
- Gaskets and sealants: Rubber gaskets, silicone sealants, and pressure plates that hold the glass in place and keep water and air from penetrating the joints.
In capped systems, the most common retention method, glazing sits on the framework supported by rubber gaskets with aluminum pressure plates behind decorative cover caps on the exterior. Structural silicone systems take a different approach, using flush-tooled silicone at each glass joint as the sole weather seal, giving the facade a smooth, capless appearance.
Stick-Built vs. Unitized Systems
There are two main ways to get a curtain wall onto a building, and the choice depends largely on the building’s height, site constraints, and budget.
Stick-Built Systems
In a stick-built system, long aluminum pieces (the “sticks”) are installed vertically between floors and horizontally between vertical members, one piece at a time, directly at the construction site. Glass and panels are then glazed into the completed framework on-site. This is the most common method for low to mid-rise buildings. Lead times for materials are shorter, upfront staging requirements are smaller, and the system handles complex facade geometries well since each piece can be adjusted during installation. The trade-off is slower on-site work and a significant need for material storage space at the job site.
Unitized Systems
Unitized curtain walls arrive at the construction site as large pre-assembled panels, already glazed and sealed in a controlled factory environment. Workers lift each unit into place and attach it to anchors on the building. Installation speed is the major advantage: unitized systems can go up in roughly one-third the time of a stick-built system, with no on-site glazing required. They’re well suited for tall buildings and projects with repetitive, regular facade patterns. The downside is a longer lead time. Factory fabrication and delivery can take six months to a year before panels arrive on-site.
How Curtain Walls Handle Weather
Keeping rain out of a building wrapped in thousands of glass-to-metal joints is one of the biggest engineering challenges in curtain wall design. Many systems use a principle called pressure equalization, also known as the rainscreen method. The idea is straightforward: the curtain wall has two layers separated by a cavity. The outer layer (the rainscreen) blocks most rain. The cavity behind it is vented to the outside through small openings, so air pressure inside the cavity matches the air pressure on the exterior surface. When pressures are equal, there’s no force driving water through any remaining gaps in the outer layer.
The cavity is typically divided into compartments to prevent air from flowing across zones where pressure differences are large, such as building corners. Combined with internal drainage channels that collect any water that does get past the outer seal and weep it back outside, this two-barrier approach is what allows glass buildings to stay dry in driving rain.
Wind and Seismic Performance
Curtain walls need to resist wind loads pushing and pulling on the facade while also accommodating building movement during earthquakes or thermal expansion. They manage this through flexibility rather than rigidity.
Metal and glass curtain walls have historically performed well in seismic events. The framing assemblies are inherently flexible, the glass panels are relatively small and strong, and the retention materials (gaskets and silicone) are resilient enough to absorb minor distortions. In seismic regions, curtain walls are often designed with slip joints between adjacent panels so the facade can accommodate the racking motion of the building frame swaying side to side without cracking the glass. Tie-back connections anchoring the wall to the structure are designed to deform under lateral forces, preventing those forces from being transmitted rigidly into the panels.
For wind, the mullion and transom framework transfers pressure loads back to the building’s floor slabs and structural frame at each anchor point. The curtain wall itself never resists these forces on its own.
Energy Performance and Insulation
Glass-heavy facades have always been a weak point for building insulation, but modern curtain wall glazing has improved dramatically. Current standards in cold climates require overall thermal transmittance (U-values) of 1.5 W/m²·K or lower for curtain wall glass, meaning the glass must resist heat flow at a level that would have been difficult to achieve a generation ago. In warmer climates, the threshold is around 2.0 W/m²·K.
Meeting these targets typically requires double-pane insulated glass units with low-emissivity coatings and argon gas fills between the panes. Warm edge spacers, which reduce heat transfer at the perimeter of each glass unit, are another common upgrade. A typical high-performance insulated glass unit today might achieve a U-value around 1.4 to 1.6 W/m²·K while also providing around 40 decibels of sound insulation.
Common Infill Materials
While glass dominates most curtain wall facades, the system is flexible enough to accept a range of infill materials. The exterior surface can be 100% glass or can mix in aluminum panels, natural stone, glass fiber reinforced concrete, and terracotta. Spandrel zones, where the wall conceals structural elements or mechanical spaces, often use opaque glass with a ceramic coating on the back, insulated metal panels, or composite panels. Louvers for ventilation openings can also be integrated into the curtain wall grid. This versatility is part of why curtain walls became the default facade system for commercial architecture starting in the mid-20th century and remain so today.