A curtain wall is a building’s outer skin, a non-load-bearing exterior wall that hangs from the structural frame like a curtain. Unlike traditional walls that support the weight of floors and roofs above them, a curtain wall carries only its own weight and transfers wind and other environmental forces back to the building’s main structure. You’ll see curtain walls on most modern commercial buildings, office towers, and high-rises, where large expanses of glass and metal panels define the exterior.
How a Curtain Wall Differs From a Regular Wall
The defining characteristic of a curtain wall is that it does nothing to hold the building up. The building’s skeleton, typically steel or concrete, handles all the structural weight. The curtain wall is attached to that skeleton at each floor level, hanging in front of the slab edges. This separation between structure and enclosure is what makes glass skyscrapers possible: because the exterior wall bears no load, it can be made of virtually any combination of lightweight materials that meet insulation, wind resistance, and aesthetic requirements.
This is different from a window wall or storefront system, which sits between floor slabs and relies on those slabs as its top and bottom support. A curtain wall is continuous past the floor line, installed outboard of the slab edge, creating one uninterrupted facade from the ground to the roofline.
What a Curtain Wall Is Made Of
The visible framework of a curtain wall consists of vertical and horizontal bars called mullions. These are most commonly extruded aluminum, though steel and timber are also used. The mullions create a grid that holds the infill panels in place.
The infill, the material that fills each opening in the grid, is where curtain walls get their variety. Glass is the most recognizable option, ranging from single-pane clear glass to double-coated solar insulating glass designed to maintain stable humidity in sensitive environments. But infill panels can also be opaque: porcelain enamel, natural stone, terracotta, fiber-reinforced concrete, metal composite panels, or even stone-aggregate-coated plywood. Many curtain walls combine two or more infill types, using transparent glass where views and daylight matter and opaque panels to conceal floor slabs or mechanical spaces.
Sealing everything together are gaskets and sealants. Neoprene gaskets, for instance, can serve as both weatherproofing and structural connectors, holding glass and wall panels without any additional fasteners. Structural silicone sealants bond glass directly to the frame in what’s called structural silicone glazing, creating a smooth, flush exterior with no visible metal caps.
Stick-Built vs. Unitized Systems
Curtain walls are installed using one of two main approaches, and the choice between them shapes the cost, timeline, and quality of the finished facade.
A stick-built system is assembled piece by piece on the construction site. Workers first install the vertical and horizontal aluminum mullions onto the building’s frame, then insert glass panels or other infill materials and seal them in place. This method is flexible and works well for smaller buildings or projects with complex geometry, but it’s slower and more vulnerable to weather delays. Quality depends heavily on the skill of the installation crew and the conditions they’re working in.
A unitized system takes the opposite approach. Large panels, each containing both the aluminum frame and pre-installed glass, are assembled and sealed in a factory under controlled conditions. These finished units are shipped to the site and lifted into place by crane, locking together like oversized building blocks. Factory assembly produces more consistent quality, and on-site installation is dramatically faster. Unitized systems are the standard choice for tall buildings and tight urban sites where minimizing on-site labor and street-level congestion matters. The tradeoff is cost: a unitized curtain wall runs roughly $105 per square foot installed, compared to about $55 per square foot for a simpler window wall system.
How It Stays Attached to the Building
Curtain wall panels connect to the building’s structural frame at each floor level using a system of hooks, brackets, and adjustable bolts. In a unitized system, each panel typically has two hooks at the top that latch onto a bracket anchored to the floor beam. One hook is fixed while the other is free to slide horizontally. This deliberate asymmetry serves an important purpose: it allows the panel to accommodate movement.
Buildings move constantly. Steel and concrete expand and contract with temperature changes. Tall structures sway in the wind. Earthquakes impose sudden lateral forces. A curtain wall that was rigidly fixed at every point would crack, buckle, or pull away from the structure. Instead, the connection system is designed so panels can shift slightly in every direction. The sliding hook absorbs horizontal movement from thermal expansion or seismic activity. The hooking connections also allow rotation to absorb forces pushing perpendicular to the wall’s surface. Sealant joints between panels are sized with specific width-to-depth ratios to flex through years of expansion, contraction, and temperature cycling without losing their seal.
Keeping Water and Air Out
A curtain wall’s most critical job, after staying attached to the building, is preventing water and air from getting inside. Most modern systems use a pressure-equalized rainscreen design. The basic idea is that water penetration is driven by pressure differences between the outside and inside of the wall. If you can equalize that pressure, water has no force pushing it through joints and gaps.
To achieve this, the curtain wall is built as two layers. The outer layer, the rainscreen, blocks most rain but isn’t perfectly sealed. Behind it sits an air gap (the cavity), and behind that is a carefully sealed air barrier. Small vents in the rainscreen allow air pressure in the cavity to match the outdoor pressure, removing the pressure difference that would otherwise drive water inward. The size and placement of those vents, the volume of the cavity, and how tightly the air barrier is sealed all determine how effectively the system performs.
Energy Performance and Building Codes
Because curtain walls are often predominantly glass, thermal performance is one of their biggest challenges. Glass conducts heat far more readily than an insulated opaque wall, which means a curtain wall can be the weakest link in a building’s thermal envelope.
Building energy codes, like the International Energy Conservation Code, set maximum U-factor requirements for curtain wall glazing based on climate zone. The U-factor measures how easily heat passes through the assembly; lower numbers mean better insulation. Recent code updates have pushed these requirements significantly tighter, demanding improved window U-factors that older curtain wall systems wouldn’t meet. Codes also limit how much of a building’s exterior can be glass, generally capping vertical glazing at 30 to 40 percent of the total above-grade wall area. Projects that exceed these limits must demonstrate equivalent energy performance through other means.
To meet modern standards, curtain wall glass is now routinely double or triple glazed with insulating gas fills between panes, low-emissivity coatings to reflect heat, and thermally broken aluminum frames that prevent the metal from conducting heat straight through the wall. These features add cost but dramatically reduce energy loss.
How Long a Curtain Wall Lasts
The aluminum and glass components of a well-made curtain wall are remarkably durable. The limiting factor is usually the sealants and gaskets that keep everything weathertight. For decades, the industry assumed structural silicone sealants had a useful life of about 20 to 25 years before they’d need replacement. Recent research has pushed that estimate much higher. Testing by the Federal Institute for Materials Research in Berlin subjected structural silicone sealants to artificial weathering and complex mechanical loading equivalent to 52 years of real-world service. The sealants still met European performance criteria for residual strength, with only minor adhesion loss at specimen corners.
Real-world evidence supports these lab findings. A significant number of structural silicone glazed curtain walls around the world have now passed 30 years of service. Specimens of first-generation silicone sealant removed from an actual facade after 23 years still passed the same performance benchmarks required of new installations. Researchers are now asking whether the true service life of these systems might extend to 75 or even 100 years, though the practical answer depends on installation quality, environmental exposure, and maintenance. Regular inspections to catch deteriorating seals before they allow water intrusion remain the most important factor in extending a curtain wall’s life.