The word curtain usually brings fabric to mind, like the shower curtain that designers hang as wall art. In commercial construction it means an exterior wall system built to keep weather out while letting daylight in. A curtain wall carries no floor load. It hangs from the structural frame the way a curtain hangs from a rod, and that is exactly where the name comes from.
This article breaks down how curtain walls work: the load path, the main system types, glazing and insulation choices, wind load behavior, and the sequence that delivers a high-performance building enclosure.
Curtain walls cover most commercial high-rise facades built since the 1960s, and a single tower can carry tens of thousands of square feet of glazing. That scale is why the details, from anchor placement to sealant selection, get engineered before the first panel ships.
What Is a Curtain Wall?
A curtain wall is a non-load-bearing exterior assembly, usually aluminum framing with glass infill, attached to the building’s structural frame. Columns and floor slabs carry all gravity and lateral loads. The wall carries only its own weight and transfers wind pressure back to the structure at each floor.
The glazing sits at the center of the system. Curtain wall construction rises and falls on the glazing panel: the glass must be held, sealed, and drained so water never reaches the interior. Most commercial towers use insulating glass units, two or three panes separated by a sealed air space, to control heat flow and condensation.
The Load Path
Loads move in a defined sequence. Wind pushes on the glass, the glass transfers pressure to the mullions, the mullions carry it to anchors at each floor slab, and the anchors pass it into the structural frame. Gravity follows the same route: each panel hangs from the frame and never rests its weight on the glass below.
The frame itself is built from mullions, the vertical members, and transoms, the horizontal members. Mullions are spaced roughly 4 to 6 feet apart on center, and a unitized panel typically spans one or two stories. Deeper mullion sections carry more load but push the sightline wider, so engineers trade stiffness against the view.
Why Non-Load-Bearing Matters
Because the wall carries no structural load, it can be light, thin, and nearly all glass. Architects wrap buildings in glazing without adding columns every few feet. The trade-off is that every movement of the frame must be absorbed by joints in the wall.
Movement Accommodation
Steel and concrete frames move with temperature changes, wind, and settlement. Curtain walls absorb that movement with slip joints, gaskets, and structural silicone. A well-designed system tolerates inter-story drift without cracking the glass or breaking the air and water seals.
| System | Assembly | Typical Use | Field Work |
|---|---|---|---|
| Stick-built | Mullions and glass assembled on site | Low-rise and budget projects | High |
| Unitized | Factory panels with glazing installed | High-rise towers | Low |
| Semi-unitized | Frame on site, glazing pre-assembled | Mid-rise buildings | Medium |
| Structural glazing | Glass bonded with silicone | Continuous glass facades | Medium |
Steel Curtain Walls and the Curtain Call
Aluminum dominates curtain wall framing, but steel holds a strong niche. Steel curtain walls that get the curtain call prove the point: steel members are stronger than aluminum, so sightlines run slimmer, spans run longer, and deflection stays lower. The price is weight and the need for corrosion protection.
When Steel Beats Aluminum
Steel earns its place in airport terminals, big-box retail, and any facade with spans too long for aluminum extrusions. Structural tube sections are fabricated off site, hot-dip galvanized, and painted. On site, the heavier members need a crane and careful sequencing, but the finished frame is stiff and precise.
Corrosion protection decides the service life. Hot-dip galvanizing coats every surface, including the inside of hollow sections, and a galvanized frame can last decades with touch-up painting at the welds and cut edges. Where coastal salt air is a factor, specify a thicker zinc coating or a duplex paint system over the galvanizing.
A steel curtain wall project follows a familiar sequence:
- Confirm the structural frame’s anchor points and allowable reactions.
- Fabricate mullions and transoms to measured shop drawings.
- Galvanize or coat every member before delivery.
- Set and align the framing with laser levels.
- Install glazing, pressure plates, and gaskets.
- Test a sample section for water and air infiltration.
Curtain Wall System Types
The choice of system comes down to building height, site access, and schedule. Curtain wall systems divide into stick-built, unitized, and semi-unitized approaches, each balancing factory work against field labor.
Stick-Built
Mullions and transoms arrive as individual pieces and are assembled on site, floor by floor. Material cost is low and shipping is compact, but field labor is high and quality depends on the crew’s skill with sealants and gaskets. Stick-built suits low-rise work where a crane cannot reach the whole facade.
Unitized
Factory-built panels arrive with glazing, gaskets, and seals already installed. A crane sets each panel and workers bolt it to its neighbor. Quality control moves to the factory floor, which is why tall towers and tight schedules favor unitized systems.
Pressure-Equalized Rainscreens
Modern unitized panels use a pressure-equalized cavity. The exterior gasket and the interior air seal create a space that matches outside pressure, so wind-driven rain cannot push through the drainage path. Water that enters drains out through weep holes at the base of each panel.
Semi-Unitized
The frame goes up stick by stick while the glass arrives in pre-assembled panels. Mid-rise buildings use this hybrid when a crane can reach the facade but factory glazing still saves time and reduces weather exposure on the job.
Costs move in the opposite direction of labor. Stick-built systems carry lower material and shipping costs but higher field labor; unitized systems shift the cost into the factory and the crane time. For a ten-story building, the unitized premium often pays back in schedule savings of a month or more.
Glazing, Insulation, and Thermal Performance
Energy performance depends on what sits between the mullions. Curtain walls combine glass, coatings, and framing to control heat flow, solar gain, and condensation, and each choice shifts the balance between daylight and energy demand.
Glazing Choices
- Insulating glass units: two or three panes with a sealed air or argon gap that cuts U-factor and condensation.
- Low-E coatings: reflect infrared heat while passing daylight, trimming cooling loads.
- Spectrally selective glass: admits visible light while rejecting much of the solar heat gain.
- Spandrel panels: opaque glazing that hides columns and slabs behind the vision glass.
Thermal Breaks and Condensation Control
Aluminum conducts heat readily, so the frame needs a thermal break: a polyamide or fiberglass strip separates the interior and exterior halves of each extrusion. Without it, cold frames condense moisture on the inside face, which stains finishes and invites mold.
Typical numbers put the stakes in perspective. A double-glazed unit with a low-E coating reaches a U-factor around 0.29, while a triple-glazed unit with argon drops toward 0.15. Each improvement cuts heating and cooling demand, but it also adds weight, which the mullions and anchors must carry.
Visible Light Transmittance
VLT measures how much daylight passes through the glass. High-VLT glass cuts lighting loads but raises glare and solar gain; low-VLT glass controls glare but darkens interiors. The right value balances daylighting against cooling demand, a decision best made with an energy model of the building.
Wind Loads and Structural Behavior
Wind is the dominant lateral load on a curtain wall. Curtain walls and wind loads are inseparable: the wall must move suction and positive pressure into the frame without exceeding deflection limits or opening the water seal.
Design Pressures
Engineers calculate design wind pressures from local codes, exposure category, building height, and shape. Corner zones and parapets see the highest suction, sometimes two to three times the pressure at the middle of the facade, so mullions near edges are often upsized.
Exposure matters as much as height. Open terrain with few obstructions puts a building in a higher exposure category, and the design pressure climbs accordingly. Two buildings of the same height can see wind loads that differ by a factor of two when one stands in open field and the other in dense downtown blocks.
Deflection Limits
Code and manufacturer limits typically cap mullion deflection at L/175 for wind and L/240 for thermal movement, where L is the span between anchors. Exceeding the limit risks glass breakage, gasket failure, and water leakage at the joints.
Testing Before Installation
Laboratories test full-size mock-ups for water penetration, air infiltration, and structural performance under static and cyclic wind loads. The mock-up is the last chance to catch a joint design flaw before it repeats across thousands of panels.
Design, Engineering, and Installation of High-Performance Enclosures
A curtain wall succeeds or fails in the first design decisions. The team must confirm the frame’s movements, match the system to the schedule, and specify glazing that meets energy targets, all before the first anchor bolt goes in.
For projects that demand maximum performance, the full sequence matters: design, engineering, and installation of high-performance non-load-bearing building enclosure systems spans shop drawings, thermal modeling, anchor design, and field quality control.
The Sequence That Delivers
- Performance targets define the system requirements.
- Structural engineering confirms frame reactions, deflections, and anchors.
- Shop drawings and thermal modeling lock the details.
- Mock-up testing verifies water, air, and structural performance.
- Installation follows the approved sequence with torque and sealant checks.
- Commissioning verifies the finished envelope before handover.
Performance targets get verified at the end. Field air-leakage tests on completed floors, water testing at the joints, and infrared scans for thermal breaks catch what the mock-up missed. The commissioning report becomes the record the owner keeps for the life of the building.
Every panel, sealant bead, and anchor bolt carries the same responsibility: keep weather out and let light in. A curtain wall meets that standard when design, engineering, and installation work as one process from the first drawing to the final inspection.