Curtain Walls & Glazing Systems

A curtain wall is a non-structural exterior wall system hung on the building's structural frame like a curtain. It does not carry any building loads other than its own weight and the wind and seismic forces acting directly on it. Curtain walls are the defining feature of modern commercial architecture — the glass-and-metal facades of office towers, airports, hospitals, and institutional buildings. Properly designed and installed, a curtain wall provides weather protection, thermal performance, natural light, and architectural expression. Improperly installed, it becomes a source of leaks, energy loss, condensation, and occupant discomfort.

Stick-Built Curtain Wall

In a stick-built system, the curtain wall is assembled piece by piece on the building:

1. Vertical mullions (aluminum extrusions, typically 2.5"–7" deep) are attached to the building structure at each floor
2. Horizontal mullions (transoms) are connected between the vertical mullions
3. Glass panels (insulated glass units, IGUs) and opaque panels (spandrel panels) are installed into the mullion grid from the exterior using glazing gaskets or structural silicone

Advantages: Flexibility for complex geometries; no large factory or staging area required; components ship compactly. Disadvantages: Slower installation (each piece installed individually at height); weather-dependent; more field labor; quality depends on field workmanship.

Stick-built systems are common for low-rise and mid-rise buildings (up to about 10–15 stories) and for buildings with complex or varying facade geometries.

Unitized Curtain Wall

In a unitized system, the curtain wall is pre-assembled in a factory as large panels (typically one story tall × one bay wide, 5'–7' wide × 10'–14' tall). The complete panel — mullions, glass, gaskets, insulation — ships to the site as a unit and is hung on the building structure by crane.

Advantages: Fastest installation (panels are hung in minutes); highest quality (factory assembly); reduced weather dependency; excellent for high-rise construction (panels can be installed from inside the building using a mini-crane or hoist). Disadvantages: Requires significant factory space and lead time; transportation of large panels; less adaptable to complex geometries; higher initial cost (offset by faster installation on tall buildings).

Unitized systems are standard for high-rise buildings (10+ stories) and are increasingly used on mid-rise projects where speed is critical.

Curtain walls must satisfy multiple performance criteria simultaneously:

Structural: Resist wind loads (positive pressure and negative suction) without excessive deflection. Mullion deflection limits are typically L/175 or 3/4" maximum (whichever is less) under design wind load.

Water resistance: Prevent rainwater from entering the building. Curtain walls manage water using the rainscreen principle — an outer surface (glass and mullion caps) sheds most water, a pressure-equalized cavity behind the outer surface prevents wind-driven rain from being pushed inward, and an inner air and water barrier (the back of the mullion) provides the final seal. Weep holes at the base of each glass pocket drain any water that enters the cavity.

Air infiltration: Limit uncontrolled air leakage through the wall. Performance is measured in CFM per square foot of wall area at a specified pressure (ASTM E283). Maximum allowable infiltration is typically 0.06 CFM/ft² at 6.24 psf.

Thermal performance: Minimize heat transfer through the wall. Overall wall U-factor includes the glass, the mullions (which are significant thermal bridges), and the spandrel areas. Thermally broken mullions (with a plastic or rubber isolator separating the interior and exterior aluminum) are required in cold climates to prevent condensation on the interior mullion surface.

Condensation resistance: Prevent moisture from condensing on interior surfaces. The curtain wall's Condensation Resistance Factor (CRF) must be adequate for the building's interior humidity and exterior temperature conditions.

Modern curtain wall glazing uses sophisticated glass technologies:

• Insulated glass units (IGUs): Two or three panes of glass with hermetically sealed air or gas-filled spaces. Standard for all commercial curtain walls.
• Low-E coatings: Metallic coatings that reduce radiative heat transfer. Solar-control Low-E (on the #2 surface, outer pane inner face) reflects solar heat; low-E for cold climates (on the #3 surface, inner pane outer face) retains interior heat.
• Tinted glass: Glass with color additives (bronze, gray, green, blue) that absorb solar energy. Reduces solar heat gain and glare.
• Reflective glass: Glass with a metallic reflective coating for maximum solar control. Creates a mirror-like exterior appearance.
• Laminated glass: Two or more layers of glass bonded with a plastic interlayer (PVB or SGP). Holds together when broken (safety glazing). Required for overhead glazing, impact-resistant glazing (hurricane zones), and blast-resistant glazing.
• Tempered glass: Heat-treated glass that is 4–5 times stronger than annealed (regular) glass and breaks into small, relatively harmless pieces. Required by code for certain locations (doors, sidelights, low panels).
• Spandrel glass: Opaque glass (with ceramic frit or opaque coating on the back surface) used at floor slab and beam locations to conceal the building structure. The space behind spandrel glass is insulated.

• Storefront: A glazing system for ground-floor commercial applications (retail, restaurant, lobby entries). Typically non-thermally-broken aluminum frames with single or insulated glass. Not designed for high-rise wind loads.
• Window wall: A hybrid between a curtain wall and a punched window. The window wall sits on the floor slab edge (bearing on the structure) rather than hanging in front of it. Common for residential high-rises.

Stick-built installation sequence:

1. Surveying and layout: Establish control lines for mullion alignment
2. Anchor installation: Attach steel embed plates or brackets to the building structure at each mullion location
3. Vertical mullion installation: Erect and align vertical mullions
4. Horizontal transom installation: Connect transoms between verticals
5. Glazing: Install glass panels from the exterior, typically from a suspended scaffold (swing stage) or boom lift
6. Sealing: Apply perimeter sealant, install pressure plates and mullion caps

Unitized panel installation sequence:

1. Install anchors/brackets on the building structure
2. Hoist panels to the installation floor (via crane, building hoist, or mini-crane)
3. Hang panels from the anchors and engage interlocking joints with adjacent panels
4. Verify alignment and seal stack joints (horizontal joints between panels at floor levels)