Curtain Walls Are Non-Structural Exterior Cladding Systems
A curtain wall is an exterior cladding system that is hung from the structural frame rather than transferring building loads like a structural wall. The term "curtain" evokes the idea of a skin draped over the structure, carrying only its own weight and transferring wind and seismic loads to the structure at floor connections. Curtain walls can incorporate glass, metal panels, stone, or composite panels. The most common type for commercial buildings is the aluminum-framed glass curtain wall. The ARE PDD tests curtain wall systems as a key Division 08 assembly - architects must understand the types, water management approach, and coordination requirements.
Curtain Wall vs. Storefront vs. Window Wall
- Curtain wall: Spans multiple floors, attached to the structure at each floor. The glass and panels fill the entire exterior skin between floor levels. Water management via internal drainage channels. Highly engineered system typically supplied and installed by specialty subcontractors.
- Storefront: Typically single-story height (floor to underside of structure above). Common for retail entries and small commercial applications. Less complex water management. More commodity product with standardized framing profiles.
- Window wall: Spans between floor slabs (floor-to-floor) but uses a different attachment approach from curtain wall - typically supported on each floor slab rather than spanning past floor levels. More economical than curtain wall; used in mid-rise residential and commercial.
Stick-Built vs. Unitized Curtain Wall
Stick-built curtain wall: All framing components (mullions, transoms) are shipped to the site individually and assembled piece by piece from the interior or from exterior scaffolding. Glazing is set into the assembled frame. Stick-built is more economical on smaller or irregular projects and allows field adjustments.
Unitized curtain wall: Factory-fabricated panel units (typically one bay wide and one story tall) are shipped to the site and hung on structural connections in sequence. The panels interlock with each other and the slab edge clips. Unitized curtain wall is faster to install (reduces scaffold time and exposure of interior to weather) but requires a higher degree of dimensional precision in the structure because the panels are prefabricated.
Water Management: Drained vs. Pressure-Equalized
Modern curtain walls use a pressure-equalized rainscreen (PER) approach. The inner glass/panel is the air barrier; the outer face is the water shedding surface, but it is not required to be completely watertight because the cavities behind the outer face are drained. Water that penetrates the outer glazing gaskets runs into internal channels in the aluminum frame and drains down through weep holes at the base of each frame module. The internal air pressure in the cavity is equalized with outside air pressure, removing the driving force that pushes water inward.
Key Exam Points
- Curtain wall: non-structural skin; spans multiple floors; attached to structure at each floor.
- Storefront: single-story height, commodity product. Window wall: floor-to-floor, economical.
- Stick-built: site-assembled, flexible. Unitized: factory-prefabricated, faster install, requires structural precision.
- Pressure-equalized rainscreen: outer face sheds water; drainage channels at framing; weep holes at base.
- Performance testing: ASTM E283 (air), E330 (structural), E331 (water) - typically specified for pre-construction mockup.
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