spandrel

A spandrel is the structural panel or infill that bridges the gap between the top of one floor and the bottom of the next in a multi-story building. It sits in the exterior wall zone where the floor slab or beam ends, and the next level begins. This space must be closed off to prevent fire spread, air infiltration, and weather damage. In steel-frame construction, spandrels typically consist of metal studs, insulation, and cladding; in concrete buildings, they may be solid concrete or a combination of framing and panels.

The term "spandrel" comes from architectural vocabulary, where it originally meant the roughly triangular space between the curve of an arch and the rectangular frame around it. In modern building systems, the usage has shifted to mean the full horizontal band of wall that wraps each floor edge, even though the geometry is no longer triangular. The spandrel is functionally distinct from the story height windows and curtain wall panels above and below it: it is always opaque and always structural or semi-structural.

Materials and assembly

Metal spandrels in curtain wall systems are typically extruded aluminum frames with insulation batts (fiberglass or mineral wool) and a moisture barrier, often faced with aluminum composite panels, brick veneer, or metal sheathing on the exterior. In some systems, particularly older ones or those with corrugated plastic or metal soffits, the spandrel may be a simple sheet product bent and fastened to the frame. Fire rating is critical: most codes require a 1-hour to 2-hour fire-rated assembly at spandrel locations to block movement of flame between floors along the exterior.

Common failure points include poor air sealing at the junction between spandrel and curtain wall mullions, leading to water infiltration and condensation at the floor line. Thermal bridging through metal studs or fasteners can cause localized surface condensation and mold growth in cold climates. Movement accommodation is also essential: the floor slab expands and contracts slightly with temperature swings, and the spandrel assembly must allow this without cracking or separating from the glass wall above.

The spandrel assembly is often the last piece installed in the exterior wall sequence, closing gaps after the structural frame and floor edges are in place but before the curtain wall is hung. Proper sequencing and coordination between structural, waterproofing, and envelope trades is essential to avoid rework and to ensure that the final assembly performs its dual role as thermal break and fire barrier without compromising the watertightness of the building envelope.