Parts of a Curtain Wall – Labeled Glass Wall Diagram

All 15 parts on the diagram

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From outside 4

Mullions The vertical members of the aluminum frame. They carry the glass and panels, and anchors fix them to the building.
Horizontal mullions (transoms) The horizontal members of the frame, fitted between the vertical mullions. Also called transoms, rails or horizontals.
Vision glass The transparent glass in front of the rooms. It is mostly insulating glass.
Spandrel panel Hides the edge of the floor and the space above the ceiling. An opaque panel that may be glass made opaque, metal or thin stone.

Across a mullion 5

Insulating glass unit Two panes of glass sealed round the edge with a space between. One unit meets the mullion from each side.
Gaskets The glass is held between an inner and an outer gasket. They seal it to the frame.
Pressure plate Clamps the edges of the glass in place. A bar fastened to the outside of the mullion. Also called a pressure bar.
Cover (cap) It fits over the pressure plate. It is the part of the frame you see from outside.
Thermal break A barrier of a material that carries heat poorly. It sits between the exterior metal and the interior metal so that less heat passes through the frame.

At a floor 6

Floor slab The concrete floor of the building. The curtain wall hangs in front of its edge and is anchored to it.
Anchor It attaches a mullion to the building. Through the anchors the loads on the curtain wall pass to the structure.
Firestop (fire safing) Slows fire and smoke passing from one floor to the next. It fills the gap between the slab edge and the back of the curtain wall.
Back pan A sheet of metal fixed and sealed to the frame. It is behind an opaque area. Insulation sits in front of it.
Setting blocks The glass's bottom edge rests on them inside the horizontal mullion.
Weep holes Small openings that let water drain to the outside. The water gets into the pocket under the glass.

How this list was compiled: Part names checked on 2026-09-30, a name kept only when two independent sources use it. (1) Whole Building Design Guide, Building Envelope Design Guide: Curtain Walls (National Institute of Building Sciences), read in the Internet Archive copy of 30 November 2023, http://web.archive.org/web/20231130015330/https://www.wbdg.org/guides-specifications/building-envelope-design-guide/fenestration-systems/curtain-walls (stick and unitized systems, vertical and horizontal mullions, vision glass, spandrel panels, insulating glass, pressure bars or pressure plates, gaskets, setting blocks, weep holes, thermal breaks, anchors at the slab edge, back pans, fire safing). (2) Wikipedia, Curtain wall (architecture) (mullions, horizontal mullions, infill and spandrel panels, IGUs, inner and outer gaskets, outer cap, thermal breaks, anchors, weep holes, firestop at the perimeter slab edge, back pan). (3) A curtain wall maker's architectural detail manual, https://www.kawneer.com/kawneer_files/shared%20files/97911-Arch_Manual/ADMD010EN.pdf (mullions, horizontals and transoms, vision and spandrel, insulating glass, pressure plates, covers, setting blocks, backpans, anchors). Licence: own work

The drawing: Drawn for this site on 2026-09-30 from plain geometry; simplified sections, no maker's profile, nothing traced. Licence: own work

How it works

The parts of a curtain wall shown here belong to the glass wall of an office building, not the curtain wall of a castle. It is a light frame of aluminum and glass hung on the outside of the floors.

Read the full guide

Outside first, then two cuts

From outside you see a grid: mullions, horizontal mullions, vision glass and spandrel panels. The cut across a mullion shows how the glass is held: gaskets each side, a pressure plate screwed on, a cover over it and a thermal break in between. The cut at a floor shows the anchor on the slab, the firestop in the gap and the back pan behind the spandrel.

In a stick system, the mullions and the glass are put up piece by piece on the building.
A curtain wall holds up only its own weight, and it passes the push of the wind to the building.

Questions and answers

What are the parts of a curtain wall called?

The frame is made of vertical mullions and horizontal mullions. It holds vision glass and spandrel panels, clamped by gaskets, pressure plates and covers. Anchors fix the mullions to the floor slabs, and a firestop fills the gap at each floor.

What is a mullion in a curtain wall?

A vertical member of the frame. The glass and panels are fixed to the mullions, and the mullions are attached to the building by anchors. The members that run across between them are horizontal mullions.

What is a spandrel panel?

The opaque panel in the band between the rows of vision glass, where the floor slab is. It can be glass that has been made opaque, a metal panel or thin stone, usually with insulation and a metal back pan behind it.

What does a pressure plate do?

It holds the glass in. The pressure plate is a bar fastened to the outside of a mullion, pressing on the edges of the glass through a gasket. A cover then goes on over it.

Is a curtain wall the same as the curtain wall of a castle?

No. On a castle it is the defensive wall between the towers. On a modern building it is a light outer wall, often of glass, that is fixed to the structure and holds up only its own weight.