A mullion is a vertical or horizontal member that divides and supports glass or panels within a window, door or curtain wall frame. It carries wind and dead loads from the infill back to the structure and gives intermediate support where two panes or leaves meet.
Strictly, a mullion is vertical and a horizontal divider is a transom, but curtain wall trades often say “vertical mullion” and “horizontal mullion”. Below: how mullions differ from transoms and muntins, the main types, materials and what to check on site.
Mullion vs transom vs muntin
| Term | Direction | What it does | Where you see it |
|---|---|---|---|
| Mullion | Vertical | Structural member between glass units, panels or window frames | Curtain wall, storefront, ganged windows, pairs of doors |
| Transom | Horizontal | Divides a frame horizontally; also the glazed panel above a door | Curtain wall horizontals, door frames with a transom lite |
| Muntin | Either | Small bars that divide the glass inside a single sash; mostly decorative today | Divided-lite and heritage windows (called glazing bars in the UK) |
Types of mullions
- Curtain wall mullions: extruded aluminum verticals that span floor to floor and anchor at each slab edge. In stick systems the glass is held by pressure plates and snap-on caps or structural silicone. In unitized systems each unit carries half a mullion, and the two halves interlock as a “split mullion” that allows movement.
- Storefront mullions: lighter sections, usually set on the floor and spanning to a head member, used for ground-floor glazing.
- Window mulls: the joint member when two or more window units are combined into one opening (“mulled windows”). Large openings may need a structural mull with reinforcement.
- Door mullions: the post between a pair of doors. A removable (often keyed) mullion lets each leaf have its own exit device and can be taken out to move equipment through the full opening.
- Stone mullions: the masonry dividers in historic “mullioned windows”.
What mullions are made of
Commercial mullions are mostly thermally broken aluminum. Long spans or heavy glass may need a steel stiffener inside the aluminum, or a steel mullion system, which is also common in fire-rated glazing. Residential windows use wood, vinyl or fiberglass mulls, and heritage buildings have stone or timber.
How a mullion carries load
Wind pushes on the glass, the glass pushes on the frame, and the mullion spans between its anchors like a small beam. Deflection often controls the mullion size more than strength, because too much bending cracks glass and breaks seals. Anchors at the slab edge carry the dead load and let the mullion slide vertically for thermal movement and floor deflection, which is why stack joints and slotted anchors matter.
Site and inspection tips
- Check mullion plumb, alignment and anchor locations against the shop drawings before glazing starts.
- Confirm which anchors are fixed and which are slotted. Slotted connections must not be welded or tightened solid.
- Inspect splices and stack joints for the specified gap and seal.
- Water test early, on the mockup and on installed work (for example a field test to AAMA 501.2).
- Protect finished caps from mortar, sealant and scratches.
Related terms
- Curtainwall: the non-load-bearing glass wall that mullions frame
- Spandrel Panel: the opaque infill between vision glass
- Low Iron glass
- Mockup
Related templates and guides
- Curtainwall Quality Control Checklist
- Interface Guidelines for Curtainwall
- Construction Safety Meeting: Glazing
- Basics of Building Envelope Construction
Frequently asked questions
What is a mullion in construction?
A mullion is the vertical member that divides and supports glass, panels or window units in a window, door or curtain wall. It takes wind and dead load from the infill and carries it back to the building structure through anchors at the floor slabs or framing.
What is the difference between a mullion and a muntin?
A mullion is a structural member between separate glass units or window frames. A muntin is a small bar that divides the glass within a single sash, mostly for appearance. In the UK, muntins are usually called glazing bars.
What is the difference between a mullion and a transom?
A mullion runs vertically and a transom runs horizontally. Together they form the grid of a curtain wall or window frame. Transom can also mean the glazed panel above a door, while curtain wall trades often call the horizontals ‘horizontal mullions’.
What is a removable mullion on a door?
It is the centre post between a pair of doors that can be unlocked and lifted out. With it in place, each leaf can latch to it and carry its own exit device. With it removed, the full opening is clear for moving equipment or furniture.
Are mullions structural?
Yes. Window and curtain wall mullions are designed for wind load and the weight of the glass, and their size is often set by deflection limits. They do not carry floor loads, but they must be anchored to the structure as shown on the engineered shop drawings.
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