Building components · Facade Support
Mullion
Explains the mullion as the primary structural member of a curtain wall, which is easy to read as a frame division.
Educational reference entry. What this element has to do, and whether any particular arrangement of it is adequate, depends on the whole assembly, the loading, the exposure, local requirements, the manufacturer's instructions and qualified professional design.
Overview
What a mullion is
In a curtain wall, mullions run vertically and are the members that span between floors. Wind pressure and suction on the glazing is collected by the mullions and delivered into the building structure at the floor levels.
They also carry the weight of the glazing and provide the drainage and pressure-equalisation channels that make a curtain wall weathertight. A mullion is therefore structural, weathering and drainage all in one extruded section.
Boundaries
How it differs from nearby elements
The distinctions that keep this entry from being a restatement of its neighbours.
- It is not a transom. Transoms run horizontally between mullions; mullions run vertically and span the structural distance.
- It is not a column. A mullion carries facade loads, not the building.
- It is not a window frame. A window frame sits within a wall opening; a mullion is a member of a facade that is itself the wall.
Terminology
Common names and aliases
These names describe the same element. The library keeps one entry per component so that a trade term or a regional name never becomes a second, thinner page.
- mullions
- vertical framing member
Dependencies
What it depends on
What has to be right elsewhere for this element to do its job.
- Its span between floors, which sets how much wind load it collects and how far it carries it.
- The brackets connecting it to the structure, which must allow for building movement and construction tolerance.
- The drainage channels within the section, which collect water passing the glazing seals.
- Thermal breaks within the section, since a continuous metal member from outside to inside would be a severe bridge.
Considerations
Topics that genuinely apply here
Subjects worth discussing about this element. These are discussion topics, never measured values, ratings or performance claims.
Mullions collect wind load across the facade and deliver it at discrete points into the frame, so those connections are as significant as the members.
Curtain walls are drained systems: water passing the outer seals is collected in the mullion and transom channels and discharged, rather than being kept out by seals alone.
A metal member spanning from outside to inside must have a thermal break, or it becomes both a heat path and a condensation surface.
The facade must accommodate movement in the building structure without loading the glazing, which is a function of the bracket connections.
Materials
Commonly formed from
Material families this element is commonly realised in. This states common practice only — it is not a statement that any of these materials suits this component in a particular building, which depends on the whole assembly and on qualified review.
Materials
Commonly incorporates
Materials this element commonly contains as a distinct constituent part, rather than being made of.
Assemblies
Building systems this element is met in
The role this component fills inside each assembly. A role describes what the element does there — it is not a statement that this component suits that system in any particular building.
Where it is met
Application contexts
Building contexts where this element is commonly encountered. Association is not recommendation: appearing here does not mean the component suits every instance of that context, or any particular one.
- Facade
Related elements
Neighbouring components
Typed relationships to other elements. There is deliberately no generic “related” category: an edge that cannot be classified does not exist.
Clarifications
Commonly misunderstood
- That curtain wall seals keep water out. They are the first stage; the drainage within the framing is what completes it.
- That mullions are frame divisions. They are the structural members of the facade.
Conversations
Questions for qualified professionals
Questions worth asking someone with the project in front of them. This page cannot answer them.
- What span do the mullions have, and how are they connected to the structure?
- How does the connection accommodate building movement and construction tolerance?
- How is water collected within the framing and discharged?
- What thermal break is within the section, and is it continuous?
Go deeper
Related Build Design Hub guides
Planning guidance behind the decisions this element involves.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this element comes up.
What this entry does not do
- No section, span, load, drainage arrangement or thermal value is stated here.
- Curtain walling is engineered for the specific building and wind loading and is designed by specialists.
Facade Support & Panels
The brackets, rails, subframes and framing members that hold a facade off a backing structure, and the panels they carry.
Browse all facade support entries →