A mullion is a structural vertical or horizontal member that separates and joins multiple individual window units together, such as when combining several window sections into a single larger window assembly, providing structural support at the connection between the separate units.
This structural function distinguishes a mullion from a muntin, which divides individual panes of glass within a single window sash rather than joining separate window units together.
Mullions carry genuine structural load in larger window assemblies, particularly in combination windows spanning significant width, making their sizing and construction a real engineering consideration rather than a purely decorative element.
What Does a Mullion Involve?
- A structural member joining separate window units
- Load-bearing capacity for combination window assemblies
- Weatherproofing at the connection between units
- Material matching the window frame construction
Why Are Mullions Important?
By providing structural connection between individual window units, mullions allow for larger, more complex window assemblies combining multiple sections, like a large picture window flanked by operable units, all functioning as one cohesive, structurally sound assembly. This structural role makes mullion sizing and construction a genuine engineering consideration for larger combination window installations.
Proper weatherproofing at mullion connections matters significantly, since these joints between separate window units represent potential water intrusion points requiring careful sealing and flashing integration, similar in principle to any other joint in a building’s exterior envelope. This structural role is worth understanding when planning any larger, multi-unit window installation. This understanding supports smarter planning for larger window installations. This understanding aids better planning. This detail matters. This matters greatly. Genuinely important detail here. Very much so indeed.
Mullion vs. Muntin
A mullion structurally joins separate window units together. A muntin divides individual glass panes within a single window sash, a decorative or historically structural element operating at a smaller scale than the load-bearing mullion.