A partition meets the window line by landing on a mullion, the vertical aluminium member that divides an office facade into separate panels of glass. The wall stops at the mullion face, a closer piece bridges the gap, and a flexible seal fills the joint so the facade can move without cracking the wall or letting sound slip around the end of it.
That junction shapes more of a perimeter layout than its size suggests. Room widths along the glass follow the mullion spacing, and the blinds, perimeter air and ceiling trim all have to be planned around the new wall. The facade also belongs to the base building, so the fixing detail is usually something the building manager wants to see first.
What a Window Mullion Is and Why Partitions Land on It
A mullion is the vertical bar between two panes or panels of glass. On an office facade it is part of a framed system: mullions run vertically, transoms run horizontally, and together they hold the glazing. In a curtain wall the mullions carry the weight of the transoms and the glass, which makes them the stiff, continuous part of the window line.
That is why a partition lands on a mullion and never partway across a pane. The mullion gives the wall a solid face to close against and a straight vertical line to seal to. A wall that stops in the middle of a glass panel has nothing to hold its seal, it looks like an error from the street, and it leaves an open path between two rooms along the face of the glass.
Most aluminium facade mullions are hollow sections, which matters for acoustics further on. The rule for the layout is simple: every wall that runs to the perimeter needs a mullion at its end.
Setting Perimeter Room Widths to the Mullion Spacing
The mullions were set out when the building was designed, and they sit at fixed centres that a fitout cannot move. Any room that touches the glass therefore comes in whole mullion bays. A meeting room drawn at a convenient width on the space plan will grow or shrink on site to the nearest mullion, and a row of perimeter offices only divides evenly when each room is a whole number of bays.
When we price and set out perimeter walls on an office partitions scope, one of the first site tasks is measuring the mullion centres and checking them against the drawing. The spacing is not always uniform. Corner bays, columns standing behind the glass and wider panels at the ends of a facade run all break the rhythm, and each one changes where a wall can finish.
Sometimes a wall has to finish between mullions, because a room size is fixed by its use or by a door position further back. The usual answers are a short return that steps the wall across to the nearest mullion, or a purpose-made closure that seals against the glass itself where the building accepts one. Both cost more than moving the wall, and both are easier to agree on paper than on site.
The same survey picks up what sits in front of the mullions. A column cover, a deep sill or a perimeter duct can hold the wall off the mullion face, and that is far easier to handle before room sizes are fixed than when the studs are being cut.
Where a glazed side wall runs back from the facade to the corridor, we set out its glass partition panel sizes from the mullion position. That way the joints fall evenly and the last panel beside the door is not a thin offcut.
How the Partition to Window Mullion Detail Is Built
The detail closes the gap between the end of the wall and the mullion, seals it, and allows for movement. Facades are designed to take up thermal movement, wind sway and deflection of the slab edges they are fixed to. A partition screwed rigidly to a mullion ignores all of that, and the joint is where cracked paint or a torn seal shows up first.
On a plasterboard wall, we usually stop the end stud short of the mullion and close the remaining gap with an aluminium closer or a proprietary mullion trim. The closer carries a compressible gasket or closed-cell foam strip against the mullion face, and the visible joint is finished with a flexible, non-hardening sealant in place of a set compound. Nothing fixes into the glass, and fixings into the mullion itself are avoided unless the building has approved that detail.
It follows the same logic as the deflection head detail at the top of a plasterboard wall. The partition meets a moving part of the building through a joint that can slide, so the wall and the facade stay sealed to each other without being locked together.
A glass partition running to the facade needs a jamb section or a short plasterboard nib to receive the glass channel. The glass cannot absorb movement itself, so the flexible part of the joint has to sit in the jamb and its seal. Choosing between a jamb and a nib uses the same thinking as other glass partition corner and junction details, with the extra condition that the facade side cannot be drilled freely.
Stopping Sound Travelling Around the End of the Wall
A partition built to a good acoustic rating can still leak speech at the window line. Sound finds the gap between the wall end and the mullion and travels around the partition along the glass. Where a hollow mullion runs past both rooms, sound can also enter the profile on one side and come back out on the other.
The gap is the part a fitout controls directly. The closer, its gasket and a non-hardening sealant close the open air path, and for rooms where privacy matters the closer can be backed with acoustic infill. Facade acoustic suppliers also make purpose-built closers for this junction and inserts for hollow mullions, and whether those suit a job depends on the room and on what the building allows inside its facade.
For a perimeter meeting room or private office, the acoustic rating on the wall drawing only holds if the facade end is built to the same standard. We treat the mullion junction as part of the wall's acoustic build and price it with the wall, so it is never left as a trim added at the end.
Blinds, Perimeter Air and the Ceiling at the Glass
The window line is a crowded strip of any office floor, and a new partition is usually the last thing to arrive there. Roller blinds hang from brackets or sit in a pocket near the head of the glass. Air-conditioning supply is often directed at the facade from linear diffusers in the ceiling, and some floors run power and data in a perimeter duct at sill height.
Blinds are the usual clash. A single blind spanning two bays has to be replaced when a wall lands between them, which means separate blinds either side of the partition and brackets positioned to clear the closer. We set this out with the blind installer before the wall goes up, because recutting blinds to suit a finished wall is slower than ordering the right widths.
Perimeter air needs the same attention. Enclosing part of the perimeter zone to add a private office changes how that room heats and cools, and the diffusers above it often need rebalancing or moving. Up at ceiling level, the partition head also has to meet the ceiling perimeter detail along the facade, where a bulkhead, blind pocket or trim may already fix the line.
Condensation at the Cold Face of the Facade
On a winter morning the facade is the coldest surface inside the office. Aluminium frames carry that cold through to the room side, particularly where the frame has no thermal break. When warm, moist room air meets a surface below its dew point, water condenses on it.
A tightly closed pocket between a partition end and a cold mullion can hold room air against the metal where nobody sees it. It is not an issue on every floor, but where it happens it tends to show as marks on the paint or a damp edge at the base of the joint.
We choose the closer and seal so the facade's own seals and drainage paths are left as they were. Where a building has a history of condensation on its frames, we raise it with the building manager before the junction detail is settled.
Working with the Building on the Facade Interface
The facade is part of the base building. It keeps the weather out, the landlord maintains it, and anything fixed to it is usually a question for the building manager before it goes on the programme. Some buildings have a standard partition to mullion detail every tenant is asked to use, and some limit fixings to the mullion face or rule them out entirely.
We include the closer detail and fixing method with the rest of the fitout documentation submitted to the building. The detail then gets looked at early, and what we build on site matches what the building has already seen.
The same junction comes back at the end of a lease. When partitions come out during a defit and make good, the closer and sealant are removed and the mullion face has to be left clean, which is straightforward when the original detail never drilled the facade.
When new walls have to land on the window line, we survey the mullions first and build the closer, seal and blind setout into the partition price, so the junction is resolved before the first stud goes up.
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