A mullion is the vertical structural member that divides adjacent window units or facade panels and carries their weight and wind load down to the building structure. In a curtain wall or window, mullions run top-to-bottom as the primary load-bearing bars, while horizontal members called transoms span between them, together forming the rectangular grid that holds glass, spandrel or infill panels in place. If you have ever looked at a glazed office tower and noticed the slim vertical lines running the full height of the glass, those are the mullions.
The word 'mullion' comes from the medieval stone bars that split arched windows into lights, but in modern construction the mullion is almost always an extruded aluminium, steel or uPVC profile engineered to precise structural and thermal tolerances. In a facade, the mullion is the single most important component: it transfers dead load, wind load and seismic movement to the floor slabs, defines the sightline width a viewer sees, and houses the glazing pockets, gaskets and drainage channels that keep water out.
This guide explains what a mullion does, how it differs from a transom, the materials and sizes used, the Indian standards that govern its design, and how thermal performance affects your energy bill. Hakimi Aluminium and Glass designs and installs engineered curtain wall glazing and structural glazing systems across Hyderabad, Secunderabad and the wider Telangana and Andhra Pradesh region.
What Is a Mullion vs a Transom?
A mullion is vertical and a transom is horizontal; the mullion is the primary load-carrying member and the transom is the secondary member that spans between mullions. The two work as a team: mullions take the wind and dead load, transoms subdivide the openings and support the horizontal edges of each glass unit.
In a stick-built curtain wall, mullions are installed first as continuous vertical runs anchored to the slab edges, then transoms are cut and fixed between them to complete the grid before glazing begins.
- Mullion: vertical, primary structural bar, carries wind and dead load to the slab.
- Transom: horizontal, secondary bar, subdivides the opening and supports glass edges.
- Both are usually the same extruded profile family so sightlines and gaskets stay consistent.
- Sightline (the visible face width) commonly ranges from 50 mm to 65 mm on commercial systems.
The same logic applies at a smaller scale in aluminium doors and windows: a large sliding or fixed unit is split by mullions and transoms so each glass panel stays within a safe, manageable size.
What Are Mullions Made Of?
Modern mullions are most often extruded from 6063-T5 or 6063-T6 aluminium alloy because of its excellent strength-to-weight ratio, corrosion resistance and formability into complex hollow sections. Aluminium is the default choice for almost every commercial facade in Hyderabad.
- Aluminium curtain wall mullion: 50-70 mm face width, 100-250 mm depth, 2-3 mm wall thickness.
- Window mullion (residential): 45-60 mm face width, 45-100 mm depth.
- Steel mullions: used for very tall or high-load spans, atriums and long unsupported runs where aluminium deflection would be excessive.
- uPVC mullions: used in some residential windows for thermal performance and low cost, though with lower structural spans.
- Finishes include powder coating (typically 60-80 microns) or anodising (15-25 microns) for weather durability against Hyderabad heat and monsoon.
The choice of alloy and finish matters for longevity. In the dusty, high-UV conditions common across Gachibowli and the Financial District, a good powder coat or anodised finish protects the mullion and keeps sightlines crisp for decades. For projects that mix glass with metal panels, mullions are often coordinated with ACP cladding so the vertical lines of the glazing and the cladding align cleanly.
What Size Is a Curtain Wall Mullion?
A typical aluminium curtain wall mullion is 50-70 mm wide on the visible face and 100-250 mm deep, with a wall thickness of 2-3 mm. The face width controls how slim the facade looks, while the depth controls how much load the mullion can resist.
- Depth increases with floor-to-floor height and wind zone, because a deeper section has a higher moment of inertia and resists more bending.
- A typical 3.6 m to 4.2 m floor height in a Hyderabad office tower needs a mullion around 150-200 mm deep.
- Very tall lobbies and double-height showrooms may need 250 mm or deeper sections, or a steel-reinforced mullion.
- Split mullions (two halves that interlock) are used in unitized glazing so factory-made panels can slot together on site.
The glass held by these mullions is usually toughened or laminated safety glass conforming to IS 2553. For frameless or minimal-frame looks, the visible mullion can be reduced dramatically using spider glazing or structural silicone, where the glass itself becomes part of the visual surface.
How Does a Mullion Carry Wind Load?
A mullion's primary job is to resist wind load and limit deflection so the glass and seals are never overstressed. Design wind pressure in India is calculated under IS 875 Part 3, and the mullion's moment of inertia is sized to keep deflection within allowable limits under that pressure.
- Deflection is typically limited to span/175 or 20 mm, whichever is smaller, for glazed curtain walls.
- The National Building Code of India (NBC) 2016 governs overall structural and safety requirements for facades.
- Hyderabad falls in a moderate wind zone (basic wind speed around 44 m/s), but tall towers in Kokapet and the Financial District still require mullions engineered for local gust and terrain factors.
- Split or shear-block mullions accommodate thermal expansion and inter-storey seismic movement between floors.
- Structural silicone glazing joints are designed per ASTM C1401, the guide for structural sealant glazing.
Getting this engineering right is not optional. An undersized mullion flexes too far in a storm, cracks sealant, and lets water in. This is why independent facade consultancy and proper structural calculation is worth the modest upfront cost on any mid-rise or high-rise project.
Do Mullions Affect Energy Bills?
Yes. A thermally broken mullion uses an insulating polyamide or polyurethane barrier between the inside and outside aluminium sections to stop heat transfer and condensation. In Hyderabad's hot climate this directly reduces air-conditioning load and electricity cost.
- Non-broken aluminium frame U-value: roughly 5.0-6.5 W/m2K.
- Thermally broken mullion frame U-value: roughly 1.8-3.0 W/m2K.
- Thermal breaks combined with low-E double glazing (DGU) cut solar heat gain across west-facing facades in Madhapur and Hitec City.
- ECBC sets maximum U-value and Solar Heat Gain Coefficient (SHGC) limits for commercial building envelopes.
- Pressure-equalised, drained-and-vented mullion cavities manage rainwater during the monsoon.
For office fit-outs, pairing thermally broken mullions with a high-performance DGU facade is one of the most cost-effective ways to hit ECBC compliance and improve occupant comfort. Interior spaces benefit too, where glass partitions with slim mullions keep daylight flowing without the heat penalty of an exterior wall.
What Are the Types of Mullion Systems?
Mullions are classified by how the facade is assembled and how the glass is captured. The two main assembly methods are stick-built and unitised, and within each the glass can be captured, structurally bonded or point-fixed.
- Stick-built: mullions and transoms erected piece-by-piece on site; lower cost, common for low- to mid-rise across Kondapur and Kukatpally. See stick glazing.
- Unitised: factory-assembled storey-high panels with interlocking mullions; faster erection and higher quality control for high-rise, via unitized curtain wall systems.
- Captured/capped: an aluminium pressure plate and cap hold the glass mechanically on all edges, using pressure plate glazing.
- Structural glazing: glass bonded to the mullion with structural silicone for a flush, frameless look.
- Spider/point-fixed: glass fixed by bolted spider fittings, with mullions replaced by tension rods or glass fins.
Choosing between these depends on building height, budget and the architectural look you want. You can see a range of completed mullion, curtain wall and facade systems in our project gallery.
How Much Does a Mullion Facade Cost in Hyderabad?
Curtain wall and mullion-based facade systems in Hyderabad are typically priced per square metre of facade area, and the mullion specification is a major cost driver. As a broad 2026 guide, expect the following installed ranges:
- Basic stick-built captured curtain wall with single glazing: around Rs 3,000-4,500 per sq ft.
- Structural glazing with DGU and thermally broken mullions: around Rs 5,000-8,000 per sq ft.
- Unitised curtain wall for high-rise towers: around Rs 7,000-11,000 per sq ft.
- Spider and point-fixed glazing for lobbies and showrooms: around Rs 8,000-14,000 per sq ft.
Prices vary with glass make (Saint-Gobain, AIS, Guardian), aluminium system (indigenous vs imported like Schuco or Reynaers), wind load, height and finish. Deeper mullions, thermal breaks and imported systems push the cost toward the top of each band. For an accurate figure on your building, the best step is a site measurement and engineered quote.
How Do You Maintain Aluminium Mullions?
A well-specified aluminium mullion system has a service life of 25-40 years with minimal maintenance, which is one of its biggest advantages over timber or unprotected steel. Most upkeep is simple cleaning and periodic sealant inspection.
- Wash the mullion faces twice a year to remove Hyderabad's construction and road dust, which is abrasive on coatings.
- Inspect structural and weather silicone joints every 2-3 years for cracking, chalking or loss of adhesion.
- Clear the drainage weep holes at transom junctions before each monsoon so trapped water can escape.
- Check gaskets and pressure-plate screws for movement after any major storm.
- Re-coat or re-seal locally rather than replacing whole runs; a good powder coat rarely needs full renewal within its design life.
If you are unsure whether an ageing facade needs resealing or replacement, or you are planning a new build, get a free quote and site assessment from our team and we will advise on the right mullion system, glass and finish for your location and budget.
