A Guide to the Application and Selection of Grilles, Muntins, and Mullions in Door and Window Design

For architects, contractors, and developers sourcing aluminum doors and windows: if you confuse grilles with muntins, or muntins with mullions, you will end up with the wrong quote, the wrong structural calculation, and often the wrong aesthetic. This guide explains how each element works, when to use it, and what to specify.

Three Different Names, Three Different Jobs

In aluminum window and door design, grilles, muntins, and mullions are three completely different components, even though the words are often mixed up in casual conversation.

• Grilles are decorative bars applied to one side of the glass or sealed inside an insulating glass unit (IGU). They give the visual impression of divided panes without actually cutting the glass into smaller pieces.
• Muntins are real dividers that split the glazing into physically separate lites. In traditional joinery they are wood or metal bars fitted into the sash. In modern aluminum systems, true divided lites are rare because they hurt thermal performance.
• Mullions are structural vertical members that separate two independent window or door units. They carry wind loads, transfer deflection forces, and join frames in large openings.

Knowing which one your project actually needs changes the glass specification, the frame engineering, the price, and the long-term maintenance.

Grilles: Decorative Grid Options That Do Not Split the Glass

Grilles exist only for appearance. They let a single large sheet of glass look like a Georgian window, a French door, or a traditional casement without sacrificing the thermal and acoustic benefits of modern glazing.

We produce three grille formats for aluminum doors and windows:

• Between-the-glass grilles. Sealed inside the IGU cavity, these bars sit between the panes. They never collect dust, never need painting, and cannot be damaged during cleaning. The cavity width determines the grille depth we can use.
• Surface-mounted exterior grilles. Bonded or mechanically clipped to the outside face of the glass. They create a stronger shadow line and a more authentic divided-lite look, but they need the same corrosion-resistant finish as the frame.
• Simulated divided lite (SDL) bars. A surface bar on the outside plus a small spacer bar on the inside, giving the illusion of a real muntin. This is the most popular compromise for residential and hospitality projects.

Common grille bar widths are 8 mm, 18 mm, 25 mm, and 38 mm. Width choice changes the character of the window: 8 mm looks refined and modern, 38 mm looks heavy and colonial. The spacing pattern is usually specified as an even grid or as a custom colonial pattern. We machine the mounting clips from 6063-T5 aluminum and powder-coat them with the same lot as the frame so the color match is consistent.

Muntins: When the Divider Is Real

A muntin is a real structural divider. In a true divided-lite (TDL) window, the sash is divided into separate small panes of glass, each held by its own frame and glazing bead. This was necessary in the age of hand-blown glass, but today it is usually a deliberate aesthetic or heritage choice.

The trade-offs of real muntins are significant:

• Thermal performance drops. Every muntin is a thermal bridge through the glass pack unless it is thermally broken, which is expensive and visually bulky.
• Glazing thickness is limited. Small lites cannot use thick laminated or triple glazing, so TDL is rarely used in high-performance or acoustic projects.
• Cost rises. More glass cuts, more glazing beads, more seals, and more labor. A TDL window can cost 40–70% more than the same size with grilles.

We still supply TDL muntins for restoration projects, listed buildings, and clients who want the authentic divided-pane feel. For aluminum systems, we typically use a 20 mm × 25 mm extruded aluminum bar with an integrated glazing bead, matched to the frame finish. If the project needs heritage approval, we can also supply slim steel-look bars with thermal isolation breaks.

Mullions: The Structural Member Between Units

A mullion is not decoration. It is the vertical frame member that joins two separate window or door units into one larger opening. In a two-panel sliding window, the vertical frame between the fixed and sliding sash is a mullion. In a four-panel curtain wall module, the vertical millions carry the dead load of the glass above and transfer wind loads to the building structure.

This is where engineering matters. A mullion must be sized for:

• Wind load. Project-specific pressure based on building height, exposure category, and local code. We typically design to a deflection limit of L/175 for standard projects and L/200 for high-rise or impact-rated work.
• Dead load. The weight of the glass, frame, and any hardware attached to the member.
• Thermal movement. Aluminum expands roughly 0.024 mm per meter per degree Celsius. Mullion joints must allow for this movement without leaking or binding.

For standard aluminum windows and doors we use 6063-T5 extrusions. For heavy structural mullions in curtain walls or large facades, we switch to 6061-T6 for its higher strength. Every mullion receives a PA66 GF25 thermal break and EPDM seals before assembly.

How Grilles, Muntins, and Mullions Affect Performance

The choice between grilles and muntins changes more than appearance.

Energy efficiency. A plain IGU with low-E coating and argon fill performs best. Between-the-glass grilles have almost no effect because they sit inside the sealed cavity. Surface grilles cast minor shadows but do not change U-value. Muntins create real thermal bridges and reduce the available glazing thickness.

Cleaning and maintenance. Between-the-glass grilles are maintenance-free. Surface grilles need the same occasional cleaning as the frame. Real muntins have many small edges where dirt collects.

Security. Grilles do not improve security. Some surface grille systems can be pried off if they are only taped. If security bars are needed, specify a separate security grille system with tamper-proof fixings.

Sound insulation. Grilles have no measurable acoustic effect. Muntins break large panes into smaller ones, which slightly changes vibration modes but is rarely the deciding factor.

Material and Finish Choices for Aluminum Systems

Most grille and muntin bars in our aluminum systems are extruded from 6063-T5 alloy. This gives a clean surface for powder coating and enough strength for normal bar lengths. For mullions that carry larger loads, we move to 6061-T6 and sometimes reinforce with internal steel plates.

Finish options include:

• Polyester powder coat. Most common. RAL color matched to the frame. Outdoor durability 10–15 years in normal environments, 5–8 years in aggressive coastal conditions.
• Anodizing. Best for metallic colors and coastal corrosion resistance. We recommend 15 µm anodizing for exterior mullions in marine atmospheres.
• Wood grain thermal transfer. A polyester base coat with vacuum heat-transfer film at 180–200 °C. Used to make aluminum grilles and muntins match timber-clad facades without the maintenance of real wood.

When grilles are clipped rather than bonded, the clips must be the same alloy and finish as the bar. Dissimilar metals set up galvanic corrosion, especially in humid or coastal climates.

Three Project Examples That Show the Difference

Caribbean residence, Guyana territory. The client wanted a French-door look for the ground-floor living room but needed aluminum for termite and salt-air resistance. We supplied aluminum French casement doors with between-the-glass grilles in a 25 mm bar width, finished in matte black RAL 9005. The grilles give the colonial aesthetic; the single IGU behind them gives the thermal performance required in that climate.

Bulgarian mountain lodge, Europe. A restoration project required the new windows to read as true divided panes from the exterior for heritage review. We used simulated divided lites with exterior 18 mm bars and interior shadow bars, installed on triple-glazed thermal break aluminum casement windows. The exterior reads historically; the interior is clean and high-performing.

Maldives boutique hotel. The facade design used four tall aluminum window units stacked vertically to create one 6-meter-high glazed wall. The vertical mullion between units had to carry the dead load of four glass panels and resist wind pressure from the open ocean. We sized the mullion in 6061-T6, added drainage paths, and used marine-grade EPDM seals. The grilles inside the units were between-the-glass, so housekeeping could wipe the glass without navigating bars.

Specification Checklist for Architects and Buyers

When you send a grille, muntin, or mullion requirement, include at least these items:

• Which element you actually need. Grille, muntin, or mullion. Use the definitions above if the drawings are unclear.
• Bar width and depth. 8 mm, 18 mm, 25 mm, or custom.
• Mounting method. Between-the-glass, surface-mounted clipped, or true divided lite.
• Grid pattern. Equal divisions, colonial pattern, or custom layout. A sketch is better than words.
• Finish and color. RAL code, anodizing class, or wood grain reference. Match it to the frame.
• Performance requirements. U-value, wind load, acoustic rating, and any impact or security standard.
• Mullion structural loads. For mullions, provide the opening dimensions, glass weight, and design wind pressure so we can size the member correctly.

Common Mistakes to Avoid

• Specifying “mullion” when you mean decorative grille. Mullions are structural and expensive. Grilles are cosmetic and relatively cheap. Confusing the two can double the budget on a large facade.
• Using true divided lites where between-the-glass grilles would work. Unless heritage approval requires it, TDL adds cost and thermal bridges for no functional benefit.
• Forgetting thermal expansion. Long mullions in direct sunlight can move several millimeters. Joints and sealant must be designed for that movement.
• Mismatching finish lots. Grille bars and frame extrusions coated in different batches can show visible color variation under certain light. We always coat them in the same lot when they are part of the same order.
• Ignoring cleaning access. Surface-mounted grilles on high-rise windows make cleaning more difficult. Between-the-glass grilles are usually the safer choice for tall buildings.

Final Takeaway

Grilles, muntins, and mullions are not interchangeable. Grilles give you the look of divided glass without the cost and thermal penalty. Muntins divide the glass for real, usually for heritage or high-end authenticity. Mullions carry loads and join separate units into larger openings. Specify the right one, and your facade will look correct, perform correctly, and cost what it should.

For aluminum windows, aluminum window systems, aluminum sliding windows, thermal break casement windows, aluminum sliding doors, and glass curtain walls, contact FRAME’s export team for drawings, samples, and a project-specific quotation.

— Written by the FRAME technical team. Last updated August 2026.