Muntin.
A muntin is a structural bar inside a single window sash that holds individual panes of glass. A 6-over-6 colonial double-hung has 12 separate panes joined by muntin bars. True-divided-light (TDL) muntins are what the colonial originally used; each pane is a separately glazed unit of glass.
On a modern window, TDL is expensive because every pane is a separate seal and a separate IGU. The cheaper alternative is simulated-divided-light (SDL): one large IGU with muntin bars applied to the inside and outside faces of the glass, which reads as divided light without the cost.
Grille, grid, mullion.
Grille (or grid) is the catch-all term for any bar pattern across a window face that is not load-bearing on glass. Internal grilles sit between the panes inside the IGU, which is the cheapest option and the easiest to keep clean. External grilles attach to the outside or inside face of the glass and can be removed for cleaning. Neither is structural.
Mullion is the vertical structural member that joins two whole window units side by side, or that runs between the head and sill of a tall window to support intermediate transom bars. Mullions are sized for load. They are not decorative; they are how a window assembly carries the wind pressure on a large opening.
Common questions.
- What is the difference between a muntin and a mullion?
- A muntin divides panes of glass inside a single window sash. A mullion is a structural member that joins two separate window units or carries load between a head and sill. Mullions are bigger and load-bearing; muntins are smaller and divide glass within one sash.
- Are muntin grids functional or decorative?
- True-divided-light muntins are structural; they hold individual panes of glass. Simulated-divided-light muntins are decorative; they sit on the face of one large IGU and create the appearance of divided panes without the cost. Internal grilles sit inside the IGU and are purely cosmetic.
- Which grille style is easiest to maintain?
- Internal grilles, which sit between the IGU panes. They never need cleaning because the surfaces facing them are sealed inside the IGU. External and surface-applied grilles need cleaning around each bar.
Project in motion
Specifying a divided-light pattern?
We build TDL, SDL, and internal grille configurations across the window range. Send your design intent and we will quote the options.