Professional specifications, installation guide & design solutions
Interior muntin grilles create decorative patterns within sealed units. Authentic divided light appearance without maintenance challenges.. Modern, efficient design with excellent weather protection. Perfect for residential and commercial applications, these glass are specified for the application rather than chosen from a catalogue. Professional installation ensures optimal performance and longevity.
A muntin quotation that says only "with grilles" is not specific enough to price or to submit for approval. Three things have to be decided, and each one changes both the appearance and the thermal result: how the bar is built, how wide it is, and how many lights the sash is divided into.
True divided lites (TDL). The sash is genuinely divided, and each opening holds its own small sealed unit. This is what an original window did, and it is the only construction that reads correctly from every angle in every light. It is also the most expensive, and it carries a real thermal penalty: every small unit has its own perimeter seal, so a 6-over-6 sash has roughly six times the linear feet of spacer that a single-glazed equivalent would. Spacer perimeter is where insulated units lose most heat and where seal failure begins, so more of it means a worse whole-window U-factor and more edges to fail over time.
Simulated divided lites (SDL). One sealed unit, with bars bonded to the outer and inner glass surfaces and a matching spacer bar suspended inside the cavity, aligned with them. The internal bar is what makes it work — without it you see straight through the joint and the illusion collapses at an angle. Done properly with a shadow bar, SDL is close enough that most people cannot tell from the pavement, and it keeps the thermal performance of a normal insulated unit. This is the usual compromise on historic district work.
Grilles between the glass (GBG). The bars sit loose inside the cavity with nothing on either surface. The glass is flat and easy to clean, and it is the cheapest option by some margin. It also reads as flat, because there is no shadow line and no relief. On a street-facing elevation in a historic district this is the construction most likely to be refused, and it is worth knowing that before it appears on a quotation as the default.
Common manufactured widths run from about 5/8″ through 7/8″, 1″, 1-1/8″ and 1-1/4″, with heavier profiles around 2″ available on some lines. The narrow end reads as modern; the wider profiles suit nineteenth-century sash, where the bar was structural and had to hold glass.
Width is not a cosmetic detail on landmark work. It is one of the first things the reviewer measures against the original, and a 5/8″ bar in an opening that historically carried a 1-1/8″ bar changes the character of the facade even though the pattern is right. Where an original survives anywhere in the building, we measure it rather than picking from a catalogue.
Patterns are written as lights in the upper sash over lights in the lower: 6-over-6, 2-over-2, 6-over-1, 1-over-1, and less commonly 9-over-9 or 12-over-12. Prairie and diamond configurations exist for particular styles.
The sequence follows glassmaking history, which is why it is a reliable dating clue. Early nineteenth-century buildings carry many small lights because large panes could not be made affordably — 6-over-6 and 12-over-12 belong to Federal and Greek Revival work. As sheet glass improved through the middle of the century, panes got larger and the count dropped, giving the 2-over-2 that is so characteristic of Italianate rowhouses. By the late nineteenth century 1-over-1 was common. A 6-over-6 installed in an 1880s Italianate house is historically wrong even though it looks traditional, and that is the kind of mismatch a reviewer catches.
Muntins reduce the visible glass area, so a heavily divided sash admits less daylight and has a slightly lower solar heat gain than the same opening unglazed by bars. With TDL there is the additional edge effect described above. With SDL and GBG the thermal penalty is small, because there is still only one sealed unit and one spacer perimeter.
The practical consequence is that the historically correct choice and the thermally efficient choice are not always the same, and on landmark work the approval decides. Where a facade is not visible from the street — a rear elevation, a courtyard — there is usually more latitude, and it is worth asking rather than assuming the whole building must match.
Glass is supplied from the primary manufacturers and processed to your specification:
Tell us the coating, thickness and make-up you need, or describe the problem and we will specify it. Collection from Staten Island is available by appointment.
Proper installation is critical for optimal performance. Our licensed contractors provide:
Installation includes sealing and flashing the perimeter of the opening, because a window is only as watertight as the junction between it and the building. Where an opening needs more than that — a failed sill, damaged masonry, a detail that was wrong to begin with — see waterproofing.
Divided lites are the detail that carries the period character of a street. Change the pattern or the bar width and the building still functions, but the block reads differently. That is why muntin specification takes up so much of a landmarks review, and why the answer differs by district. What follows describes the character of each area rather than any specific project.
Designated on 14 August 1973 and extended in 2010, the district covers roughly 26 blocks and about 500 buildings, and was made a National Historic Landmark in 1978. The cast-iron loft facades were built around large openings and slender structural members — that was the point of iron construction. Divisions here tend to be sparse and the glass generous, so the specification question is usually about matching an industrial sash profile rather than reproducing a domestic 6-over-6.
The oldest surviving commercial fabric in the city, with Federal and Greek Revival counting houses and warehouses. Buildings of that period carry many small lights, and the bar profiles are heavier than anything a modern catalogue offers as standard. This is the area where true divided lites are most often genuinely required rather than preferred.
The rows around Washington Square are among the best-preserved Federal and Greek Revival houses in Manhattan. The 6-over-6 pattern with a substantial bar is characteristic, and the survival rate of original sash is high enough that a reviewer often has an intact neighbour to compare against. Getting the width wrong is more visible here than almost anywhere.
Prospect Park West and the streets running off Grand Army Plaza hold some of Brooklyn's most intact brownstone rows, mostly Italianate and neo-Grec. The dominant historic pattern is 2-over-2 with a wide bar, reflecting the larger panes available when they were built. Replacing a 2-over-2 with 1-over-1 to gain glass area is a common request and a common refusal, because the horizontal line across the facade is part of what makes the row read as a row.
Designated 25 September 1973, a compact district of roughly 134 rowhouses bounded by Carroll, President, Smith and Hoyt Streets, mostly built between 1869 and 1884 and set behind the deep front gardens the area is named for. Late Italianate and neo-Grec, with the same 2-over-2 vocabulary as Park Slope. We cover the approval process for this district in detail on our Carroll Gardens landmark window page.
The rows around Mount Morris Park and through Hamilton Heights and Sugar Hill are among the finest surviving in the city, and they span a wider period than the Brooklyn districts — Italianate through Romanesque Revival and Queen Anne. That variety means there is no single correct pattern for Harlem. The specification follows the individual building and its row, which makes surveying the existing sash more important here than working from a neighbourhood rule.
New York City's first designated historic district, and the deepest concentration of early nineteenth-century houses in the borough. Federal and Greek Revival stock with small-light patterns and heavy bars, alongside later Italianate work — often on the same street, which is exactly why pattern is decided building by building.
We measure the existing sash before quoting, including bar width and depth, meeting rail height and the reveal within the opening. Where an original survives elsewhere in the building we use that as the reference. Where the approval specifies a construction, we build to it rather than substituting a cheaper grille and hoping it passes.
We supply and install divided-lite units across Manhattan, Brooklyn, Queens, the Bronx and Staten Island, and throughout Northern New Jersey, the Jersey Shore, Philadelphia and the Lehigh Valley. If you have a landmarks approval in hand, or your architect is preparing one, send us the drawings and we will quote against what it actually requires.
Recent project: Landmark window replacement on the Upper West Side, Manhattan — Pella Reserve, matched to the approved divided-lite profile.
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