WindowsGrids, Mullions, Panes, and Lites

September 11, 2020by jason0
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The structure of ordinary windows in your home can actually be quite complicated. Of course, the glass is the part we all think about when we imagine a window and that’s an important part. But there’s a lot more that goes into a window. In a very modern home, the windows may consist of almost purely glass but other styles of architecture often call for more “accessories” in their fenestration…or window selection. Breaking up the window into smaller panes is very common. There are quite a few ways to achieve this look. 

Traditional wooden windows would have had wooden dividers to make each piece of glass smaller than the size of the whole window. Before modern glass manufacturing, it was very difficult and expensive to make and transport larger sheets of glass. So you could bring your cost down to dividing that large window into smaller pieces or panes. These individual, framed pieces of glass can also be referred to as lites. Often there would be one or two larger lites and other parts of the window would be broken up into smaller panes and bordered with wood, called mullions. This required each piece of glass to be glazed, or sealed, into its surrounding wood pieces. Quality glazing can easily last a hundred years…but it does present a possible failure point. With traditional wood window panes, the wood surface of the dividing pieces is exposed on both the interior and exterior surfaces of the window, creating another element requiring maintenance…new paint at least every 8-10 years, and much sooner in harsh climates or direct afternoon sun. For all their cost and maintenance needs, these old-fashioned wood windows with wooden dividers are certainly beautiful and are a show of true craftsmanship. Often, the divisions which were made to save costs were used to create stunning geometric patterns…diamonds, complex borders, upper sash designs mirroring that of the lower sash.

There is a very practical and inexpensive modern equivalent to traditional paned wooden windows. The lites are not truly divided. Instead, the dividers are placed between the panes of dual-glazed glass. This approach is inexpensive because these days making large sheets of glass is very inexpensive and the system of grids is sandwiched between an interior and an exterior sheet of glass. Usually modern windows have at least two layers of glass, anyway, so tucking the mullions between the sheets of glass adds very little cost. And because the glass is not really divided into lites, the glass on both sides can be cleaned as single sheets of glass…no need to work around those mullions extending above the glass. This type of grid is most often used in windows made from vinyl. Vinyl can be a very practical and durable material for windows…and it’s not always a “cheap” option, for sure. But it is still the least expensive window frame material. And this is what the grids in a vinyl window are made from. They are color-matched vinyl. The width of the internal grid can be selected but within limits because the gap between the two panes of glass is only so wide so the internal mullions must be thinner than that. And for width, you can usually choose from two widths depending on the architectural look you’re trying to match. The disadvantage of windows with internal grids is really just the fact that those who know, can tell that they are the cheap option for mullions. It’s the equivalent of a t-shirt printed on the front like a tuxedo. Everyone knows it’s not real. From the curb you can easily see that the glass is one large sheet…and not individual divided lites.

Another modern take on window grids is called simulated divided lites. This type of grid still takes advantage of our modern ability to pour large sheets of glass inexpensively. And it also takes advantage of the high insulation value of a dual-glazed window. But the dividers are placed on the exterior of the glass and can match whatever material the rest of the window is made of whether it be wood, aluminum-clad wood, vinyl, Fibrex, etc. And from a distance you don’t see one uninterrupted sheet of glass. You see the mullions on the exterior of the window. Because there is a gap between the sheets of glass and mullions applied to both interior and exterior surfaces, you would normally be able to see the back side of the mullions from each side of the window, which would ruin the illusion of traditional wooden divided windows. To solve this problem, manufacturers include a (usually) metal bar between the sheets of glass which exactly matches the exterior mullions. If you look closely you can see that the mullion is not a single piece dividing the glass but from a distance the look is very convincing. Windows with simulated divided lites are definitely more expensive than internal grids. 

But the added cost for this type of grid varies with the cost of the material the rest of the window is made of. So, this type of grid on a quality vinyl window would not add as much cost as it would to an aluminum-clad window, for example. If you’re in the market for a more expensive Fibrex window, then your budget probably allows for this more expensive grid solution, which will also be a better fit with the architecture of the home you install those beautiful Fibrex windows in. As with most any aspect of choosing and buying windows, your project will determine your budget. If you’re working on a series of inexpensive apartments from the 1980s then you’ll probably be looking at vinyl windows and you’ll probably lean toward the less costly internal grids, which is a great choice. They’re durable and low-maintenance. See our article on glass and fill selections because you’ll want to be sure you get a window with high quality construction because a broken seal in a dual-glaze window is a real hassle and unwanted expense! And if you’re in the market for a higher-end window for your own home, whether it’s a historic property, new construction, or a quality remodel…then you’ll be looking for simulated divided lites or perhaps even true divided lites in full wood construction, but only if you’re in a situation where single-glazed windows are required by a historic committee. In any of these cases, though, you’ll already be in a position to have a budget suitable to the project and you’ll certainly be happy with the new window products you install!

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