What is a window mullion?

If you’re planning on replacing windows or patio doors, chances are you’ll hear the term “mullion” a lot from your window renovation contractor. But, what is a window mullion? 

Well, the answer is so simple that you’d probably want to smack your forehead. Let me explain. As you look out a window, often, your view gets blocked, albeit partially, by a vertical beam, right? This structural feature divides the window opening into smaller panes, enhancing both functionality and aesthetic appeal. We bet you noticed the vertical bars many times but didn’t know their exact name!

Basically, these vertical structures divide the glass window panes. The design element is especially connected with Gothic architecture – found in many Gothic cathedrals and residences.

The History and Evolution of Mullions

Mullions have a rich history, especially prominent in Gothic cathedrals, where large windows required additional structural support. Before the Victorian era, manufacturing large sheets of glass was impractical and expensive. Smaller panes of glass were joined using mullions, making them a practical solution for large windows.

Today, mullions are more about design than necessity. They add a touch of elegance to any space while contributing to the window’s overall energy efficiency and structural integrity.

What do mullioned windows look like?

Mullioned windows have a distinctive shape; they’re usually arched at the top. These gridded windows can be square or rectangular. And, the grids are made up of spaced glass panes instead of large window glass.

Do you know why these mullions came into existence? Again, the answer is simple. Before the Victorian era and the introduction of inexpensive glass panes, it was impractical or prohibitive to manufacture big sheets of glass. So the window installers made use of these vertical elements to join separate pieces of glass in a window or door. Today, their function is more decorative than functional. However, they do have their benefits. You’ll read about them below.

Importance of windows mullions

Understanding how the mullion is used necessitates some knowledge of its history. As mentioned above, the technology to produce big sheets of glass only came into posting in the Victorian Era. By dividing large window units into smaller sections, mullions help distribute the load, maintaining the integrity of the entire window frame. Before this, individual panes of glass – held together by supportive mullions.

Apart from adding an aesthetic value, mullions also serve to support the glazing of a window. Moreover, they are an essential part of sustaining the structural integrity of a window arch. In most window installations, these window mullions are coupled with transoms for further stability.

Additionally, mullions contribute to energy efficiency by reducing the size of individual glass panes, which helps with insulation. Materials including wood, metal, and vinyl are commonly used for mullions, depending on the window style and overall design goals.

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Difference between a mullion and a transom

There’s a significant difference between what is a window mullion and what a window transom is. Mullions are vertical crossbars that run through a door or a window while a transom is usually a horizontal structural beam or bar. For example, in windows and doors, a transom might separate a door from the small window above it, often adding a decorative and functional element. Simply put, a transom window is a small window above a door (usually a front door) or a window.

Repairing and replacing window mullions

Unfortunately, replacing or repairing a window mullion is not as simple as it looks. This is due to the fact that it’s almost impossible to keep the glass panes in place when replacing the mullions.

It’s best to hire a professional to help you repair the window wood mullion. So, unless you’re a competent and skilled DIYer with the right tools, you shouldn’t attempt such an extremely delicate process. You may end up causing some serious harm to your window. 

Read more: Things when planning window replacement

Window mullion replacement cost

If you’re really looking to update your old windows, it’s better if you replace the windows entirely. 

The professional you hire to work on your mullioned windows will recommend you the best course of action. Generally speaking, replacing windows or door will cost you between $200 and $500. A full window replacement, on the other hand, will cost you anywhere from $2,000 to $3,800. Of course, the prices vary according to the type of window you want, its wood species, the design, and accessibility.

Planning for window installations? Get the best estimate with us.

Key takeaway

Window mullions are more than just decorative features—they’re essential for structural support, energy efficiency, and maintaining the architectural integrity of windows and doors. Whether you’re inspired by the grandeur of Gothic cathedrals or planning to enhance your home’s aesthetic appeal with smaller panes. Mullions play a crucial role in window design. Understanding their function and history helps you make informed decisions when planning your next window installation or replacement.

Read more: A home window installation guide

Written by Ramona Sinha. April 24, 2022

Ramona is the Senior Content Writer for Kukun. This experienced blogger uses simple and succinct words to decipher the complex phenomenon called life. She has written articles covering a broad range of topics, such as real estate, lending and mortgage, finance, business, taxation, home designs, home improvement projects, decor concepts, and more. An avid traveler, she’s a digital nomad at heart and an animal lover from the depth of her soul.