You’re browsing for new windows and come across “what are mullion windows” in your search. What exactly are these windows and what makes them different? As you research window options for your home, it’s important to understand the key features and benefits of different styles. Mullion windows have a distinctive look that can add visual interest and architectural flair. But they also provide practical advantages. 

This article will walk you through everything you need to know about mullion windows – from their unique multi-pane design to considerations for installation and energy efficiency. You’ll learn how they can bring style and functionality to your home. With the right background knowledge, you’ll be prepared to decide if mullion windows are the right choice to bring more light, beauty, and character into your rooms. So read on to become a mullion window expert.

What Exactly Are Mullion Windows?

Mullion windows, also known as divided light windows, are windows that are separated into smaller panes by horizontal and vertical bars. The bars that divide the window into sections are called mullions. Mullion windows date back to the 14th century and were originally used in churches, cathedrals, and other large buildings. Today, mullion windows are still used in modern architecture and home design.

Benefits of Mullion Windows

There are several benefits to using mullion windows in architecture and home design. Mullion windows provide architectural interest and can enhance the aesthetic appeal of a building’s facade. The divided panes also give the windows a traditional look that is reminiscent of historic architecture. In addition to their visual appeal, mullion windows offer practical benefits. The smaller panes make the windows more resistant to damage – if one pane breaks, the others remain intact. The mullions also provide additional structural support to large window openings.

Mullion Window Materials and Styles

Mullion windows are available in a variety of materials, including wood, aluminum, vinyl, and fiberglass. Wood mullion windows provide an authentic historic appearance but require frequent maintenance. Aluminum, vinyl, and fiberglass mullion windows are low-maintenance and energy-efficient alternatives. Mullion windows come in a range of styles, including double-hung, casement, awning, and fixed windows. Double-hung and casement mullion windows allow for ventilation since the window sashes open. Fixed mullion windows do not open but provide an architectural element.

Cost Considerations

The cost of mullion windows can vary depending on the material, number of divided panes, and overall window size. In general, you can expect to pay between $400 to $1500 per window for high-quality mullion windows made of wood, aluminum or vinyl. The more divided panes and the larger the window size, the higher the cost. However, mullion windows can add curb appeal and value to a home, so the investment may be well worth the additional upfront cost.

The History and Origin of Mullion Windows

Mullion windows have been used in architectural design for centuries. The word ‘mullion’ dates back to the 15th century and refers to the vertical bar separating window panes. Originally made of stone, mullions provided structural support for large windows.

The Emergence of Glass and Lead

With the emergence of glass manufacturing in the Middle Ages, mullioned windows began incorporating multiple small panes held in place by grooved lead strips, known as cames. The lead allowed for more intricate window patterns with diamond-shaped panes and other decorative forms. These windows were most common in Gothic architecture, especially in cathedrals, churches and universities across Europe.

Revival Styles and Modern Use

In the 19th century, mullioned windows experienced a revival as part of Gothic Revival and Arts and Crafts architecture. Today, mullioned windows are popular in residential and commercial spaces aiming to achieve a traditional look. Now made from wood, steel and vinyl, mullions provide aesthetic appeal and the illusion of small panes while requiring less maintenance than true divided-light windows.

Benefits of Mullion Windows

Mullion windows offer several benefits, including:

Next time you see those signature intersecting glazing bars, you’ll know you’re looking at a window with a rich history and timeless style. Mullion windows have earned their place as an architectural classic.

Types of Mullion Windows

Casement Windows

Casement windows are hinged on one side and open outward, to the left or right. They typically feature hand cranks or crank handles to open and close the window. Casement windows provide excellent ventilation since they open outward. They also offer an unobstructed view since there is no center frame. However, casement windows can be difficult to clean from the outside since they open outwards.

Double-Hung Windows

Double-hung windows feature two movable sashes (glass panels) that slide up and down in a frame. The sashes are balanced by a counterweight system that makes them easy to open and close. Double-hung windows provide good ventilation when open and an unobstructed view. However, they can be more difficult to clean compared to casement windows since access to both sides of the glass is required.

Awning Windows

Awning windows are hinged at the top and open outward. They are often long, narrow windows located above doors or larger windows. Awning windows let in light and air while keeping rain and snow out. They provide ventilation even when open slightly. However, awning windows can only be opened partially and do not provide an unobstructed view like casement or double-hung windows when fully open.

Fixed Windows

Fixed windows, also known as picture windows, do not open. They provide an unobstructed view and let in plenty of natural light. However, fixed windows do not offer ventilation. They are often combined with operable windows like casements or double-hungs to provide both natural light and fresh air.

The four main types of mullion windows—casement, double-hung, awning, and fixed—each have their advantages and disadvantages with regards to view, ventilation, and ease of use. The ideal solution for any application will depend on balancing these factors to meet the needs of the space.

Benefits of Installing Mullion Windows

Increased Natural Light

Mullion windows are an excellent choice if you want to maximize the amount of natural light that enters your home. The narrow frames and multiple panes mean more glass and less framing, allowing light to flood into the space. The increased light exposure can help reduce your energy usage since you won’t need as much artificial lighting during the day. More natural light is also beneficial for your health, mood, and productivity.

Enhanced Ventilation

The split panes and narrow frames of mullion windows make them very effective for ventilation and airflow. When opened, they allow fresh air to circulate into the home while still providing an open, unobstructed view. This can help reduce your dependence on air conditioning and create a comfortable interior environment. The enhanced airflow also helps prevent the buildup of heat, humidity, and stale air in the home.

Improved Energy Efficiency

If you choose mullion windows made of insulating glass like double-pane or triple-pane, they can significantly improve the energy efficiency of your home. The multiple panes of glass and narrow gaps between them create insulated air pockets that help reduce heat transfer. This means the windows keep more heat inside during winter and prevent heat gain in summer. The energy efficiency benefits can lower your utility bills and make your home more comfortable overall.

Timeless, Architectural Appeal

Mullion windows have an architectural style that provides a timeless, custom look to any home. Their narrow profiles and divided panes give a historic feel reminiscent of old-world craftsmanship. They are an attractive design choice for any homeowner looking to add architectural interest, character, and curb appeal. The windows help enhance and complement both traditional and modern home styles.

Mullion windows offer significant benefits for your home including increased natural light, enhanced ventilation and airflow, improved energy efficiency, and timeless architectural appeal. By installing mullion windows, you can create an interior environment that is bright, airy, comfortable, and aesthetically pleasing. The windows are an investment that provides both short-term and long-term value.


Mullion windows offer a unique combination of aesthetic appeal and functionality that can elevate the character of your home. With their distinctive grid patterns and natural ventilation capabilities, these windows can add architectural flair while regulating light, airflow, and views. By understanding the pros, cons, costs, and installation considerations, you can determine if mullion windows are the right choice for your next home improvement project.At Rhino Livermore Window Replacement & Siding, we are committed to helping you achieve the perfect blend of style and functionality for your home. Our experienced professionals can guide you through the process of selecting and installing mullion windows that meet your specific needs and enhance your home’s unique style.Whether you’re looking to upgrade your existing windows or design a new home, our team is here to help. Contact us today to learn more about how our expert services can help you achieve the perfect blend of style and functionality for your home.

Frequently Asked Questions

What do mullion windows look like?

Mullion windows feature vertical or horizontal bars that separate and support individual panes of glass within a window. These bars, known as mullions, can give the window a more structured and traditional appearance. The design often creates a grid pattern, adding both aesthetic and functional benefits.

What is the difference between a mullion and a transom window?

A mullion is a vertical or horizontal bar between panes of glass in a window, providing structural support and dividing the window into smaller sections. A transom window, on the other hand, is a horizontal window placed above a door or another window. It is usually used for ventilation or decorative purposes, often featuring decorative glass or patterns.

What is the difference between mullion and muntin?

Mullions are the structural elements that divide adjacent window units, providing support and separation. Muntins, however, are the strips of wood, metal, or plastic that divide a single window sash into smaller panes of glass. Essentially, mullions separate entire window units, while muntins divide the glass within a single window.

Where is the mullion on a window?

The mullion is located between separate window units, either vertically or horizontally. It is the bar or post that divides and supports the individual sections of glass or separate windows, forming a framework within the overall window structure.

What is the purpose of a window mullion?

The purpose of a window mullion is to provide structural support to the window and to divide it into smaller sections for aesthetic reasons. Mullions help in maintaining the integrity and strength of larger window assemblies, allowing for the combination of multiple smaller windows into a larger, cohesive unit.

What is the difference between a frame and a mullion?

A window frame is the outer structure that encloses and supports the entire window assembly, including the glass, sashes, and other components. A mullion, however, is an internal support bar that divides and supports the individual window panes within the frame. Essentially, the frame surrounds and holds the entire window assembly, while mullions are the dividers within that assembly.

Leave a Reply

Your email address will not be published. Required fields are marked *