Casement Window Mullions: A Comprehensive Guide to Design and Functionality

Casement windows are a popular choice for homeowners due to their versatility, energy efficiency, and timeless beauty. One key element of casement windows that often goes unnoticed is the mullion. Mullions are the vertical or horizontal bars that divide the window into smaller panes. In this comprehensive guide, we will explore the role of mullions in casement windows, their design options, and how they contribute to the overall functionality and aesthetics of your home.

What Are Casement Window Mullions?

Casement window mullions are the structural elements that divide a window into smaller panes, called lites. Mullions can be made of various materials, including wood, metal, or PVC. They serve both functional and decorative purposes, adding visual interest to the window and supporting the weight of the glass panes.

Types of Casement Window Mullions

There are several types of mullions available for casement windows, each offering a unique look and functionality. Here are some common mullion types:

1. True Divided Lite (TDL): This type of mullion divides the window into individual glass panes. Each pane is held in place by the mullion, creating an authentic and traditional look.

2. Simulated Divided Lite (SDL): SDL mullions are applied to the exterior and interior surfaces of a single glass pane, creating the appearance of divided lites without actually dividing the glass. This type of mullion offers a similar aesthetic to TDL but with improved energy efficiency and easier maintenance.

3. Grilles Between Glass (GBG): GBG mullions are sandwiched between the glass panes of a double- or triple-glazed window. This type of mullion provides the divided lite appearance without affecting the window's exterior surface, making it easy to clean and maintain.

Design Options for Casement Window Mullions

Mullions can be customized to suit the architectural style and personal preferences of the homeowner. Some popular design options include:

1. Colonial: This classic design features evenly spaced, rectangular lites, creating a symmetrical and traditional look.

2. Prairie: Inspired by the Arts and Crafts movement, Prairie-style mullions feature a perimeter of smaller lites surrounding a larger central pane.

3. Diamond: Diamond-shaped mullions create a unique and eye-catching pattern, often found in Tudor-style homes.

4. Gothic: This design features pointed arches and intricate patterns, reminiscent of Gothic architecture.

5. Custom: Homeowners can work with a window manufacturer to create custom mullion patterns to suit their unique tastes and design preferences.

The Role of Mullions in Casement Window Functionality

While mullions add visual interest to casement windows, they also serve a functional purpose. Some benefits of mullions include:

1. Structural Support: Mullions provide support to the glass panes, ensuring stability and durability.

2. Energy Efficiency: Mullions, especially those in double- or triple-glazed windows, can help improve the window's energy efficiency by reducing heat transfer.

3. Sound Insulation: Mullions can help reduce noise transmission, providing a quieter and more comfortable living environment.

4. Privacy: Mullions can create smaller panes of glass, allowing for increased privacy without sacrificing natural light.

Choosing the Right Mullions for Your Casement Windows

When selecting mullions for your casement windows, consider the following factors:

1. Architectural Style: Choose a mullion design that complements the architectural style of your home.

2. Material: Mullions are available in various materials, each with its own benefits and drawbacks. Wood mullions offer a traditional look but may require more maintenance, while metal or PVC mullions are low-maintenance and durable.

3. Budget: The cost of mullions can vary depending on the material and design. Consider your budget when making your selection.

4. Energy Efficiency: If energy efficiency is a priority, opt for mullions that contribute to improved insulation, such as those in double- or triple-glazed windows.

In summary, casement window mullions play a crucial role in both the design and functionality of your windows. By understanding the different types of mullions and their benefits, you can make an informed decision when selecting the perfect mullions for your casement windows. With the right mullions in place, your windows will not only look beautiful but also provide the comfort, energy efficiency, and privacy you desire.

Casement Window


Casement windows are the most common style of window. They feature hinged panels that swing open and closed. Casement windows are available in single panels or multiple panels. The direction a door swings, either to the right or the left, is called the handing. In addition to handing, window are identified as either inswing or outswing depending on whether they open into the house or towards the outside.


Our team of experts will work with you to develop a bespoke plan that fits your needs. We take a holistic approach to the planning process, combining our technical expertise with our knowledge of customer requirements and project constraints.


Freedom to dream "truly custom-made"

Doorwin provides one-stop solution for professional windows doors manufacturer and installation. We have been serving customers with our high quality and branded products in United States, Asia and worldwide.


This newly built private house is in Sydney, Australia. The area occupied by the fenestration system in our scheduled design drawings was up to more than 110 square meters. A wide range of design details made the perfect mix of modern aesthetics and traditional sophistication, the simulated vertical hung windows with tilt & turn opening systems in this home are absolutely breathtaking. SDLS (Simulated Divided Lites with Spacer Bar) brings some historical vintage feeling to the whole building without losing any energy saving performance. Long narrow fixed windows creating larger-than-life expenses, coupled with German high-end systems made this project stand out from the rest.

Simulated Hung Window with Tilt & Turn Openings

The client's idea was to maintain a certain vintage feel, and our aim was to meet the energy efficiency needs and standards of modern houses as much as possible while maintaining a retro design. Doorwin’s 61S simulated hung windows with tilt & turn openings is a respectful modernization of a classic architectural masterwork.

Why choose the tilt and turn opening method?

Because it generally has better air seals than sliding, single hung, or double hung window frames. Tilt and turn windows form a complete seal when fully closed, this means they are less prone to water leakage, and cold draughts and you’ll keep more of your heating indoors where it belongs. They can be opened in the upper tilt way to ventilate at a minimum opening status and are easy to clean outside of the glass; when there’s a good day, they can be opened as normal inward casement windows.

Large Energy Efficient Fixed Windows

Our well-crafted bespoke systems enhance the functionality and panoramic view of this modern house.

Huge, fixed glass walls up to seven meters long and two meters high bring great lighting and open views, making it really refreshing to the mind. Five meters high long narrow windows give the whole building a contemporary aesthetic.

Every window’s placement and shape not only add to the uniqueness of this home but also provide a more energy-saving ability for the owners.

Doorwin energy-efficient integrated glazing units feature double-pane glass, warm-edge spacers, argon gas fill, Low-E coatings, and fully tempered glass, they can not only create a comfortable and natural indoor environment but also save electricity bills and heating bills of more than 100 USD for the owner, it’s a real bargain compared to choosing a regular glazing set.

Custom Designed Thermal Break Aluminum Entry Door

Aluminum Door offered an air of uniqueness and elegance to a home. With their luxurious patterns and beautiful finishes, it provides this house with a bounty of aesthetics and security. Aluminum entry doors allow you to resist any weather extremes, corrosion, and pests. So aluminum is one of the most resilient and durable materials you can select for front doors considering the price/performance ratio.

With pieces of double panes, and Low-E tempered glass. The door provides a good insulation performance.

Custom Designed Grilles and Transom

Divided lite windows and doors offer unique architectural solutions for buildings without compromising energy efficiency or aesthetics. Doorwin provides multiple muntin and glass options to customize your divided lite windows and doors. Further customization is available with a variety of colors and materials, creating an infinite number of design combinations to best suit your style.

In this project, we use SDLS (simulated divided lites with spacer bars), these bars permanently adhere to both sides of the glass, and spacer bars are installed between the glass, this is the most energy-efficient way to create the look of classical décor grilles. The windows in the bathroom bedroom and study have an Arch Transom Design, making these rooms, which are often used by the owner, have some traditional aesthetics.

BTW, Doorwin offers free design assistance, so if you need a custom design like this one, contact us right away and we will provide you with the most aesthetically and functionally pleasing design solution.

German Imported Hardware System

For windows and doors, hardware is as important as the engine is to the car. The continuous technological innovation of the European hardware industry has strongly promoted the performance improvement and functional innovation of windows and doors. In this project, Doorwin uses Hoppe™ multi-point locks, hinges, and handles from Germany.

Multi-point lock systems provide not only the burglar-resistance but also a tighter compression seam, making the window keeps good energy-efficient performance. Together with Technoform Bautec’s insulation edge bond and sealant strips, these make up our products' top insulation and energy-saving performance.

Save Your Money On Installation Costs

We have several international patents to reduce clients’ labor costs, such as our unique flanges and assembly type tubular design, which is different from the other products on the market. What sets us apart among Chinese counterpart is that all our products are equipped with nail fin structures for quick and easy installation. Labor costs during installation may exceed of the window and door price, therefore, our exclusive patent allows users to not only get a significant increase in installation efficiency, but also greatly reduce the labor cost of window and door installation. A cost that may bring you unexpected savings that far exceed the price difference you may care about now.

Free Design Assistance

With our own expertise, we provided the no-cost design development and contract documentation with CAD & BIM drawings, including elevation, details about the project, and installation help.

Before production, we will provide accurate sample drawings, which will clearly show the cross-sectional pattern, size, opening method and direction, pattern of the warm spacer, and the installation nodes. We will not go into production until you have signed off on the drawing to ensure that you receive a final product that meets the exact requirements of your project.

About Doorwin

Doorwin's professional packing system guarantees all products are delivered safely and without any damage. We apply a four-point packing system. Firstly, adhesive plastic film is applied on interior wood frames and exterior aluminum frames to avoid scratches on the surface. Then, a paper board is applied after we apply air bubble film surrounding the whole windows/ doors to avoid any destruction or clash. Lastly, we place them into a durable non-fumigation plywood crate to avoid any possible damage during transportation.

With years of experience shipping our products worldwide, we could guarantee to ship our products to their final destinations intact and without any damage.

About Doorwin Factories

Doorwin is one of the most professional window & door manufacturers are known for its great customized products. In Doorwin, a variety of colors, styles, and sizes of products are available to meet the client’s unique requirements. We own three ISO9000-certified modern factories in China. With a factory area of 20,000 m2 and a production capacity of 100,000 m2 per year. Doorwin Group has the scale to ensure timely delivery and the flexibility to reduce manufacturing lead times to as short as 18 days.