Windows with Mullions

Naturally, window manufacturers did not leave the problem unresolved and, applying the principle 'new is well forgotten old,' introduced a new design to consumers: windows with mullions.

But today, using such a technological solution isn't always about reinforcing the window. Often, modern PVC windows appear as large dark spots on the light and colorful facade of a new home. Therefore, window manufacturers have been creative here, offering consumers an original way to solve this issue by designing windows with decorative mullions.

What are windows with mullions?

A mullion is a decorative or structural grille on windows, designed to strengthen or beautify PVC, wooden, and other windows. While structural mullions are used to reinforce large-area windows, decorative mullions serve as an aesthetic element to visually divide wide or tall windows into smaller sections.

This design looks very attractive, which likely explains why windows with mullions are so popular.

They are most commonly used in windows for country houses, villas, and cottages. However, if you decide to install them, it’s wise to heed professional advice and learn what experts say about using mullions. First, let’s discuss the different types of mullions and how they differ.

Types of Mullions and Their Construction

Window mullions come in various designs: with straight or routed edges, wide, narrow, or curved. The way mullions connect to each other can vary by angle, allowing for window patterns of virtually any shape.

These mullions are an integral part of the window frame. They divide the frame into smaller sections, each housing a separate glass unit. Made from the same material as the window profile (PVC, wood, aluminum), they contribute to structural integrity.

These elements are the most popular. They are also known as false mullions or faux glazing bars. They come in types: Venetian mullions, inter-pane mullions, and surface-mounted (applied) ones.

Decorative mullions can be wide or narrow and are made from various materials. When choosing wide or thin decorative mullions, keep in mind that wide mullions can only bend with a radius up to 8 cm.

• Inter-pane mullions consist of aluminum or plastic strips arranged inside the glass unit in a specific pattern. They are installed during window manufacturing, so they cannot be added to a finished and installed unit.
• Venetian mullions, also known as faux glazing bars, are glued to the outer or inner surface of the glass or both. They are typically made from plastic, but wooden and even aluminum versions exist. They can be removed when needed, as they are attached using double-sided tape or sealant.
• Surface-mounted (applied) mullions are separate frames attached to the window from the exterior side. They are only suitable for wooden windows. The main advantage is that they don’t touch the window, so they reduce dirt accumulation from rain, and windows need less frequent cleaning.

How Do Mullions Affect Window Heat Conductivity?

Window manufacturers state that a high number of mullions significantly increases window heat conductivity, which in turn reduces insulation quality. A window with mullions has a thermal conductivity coefficient 10–20% higher than a window without them. Therefore, such windows will feel colder.

However, the type of mullion matters. Structural mullions are the coldest. But note that even inter-pane mullions reduce insulation, as they are made from aluminum, which conducts heat. Those who choose Venetian or surface-mounted decorative mullions need not worry: they have no significant impact on thermal performance.

Are There Any Drawbacks to Windows with Mullions?

The main drawback of structural and Venetian mullions is their increased dirt accumulation. During rain, dirt collects at the joints between mullions and glass. Keep in mind that inter-pane mullions do not affect dirt buildup, while surface-mounted ones, although not obstructing water runoff, must be removed before cleaning.

Strength is also important when choosing mullions. Naturally, the best structural solution is inter-pane mullions, which last as long as the window itself. Less durable are Venetian and surface-mounted mullions.