Mulled vs. Non-mulled Windows: What’s the Difference and Which Is Better for Your Home?

If you’ve started getting quotes for replacement windows, you may have heard terms like “mulled” and “non-mulled” windows and thought… wait, what does that even mean?

You’re not alone.

This is one of those behind-the-scenes window terms that contractors use all the time, but most homeowners aren’t familiar with. Choosing the wrong window configuration can affect how your windows look, how they’re installed, and even how much your project ends up costing. And if you don’t fully understand the difference, it’s easy to feel unsure about what you’re actually being quoted.

The good news? It’s actually pretty simple once you break it down.

Energy Swing Windows has been helping Western PA homeowners replace their windows for over two decades, so we understand how to find the best solution for your project.

In this guide, we’ll break down exactly what mulled and non-mulled windows are, how they impact your project, and how to confidently decide which option makes the most sense for your home.

Key Takeaways: Mulled vs. Non-mulled Windows

• Mulled windows are multiple windows joined together
• Non-mulled windows are standalone units
• Neither option is inherently better, it depends on your home and installation
• Installation quality plays a bigger role than configuration

Table of Contents

• What Is a Mulled Window?
• What Is a Non-Mulled Window?
• Mulled vs. Non-Mulled: Key Differences
• Why Windows Are Mulled
• When to Mull vs. Not Mull
• Pros and Cons
• Installation Differences
• Cost Differences
• Replacing Mulled Windows
• Energy Efficiency
• What They Look Like
• Should You Mull Windows?
• Questions to Ask
• Final Thoughts
• FAQs

What Does “Mulled Window” Mean?

A mulled window is when two or more windows are joined together to form one larger unit.

These windows can be:

• Side-by-side (most common)
• Stacked vertically
• Combined in custom configurations

For example:

• Two double-hung windows next to each other
• A large picture window with casement windows on each side
• Three windows across the front of a home

In many homes, especially older ones, you’ll find mulled windows in large openings where a single window wouldn’t make sense.

What Does “Non-mulled Window” Mean?

A non-mulled window is simply a standalone window unit.

It has:

• One frame
• One opening
• No other windows physically attached to it

Even if a window has multiple moving parts (like two sashes in a double-hung), it still counts as one non-mulled window because it’s all within a single frame.

What Is the Main Difference Between Mulled and Non-mulled Windows?

At a high level, the difference comes down to whether windows are connected together or installed separately.

Mulled vs. Non-mulled Windows

Quick takeaway:
Mulled windows are individual window units joined together to form one larger unit installed in a single opening, while non-mulled windows are separate units installed individually within the same opening.

Why Do Builders and Contractors Mull Windows Together?

There are a few common reasons windows are mulled:

1. To Fill Larger Openings

If you have a wide opening (like 60 inches or more), it often makes more sense to use:

• Two or more windows mulled together
  instead of
• One oversized window that may look out of place

2. For Aesthetics

Mulled windows can create:

• A more balanced look
• Better symmetry across the home
• Opens up the room letting in more light while providing an enhanced view.

3. To Match Design Intent

Builders often mull windows to:

• Maximize natural light
• Create focal points (like front-of-home picture windows)

When Should Windows Be Mulled vs. Left Non-mulled?

There’s no one-size-fits-all answer, it depends on your home and goals.

Mulled windows often make sense when:

• You have a large opening to fill
• You want a consistent, symmetrical look
• You’re matching an existing design

Non-mulled windows are often better when:

• Are dealing with existing single window openings.
• You’re working with standard-sized openings
• You want easier installation and repair options

A good rule of thumb:
If your current windows are mulled, many homeowners choose to replace like-for-like unless there’s a strong reason to change. 

Are Mulled Windows Better Than Non-mulled Windows?

Not necessarily, this is where a lot of confusion comes in.
Neither option is “better” across the board.
It really depends on installation quality and your goals.

How Do Mulled Windows Affect Installation?

Mulled windows can be installed in two main ways:

1. Factory-Mulled Windows

• Built as one unit in the factory
• Installed as a single assembly
• Typically faster and more consistent
• More difficult to get the installation right

2. Field-Mulled Windows

• Connected together on-site during installation
• May require added materials (like the wood framing between units)
• Can take more time depending on the setup

The installation method can impact:

• Labor time
• Structural support
• Overall performance

Wondering how Energy Swing installs windows? Check out this article that breaks down our installation process from start to finish.

Do Mulled Windows Cost More Than Non-mulled Windows?

In many cases, yes but not for the reason you might think.

It’s not just about “mulled vs. non-mulled” it’s about:

• Number of windows
• Installation complexity
• Finishing work

For example:

• Two mulled windows will usually cost more than one single window (because you’re buying two units)
• But the installation method and finishing details can also impact the final price