The Brutal Truth: Why Glass Matters More Than the Frame

If you’re researching thermally broken frames, our 6 minute guide “Thermally Broken Steel Windows Explained: Do You Need Them for Your Project?“ explains how thermal breaks work and when they may be beneficial.  

This blog explores why glazing specification is often the most influential factor in determining the thermal performance of a steel window system. 

The biggest misconception about energy-efficient steel windows

If you’ve spent any time researching energy-efficient steel windows, you’ve probably noticed one thing.

Everyone talks about the frame.

Thermally broken.

Insulated.

High-performance.

European.

The conversation almost always starts with the frame. Yet in most steel window systems, the frame represents only a small proportion of the total window area, while the glazing makes up the vast majority. Because U-value and SHGC measure the performance of the complete window system, the glazing specification often has the greatest influence on the final result.

These performance metrics need to be considered together rather than viewed in isolation. Understanding the difference between U-value, SHGC and glazing performance is key to selecting the right steel window specification. 

But here’s the brutal truth.

For most projects, the glass has a greater influence on thermal performance than the frame.

That doesn’t mean the frame isn’t important.

It absolutely is.

But if you’re focusing only on whether a frame is thermally broken, you may be overlooking the component that usually has the biggest impact on your window’s energy performance.

Look at the View

Take a moment to picture a steel window.

What occupies most of the opening?

Glass.

In many architectural steel windows and steel doors, the glazing accounts for more than 80% of the total surface area, while the frame forms only a narrow perimeter.

Heat doesn’t know where the marketing brochure starts and stops.

It moves through the entire window.

So it makes sense that the component covering the greatest area often has the greatest influence on how the window performs.

The Numbers Tell the Story

When energy assessors evaluate a window, they don’t assess the frame in isolation.

They assess the complete window system.

Two key measures determine thermal performance.

U-value

The U-value measures how readily heat moves through the complete window.

That includes:

• the glass
• the frame
• the spacers
• the seals

A lower U-value means better insulation.

SHGC

The Solar Heat Gain Coefficient (SHGC) measures how much of the sun’s radiant heat enters the building.

The ideal SHGC depends on the building’s orientation, climate and design objectives.

Because these measurements apply to the entire window system, improving the glass specification often has a significant influence on both values.

Understanding how U-value and SHGC work together is essential when comparing window systems and making informed specification decisions. Read our guide: Stop Comparing U-Values Alone — Why SHGC and the Complete Window System Matter. 

Why Glass Makes Such a Difference

Not all glazing performs the same.

A window can be specified with:

• low-E coatings
• double glazing
• triple glazing
• solar control glass
• laminated glass
• gas-filled insulated glass units

Each of these influences how heat enters and leaves the building.

Changing the glazing specification can significantly alter the window’s overall U-value and SHGC while maintaining exactly the same architectural appearance.

Whether you’re specifying fixed windows, casement windows, awning windows, sliding doors, bi-fold doors, pivot doors or hinged steel doors, the same principle applies: the thermal performance of the complete window system depends on selecting the right combination of frame and glazing to suit the project’s performance requirements.

Here’s What Most People Get Wrong

One of the most common assumptions is that upgrading to a thermally broken frame automatically creates a high-performance window.

It doesn’t.

Imagine two projects.

The first uses a thermally broken frame paired with a basic glazing specification.

The second uses a standard steel frame paired with glazing that’s been carefully selected for the building’s energy targets.

Depending on the project, the second window system may achieve a better overall thermal outcome because the glazing has been optimised for the building.

That’s why Metro Steel Windows doesn’t start with the frame. We start with the performance requirements. 

The Best Question Isn’t “Which Frame?”

The better question is:

“What combination of frame and glazing delivers the performance this building requires?”

Sometimes the answer will be a thermally broken steel frame.

Sometimes it won’t.

The right specification depends on the performance requirements of the building, not on a single product. 

Whether a project ultimately uses the Metro M40 System or the thermally broken Secco OS2 75 System, the most appropriate solution depends on the project’s energy performance objectives. In either case, the glazing specification remains one of the most influential factors in determining the thermal performance of the complete window system. 

The Brutal Truth

Thermally broken steel frames absolutely have their place and can make an important contribution to the overall thermal performance of a window system.

But they aren’t a silver bullet.

The biggest gains in thermal performance often come from selecting the right glazing specification and ensuring the complete window system has been designed to suit the project’s orientation, climate and energy performance requirements.

The frame matters.

The glass matters more often than many people realise. 

But when it comes to thermal performance, the two should never be considered in isolation.

That’s the conversation we believe every project should start with.