What Is a Thermal Break in Windows? A Guide to Thermally Broken Steel Systems

What is a thermal break in windows?

A thermal break is an insulating barrier built into a window frame that reduces heat transfer between the inside and outside of a building.

Steel is an excellent structural material, but it is also an excellent conductor of heat. Without insulation, a steel frame will allow heat to pass directly through it. This process is known as thermal bridging.

A thermal break interrupts this path.

In a thermally broken steel window, the inner and outer sections of the frame are separated by a non-conductive material (often a high-performance polymer). This barrier dramatically reduces heat transfer, improving insulation and energy efficiency.

In simple terms:

• Non-thermally broken steel = heat passes straight through
• Thermally broken steel = heat transfer is significantly reduced

Understanding heat transfer and thermal bridging in steel frames

Heat naturally moves from warm areas to cold areas. In glazing systems, heat loss occurs in three main ways: through the glass; through air leakage; through the frame.

In traditional steel windows, the frame becomes a major source of heat loss because steel conducts heat so efficiently.

This creates several problems:

• Increased overall U-values
• Cold internal frame surfaces
• Risk of condensation
• Reduced occupant comfort

Thermal bridging is particularly problematic in large expanses of glazing or exposed elevations – exactly the types of projects where steel is often specified for its visual appeal.

Without a thermal break, a steel window may deliver aesthetic excellence – but compromise performance.

Why traditional steel systems struggle with insulation

Many older or Crittall-style steel systems were developed in an era when energy regulations were less demanding. These systems often:

• Lack a thermal break entirely
• Use single glazing or limited double glazing capacity
• Struggle to meet modern insulation standards

As Building Regulations have evolved – particularly Part L, which governs energy efficiency – these traditional steel windows frequently fall short.

This is especially relevant when comparing modern thermally broken systems to older-style products often associated with heritage or industrial aesthetics. While the visual language may be similar, the performance is not.

How MHB Steel solves the thermal break challenge

MHB Steel’s patented systems represent a modern evolution of steel glazing – retaining the refined, slender sightlines architects value, while integrating advanced thermal performance.

Unlike traditional steel profiles, MHB’s solid steel systems are thermally broken, incorporating an engineered insulating core within the frame. This creates a true separation between internal and external steel sections.

The result:

• Significantly improved thermal performance
• Compatibility with high-performance double or triple glazing
• Compliance with current insulation standards
• No compromise on slim aesthetics

Importantly, MHB Steel achieves this while maintaining the elegant proportions that designers associate with classic steel windows. MHB’s patented solid steel, thermally-broken system offers a viable, regulation-compliant alternative to older non-thermally broken systems.

For architects balancing conservation aesthetics with contemporary energy standards, this is a crucial step forward.

Thermal breaks and Part L Building Regulations

Part L of the Building Regulations sets minimum standards for energy efficiency in residential projects.

For new builds and significant refurbishments, glazing systems must achieve specific U-value thresholds. Non-thermally broken steel windows typically struggle to meet these requirements.

Thermally broken window systems such as MHB Steel are engineered specifically to align with modern compliance standards, enabling:

• Approval under current regulations
• Greater design freedom in new-build architecture
• Performance-led heritage refurbishments
• Reduced risk at specification stage

For architects, this clarity is essential. Selecting a compliant system early in the design process avoids costly redesigns later.

Balancing performance with aesthetics

There is often an assumption that enhanced thermal performance means thicker frames and compromised design. MHB Steel challenges that notion.

By integrating thermal break technology within solid steel profiles, it delivers:

• Ultra-slim sightlines
• High structural capability
• Design flexibility and freedom 
• Large-format glazing potential
• Excellent thermal performance

For projects requiring expansive glazing, conservation sensitivity or industrial-inspired detailing, thermally broken steel allows you to maintain architectural intent without sacrificing compliance.