What is a thermal break and why it matters

• 30 March 2026
• News

When specifying steel windows, the term ‘thermal break’ frequently appears in technical literature and performance data. Understanding how thermal break windows work is essential for architects, contractors and property owners concerned with energy efficiency, condensation control and long term building performance.

Steel frames are valued for their strength and slender sightlines. These characteristics allow large areas of glazing and the distinctive proportions associated with traditional steel windows. However, steel is also highly conductive, which means heat can move through it very easily. Without careful design, a metal frame can create a direct route for heat to move between the exterior of a building and the interior space.

Defining the thermal break

A thermal break is an insulating barrier incorporated within a metal window frame to interrupt the direct path of heat transfer. In steel window systems this usually takes the form of a strip of material with very low thermal conductivity, positioned between the inner and outer sections of the frame.

Materials commonly used for this purpose include polyamide or high density polyurethane. These materials conduct heat far less readily than steel, so placing them between the interior and exterior steel sections slows the transfer of heat through the frame. This barrier reduces thermal bridging and improves the overall thermal performance of the window.

Some steel systems achieve improved thermal insulation through a different approach. Carefully designed voids within the profile create separation between interior and exterior steel elements. Air has very low thermal conductivity, so a void acts as an effective insulating layer within the frame. This principle is used in systems such as W20 TE, where the profile geometry helps reduce the transfer of heat without incorporating a polymer thermal break.

Both approaches aim to reduce thermal bridging through metal frames and improve the energy performance of the building envelope.

How thermal break windows improve thermal performance

Heat moves through solid materials by conduction. The rate of this transfer depends on the thermal conductivity of the material involved. Steel has a conductivity of roughly 50 W/mK, which makes it an efficient conductor of heat.

A thermal break introduces a material with much lower conductivity into the path between the interior and exterior steel sections. Polyamide, for example, has a conductivity of around 0.25 W/mK. This substantial difference slows the movement of heat through the frame and improves the thermal insulation of the window.

Reducing heat transfer in this way helps lower the overall U-value of the window assembly. Modern thermally broken steel systems such as Crittall’s T60 can achieve whole window U-values as low as 0.8 W/m²K when paired with appropriate glazing. Lower U-values indicate better thermal performance and reduced energy loss through the building envelope.

Effects on interior comfort

Interrupting the conductive path through the frame has practical effects inside the building. When heat loss through the window frame is reduced, the internal surfaces of the frame remain closer to the interior temperature of the room.

During colder months this helps prevent steel frames from becoming excessively cold. When surface temperatures remain higher, the likelihood of condensation forming on the frame is significantly reduced. Condensation occurs when warm indoor air meets a cold surface and moisture in the air condenses into water droplets.

Maintaining warmer internal surfaces therefore contributes to a more stable indoor temperature and a more comfortable living environment. Occupants are less likely to experience cold downdraughts near windows, and the risk of moisture related maintenance issues is reduced.

Energy efficiency and building performance

Thermal break windows also influence the overall energy efficiency of a property. When heat loss through window frames is limited, less energy is required to maintain a stable indoor temperature. This can reduce overall energy consumption and help control energy bills, particularly during colder months.

Improved thermal insulation within steel frames also supports compliance with modern building regulations, including the thermal performance targets set out in Part L. Lower U-values contribute to better energy performance across the building envelope and support the design of low energy buildings.

Considerations for specification

For architects and specifiers, assessing thermal break windows involves more than confirming the presence of an insulating barrier. The design of the window frame, the type of thermal break technology used and the glazing specification all influence the final thermal performance of the system.

Key considerations include the position and thickness of the thermal break material, the way the system manages thermal bridging at joints and corners, and compatibility with high performance glazing such as double or triple glazing. These factors determine how effectively the window limits heat transfer between the interior and exterior.

Understanding these details helps ensure that steel window systems deliver the intended energy performance while maintaining the structural integrity and slender profiles that define steel fenestration.

For projects that combine the classic appearance of steel frames with contemporary performance standards, understanding how thermal breaks and other thermal control strategies work is an important part of making an informed specification decision.

• To find out more about thermally broken steel, contact Crittall Windows here.