Thermal Bridging Explained: Its Impact on Cost, Compliance and Building Performance

In recent years, a great deal of focus has been placed on improving building fabric efficiency with the aim of reducing operational energy emissions and as such operational greenhouse gas emissions. Initially, these improvements focused on the reduction of U-values to reduce heat transfer through the mostly flat surfaces of building elements, this is reflected in alterations to building regulations Part L in 2021, which mandated improved U-values for all elements in new build properties. However, thermal transmittance occurs not only through a building elements such as walls, roofs, floors and windows but also at the junctions where these elements meet. These thermal bridges are responsible for a significant amount of all building heat loss. As such thermal bridging has become one of the most important topics in building performance, with the quality of designed thermal bridges becoming a requirement to pass gateway 2. Yet many in the construction industry still see it as a small design detail rather than a major factor in energy efficiency, compliance, and cost. As regulations tighten and expectations around building performance continue to rise, understanding thermal bridging and addressing it early has never been more important.

This article provides a clear, practical explanation of thermal bridging for anyone working in construction, architecture, development, or project management. It outlines what thermal bridging is, why it matters, and how it affects project cost, timelines, and compliance. It also explains why accurate psi-value calculations are now essential for delivering buildings that meet performance requirements without unnecessary redesign or overspend.

What exactly is a thermal bridge?

A thermal bridge is a point in the building where heat transfers more easily than it should due to a non-continuous insulating layer or because a more conductive material like steel or aluminium creates a direct path for heat to move through the structure, bypassing the insulation layer either partially or entirely. These junctions and bypasses are not rare or unusual occurrences; they are present in every building. The most common junctions are wall junctions with floors and roofs, corners, window sills and lintels, door jambs, balcony supports which penetrate the insulation and ground-floor junctions where the slab meets the external wall.

At these points, heat escapes faster than it does through the surrounding insulated areas. Even though the junctions themselves may look small on drawings, collectively they can have a significant impact on overall performance. Junction heat losses typically account for 15-30% of a building’s total heat loss. A psi-value (Ψ-value) is the measurement we use to quantify how much extra heat is being lost at each of these junctions, helping us understand exactly where the weak spots are and how significant their impact is.

Why does it matters?

Thermal bridging directly affects how a building performs in practice. When heat escapes quickly through junctions, a building requires more energy to maintain comfortable internal temperatures. This reduces the overall fabric efficiency of the building envelope by bypassing the low u-value insulating layers within building elements. increases heating demand, raises running costs for occupants, and affects the overall efficiency of the building fabric. Junctions with poor thermal performance can also result in colder internal surfaces and surfaces throughout build ups, potentially resulting in surface or interstitial condensation and mould growth, and the long-term maintenance issues and occupant health problems.

From a regulatory standpoint, thermal bridging plays a central role in how a building performs in SAP and EPC calculations. As the industry shifts towards higher standards and stricter performance requirements, the influence of thermal bridging has become more pronounced. Good U-values alone are no longer enough; the performance of junctions must also be accounted for accurately. As a result, the supply of Psi value calculations demonstrating the quality of thermal bridge design must now be supplied to the building safety regulator as part of the Gateway 2 submission process.  

Figure 1.1 External wall and slab junction and thermal modelling results

Although the junction detailing in figure 1.1 appears robust in the construction drawing, the thermal analysis reveals concentrated heat flow around the steel masonry support as show by the green and blue lines denoting heat transfer though the structure. This element creates a conductive path through the insulation layer, resulting in a clear thermal bridge at the steel masonry support, where heat is travelling quickly from inside to outside.

Cost implications

Thermal bridging has a direct impact on project cost, both during design and construction. When psi-values are not calculated, energy assessors are required to use conservative default values within SAP. These defaults assume that junctions perform poorly, which reduces the calculated performance of the building.

This often forces the design team to compensate by increasing insulation thickness, upgrading materials, or adding additional renewable technologies such as photovoltaic panels. These upgrades are significantly more expensive than designing junctions properly in the first place. In many cases, teams find that accurate psi-value calculations would have avoided these costs entirely.

If thermal bridging is identified too late, particularly at as-built stage, the consequences are even more expensive. Redesigning junctions, adding thermal breaks, modifying slab edges or altering facade details during construction is not only costly but also disruptive and time-consuming. Addressing thermal bridging early is always the most cost-effective approach.

Impact on project timeline

Thermal bridging issues are a common cause of delays in achieving compliance. If psi-values are not provided, SAP and SBEM assessments may fail, which prevents EPCs from being issued and can delay completion and handover. For developers, this can have serious financial implications, particularly when sales or occupancy depend on completed EPCs.

When a building fails its energy assessment, the team must return to the drawings, revise specifications, and rerun calculations often under tight time pressure. This can push projects off programme and place additional strain on design and construction teams. The earlier thermal bridging is assessed, the lower the risk of last-minute redesign and programme delays.

Compliance and regulatory requirements

As Part L and energy performance regulations tighten, the quality of junction is becoming increasingly important. Thermal bridging contributes directly to emissions calculations, fabric energy efficiency and EPC ratings. Default assumptions in SAP are designed to be conservative. Without calculated psi-values, a building may appear significantly less efficient than it is in reality and may fail key compliance checks even if high efficiency insulation and building services are specified. Accurate psi-values help the assessment reflect the true performance of the building, reducing the risk of non-compliance and overspecification.

Why accurate Psi-value calculations are essential?

Accurate psi-value calculations show exactly how a junction performs, rather than relying on assumptions or default values. By modelling the junctions at early stage, the design team can see where heat is being lost and make any necessary adjustments while the details are still easy to change. This reduces the risk of costly revisions later in the project.

In many cases, projects experience a marked improvement in SAP outcomes simply by replacing default assumptions with calculated psi-values. This can reduce the need for additional insulation or renewable technologies, improve EPC ratings and help achieve compliance without expensive upgrades. Early modelling and psi-value calculation provides a more reliable pathway to meeting performance requirements.

Conclusion

Thermal bridging is not just a technical detail — it plays a major role in a building’s energy performance, cost and ability to meet compliance targets. When junctions are considered early in the design process, project teams can avoid unnecessary upgrades, prevent late design changes and reduce the chance of delays during assessment or handover. Accurate psi-value calculations are essential for this, giving a true picture of how the junctions perform and adjusting accordingly, helping the project progress more smoothly from design through to completion.

Written by Rishabh Patel