What is Thermal Bridging?

Thermal bridging occurs when materials with high thermal conductivity provide an uninterrupted path for heat to flow through building fabric. Unlike the insulation surrounding it, these conductive pathways—typically steel, concrete, or masonry—allow heat to transfer more readily between the interior and exterior of a building. This direct route bypasses the thermal resistance of insulation layers, reducing the overall effectiveness of energy retrofit measures.

In retrofit projects, thermal bridges are a primary concern because they can account for 20-30% of heat loss through the building envelope, depending on design and construction quality. This is particularly relevant in buildings constructed before modern building standards, where thermal bridges were rarely considered during original design.

Common Thermal Bridge Types in UK Buildings

Structural Elements

• Steel beams and columns penetrating insulation layers
• Concrete lintels above windows and doors
• Reinforced concrete floor slabs in multi-storey buildings

Building Junctions

• Wall-to-roof connections
• Wall-to-floor interfaces
• Window and door frames
• Balcony attachments

Service Penetrations

• Pipework and ductwork passing through insulation
• Electrical conduits and cables
• Fixings and fasteners

Why Thermal Bridging Matters for Retrofit Design

When designing retrofit measures, professionals must account for thermal bridging to achieve predicted energy savings. Building Regulations Part L and the PAS 2035 standard both require detailed consideration of thermal bridging during retrofit planning. Ignoring these pathways can result in:

• Actual U-values significantly higher than calculated values
• Failure to meet retrofit performance targets
• Increased operational energy costs for building occupants
• Greater risk of surface condensation and mould growth
• Reduced comfort levels within the building

The PAS 2035 retrofit standard specifically emphasises the importance of reducing thermal bridging as part of a holistic retrofit approach. Poorly designed thermal bridge remediation can undermine even high-specification insulation improvements.

Quantifying Thermal Bridging

Thermal bridges are quantified using psi-values (Ψ), measured in W/mK. This value represents the additional heat loss per metre of thermal bridge length per degree Kelvin temperature difference. Linear thermal bridges—such as those at building junctions—are calculated this way.

Point thermal bridges (individual fixings or penetrations) are quantified using chi-values (χ), measured in W/K. During retrofit design, professionals should identify all significant thermal bridges and include their combined effect in overall fabric performance calculations.

Many UK buildings contain poorly-detailed thermal bridges that contribute substantially to overall heat loss. Retrofit design should prioritise addressing the most significant bridges first, typically structural elements and major junctions rather than minor penetrations.

Mitigation Strategies

Design Approaches

• Specifying continuous insulation layers that avoid structural penetrations
• Using thermal breaks in metal components (windows, door frames, railings)
• Redesigning junctions to minimise conductive pathways
• Selecting lower-conductivity fixing systems

Installation Quality

• Ensuring insulation continuity across junctions and around penetrations
• Proper sealing of gaps and cavities
• Site supervision to verify thermal bridge details are constructed as designed
• Using specialist membranes and sealants to maintain airtightness

Material Selection

• Choosing insulation materials with appropriate conductivity ratings
• Considering aerogel or vacuum panel solutions for constrained spaces
• Using mineral fibre for fire performance without sacrificing thermal resistance

Documentation and Compliance

Under PAS 2035, retrofit designers must document thermal bridge treatment in retrofit specifications. This includes identifying all significant thermal bridges, calculating their impact on overall fabric performance, and detailing mitigation measures for each. This documentation becomes part of the retrofit standard assessment and compliance record.

Professionals should use recognised calculation methods, such as those published by CIBSE or the UK Building Regulations, to ensure consistency and compliance. Where non-standard thermal bridge details are proposed, independent verification may be required.

Looking Forward

As retrofit activity intensifies across the UK, thermal bridging becomes an increasingly critical design consideration. Buildings achieving deep energy retrofits must address thermal bridges comprehensively to realise predicted performance improvements. This requires coordinated design between thermal modelling specialists, structural engineers, and site teams to ensure thermal bridge strategy is properly integrated throughout the retrofit project.