Understanding Thermal Bridging

Thermal bridging occurs when materials with high thermal conductivity create pathways for heat to bypass insulation layers. In retrofit projects, these bridges significantly reduce the effective performance of insulation systems, leading to higher energy bills, condensation risk, and potential structural damage.

Unlike air leakage, which is often visible and immediately apparent, thermal bridging operates silently. Heat flows through conductive pathways—typically metal fixings, concrete lintels, or poorly detailed junctions—transferring internal warmth to the external environment far more efficiently than through properly insulated sections.

Common Sources of Thermal Bridges in Retrofit

Structural Elements

• Concrete floor slabs and basement walls
• Steel beams and columns
• Reinforced concrete lintels above windows and doors
• Thermal mass elements where insulation cannot be applied effectively

Building Services and Fixings

• Metal wall ties and cavity closers
• Fixings for insulation boards and battens
• Service penetrations (pipes, electrical conduits)
• Brackets supporting external installations

Junctions and Details

• Window and door frames at reveals
• Balconies and canopies projecting through insulation layers
• Roof-to-wall connections
• Floor-to-wall transitions

Why Thermal Bridging Matters in Retrofit

The impact of thermal bridging becomes more pronounced in retrofit scenarios. When improving a building's fabric performance, unaddressed bridges can account for 20-30% of total fabric heat loss in some cases. This directly affects compliance with energy performance standards and undermines the return on investment for insulation measures.

From a PAS2035 perspective, thermal bridging must be systematically assessed and addressed. Buildings upgraded without proper thermal bridge mitigation may fail to meet improvement targets, and occupants may experience comfort issues including localised cold spots, condensation, and mould growth.

Quantifying Thermal Bridges

Professional assessment typically involves calculating linear thermal transmittance (Ψ-value, measured in W/mK) for each bridge. This value represents the additional heat loss through a junction compared to the main wall assembly. UK Building Regulations and SAP calculations require these values to be included in energy modelling.

Common Ψ-values for typical bridges range from 0.05 to 0.60 W/mK depending on design and materials. Cumulative effect is significant: a typical semi-detached house might have 100+ metres of thermal bridges, equivalent to losing heat through several additional square metres of uninsulated wall.

Mitigation Strategies

Design-Stage Solutions

• Continuous insulation: Locate insulation externally to wrap over structural elements wherever possible
• Thermal breaks: Use low-conductivity materials (cork, timber, plastic) to interrupt conductive pathways
• Insulated fixings: Specify thermally broken wall ties and brackets designed to reduce heat transmission
• Robust detailing: Plan junctions carefully to minimise exposure of conductive elements

Construction-Stage Solutions

• Install insulation continuously without gaps or compressions
• Use non-metallic fixings where structural requirements permit
• Carefully seal and insulate around service penetrations
• Apply high-performance sealants and gaskets at critical junctions
• Verify installation quality against design intent through site inspections

Post-Installation Assessment

Thermal imaging surveys can identify problem areas where cold spots indicate thermal bridging. These surveys are particularly valuable for verifying installation quality and identifying remedial measures needed before completion.

Key Considerations for Retrofit Projects

Retrofit presents unique challenges because existing structural constraints often prevent the optimal solutions used in new construction. External insulation is preferable to internal approaches as it avoids thermal bridges at structural elements, but may be restricted by conservation considerations, building aesthetics, or neighbour boundaries.

Where external insulation is applied to cavity walls, ensure cavity closers and wall ties are properly specified. For properties with solid walls or hybrid approaches, carefully detail every junction and service penetration. Balconies and other projecting elements require particular attention—insulating the main wall while leaving uninsulated concrete canopies protruding creates significant bridges.

The cost of addressing thermal bridges during retrofit—whether through design refinement, better materials, or additional detailing—is typically modest compared to the long-term performance gains and occupant comfort benefits achieved.