Understanding Thermal Bridging in Retrofit

Thermal bridging is one of the most significant challenges in building retrofit. It occurs when materials with high thermal conductivity create a direct path for heat to escape through the building envelope, bypassing insulation. In retrofit projects, addressing thermal bridges is critical to achieving the energy performance targets required by regulations and to prevent condensation and mould growth.

What is a Thermal Bridge?

A thermal bridge (or cold bridge) is any element in the building fabric that conducts heat more readily than the surrounding materials. Common examples include:

• Steel or concrete structural elements passing through insulation layers
• Mortar joints in masonry walls
• Window frames and reveals
• Balconies attached to external walls
• Lintels above openings
• Junctions between walls, roofs and floors
• Cavity wall ties and fixings

Heat follows the path of least resistance. Where a material with poor insulation properties interrupts an insulation layer, heat will preferentially flow through that route, significantly reducing the effective thermal performance of the wall or roof assembly.

Why Thermal Bridges Matter in Retrofit

In new build, thermal bridge risk is managed through design and specification. In retrofit, you often work with existing structural elements that cannot be removed. This creates particular challenges:

1. Limited ability to redesign: Existing structure is fixed; you must work around it.
2. Hidden elements: Discovering thermal bridges during works is common and often requires mid-project problem-solving.
3. Performance guarantees: Unmanaged bridges will degrade predicted U-values and SAP ratings, affecting compliance and client satisfaction.
4. Surface temperature risk: Thermal bridges create cold spots on internal surfaces, increasing condensation and mould risk—a critical concern in occupied retrofit properties.

Quantifying Thermal Bridges

Thermal bridge impact is expressed as a linear thermal transmittance (Ψ-value), measured in W/mK. This value is multiplied by the length of the bridge and temperature difference to calculate heat loss.

For retrofit insulation upgrades, you should:

• Identify all significant thermal bridges during the survey phase
• Obtain Ψ-values from manufacturers or recognised standards (BS EN ISO 10211, Thermal Bridges in Building Construction)
• Include these in SAP calculations and U-value assessments
• Prioritise reducing high-impact bridges that appear repeatedly (e.g., cavity tie bridges in cavity walls)

Modern building control and retrofit guidance increasingly expect thermal bridge effects to be calculated rather than assumed.

Retrofit Strategies for Thermal Bridge Reduction

External Wall Insulation (EWI)

EWI is highly effective because the insulation layer can be placed on the outer face of structural elements, wrapping around most bridges. However, specific details require care:

• Lintels and sills: Use low-conductivity lintel covers or extend insulation around openings
• Balconies: Consider disconnection brackets or thermal breaks at the attachment point
• Cavity ties: Cannot be eliminated but their impact is reduced when embedded in external insulation

Internal Insulation

Internal insulation is more challenging for thermal bridges. Structural elements remain on the external face, conducting heat to the outside. Where internal insulation is used:

• Use thicker insulation to compensate for unmanaged bridges
• Apply vapour control carefully to prevent interstitial condensation
• Consider hybrid approaches: internal insulation with external wrapping of key elements

Detail-Level Solutions

Regardless of insulation strategy, specific details must be designed:

1. Window reveals: Insulate the reveal depth and frame edge; avoid thermal breaks alone without bulk insulation
2. Floor edges: Where floors meet external walls, ensure insulation continuity or use thermal breaks in joists
3. Roof-wall junctions: Extend insulation and vapour control seamlessly across the junction
4. Service penetrations: Seal and insulate around pipes, ducts and cables passing through the envelope

Design and Specification Best Practice

To manage thermal bridges effectively in retrofit:

• Commission a detailed thermal bridge survey and analysis during design
• Produce annotated drawings showing how each bridge is addressed
• Specify materials by thermal conductivity value, not product type alone
• Use recognised detail libraries (e.g., Accredited Details, manufacturer guides) as a baseline
• Brief installers on thermal bridge criticality and sign-off requirements
• Inspect junctions and details during installation; do not assume compliance from specification alone

Building thermal simulation software (e.g., 2D/3D modelling in accordance with ISO 10211) can validate whether proposed details achieve acceptable internal surface temperatures and prevent condensation risk.

Key Takeaways

Thermal bridging in retrofit cannot be ignored. It affects energy performance, condensation risk, and building durability. Successful retrofit requires:

• Early identification of bridges through survey and analysis
• Strategic choice of insulation approach to minimise unmanaged bridges
• Detailed specification of junctions and transitions
• Quality assurance during installation
• Integration of bridge data into performance calculations and compliance documentation

With systematic attention to thermal bridging, retrofit projects can achieve reliable, durable improvements in thermal performance and occupant comfort.