Thermal bridging occurs when materials with high thermal conductivity create pathways for heat to bypass insulation layers. In retrofit projects, thermal bridges are a primary cause of performance shortfall—the gap between predicted and actual energy savings. Understanding where they occur and how to address them is essential for delivering compliant PAS2035 work.
When retrofitting existing buildings, you inherit structural details designed before modern thermal standards existed. Concrete lintels, steel beams, masonry ties, and poorly detailed junctions create uninsulated routes for heat loss. Left unaddressed, thermal bridges can account for 10–30% of total heat loss through external walls, negating the benefit of cavity wall insulation or external wall systems.
PAS2035 requires that retrofit designs minimise thermal bridging through specification and detailing. This isn't optional—it's a core principle of the standard, particularly for major works where insulation is being upgraded.
Visual inspection is your first tool. During site surveys, look for:
Thermal imaging confirms suspicions. Infrared thermography identifies cold spots on internal surfaces where thermal bridges exist. During winter, exterior thermal bridges show as localised cooler areas on external facades. This evidence is valuable for design verification and client communication.
Calculation quantifies impact. Building regulations now require thermal bridge calculations using linear transmittance (Ψ-value) for major structural elements. Accredited software and standardised calculation methods (such as ISO 10211) provide psi-values that feed into whole-building U-value calculations. Ignoring thermal bridges in calculations can result in non-compliant designs.
Key point: Always calculate thermal bridge contribution to overall fabric heat loss. A single unmitigated lintel can reduce whole-wall performance by 2–5% depending on its extent and linear length.
Breaking the conductive path is the most effective approach. Options include:
These solutions work best when integrated into the retrofit specification from design stage.
Externally applied insulation systems (EWI) can wrap thermal bridges if designed correctly. The insulation layer should extend across lintels, floor edges, and structural features where possible. However, wrapping creates its own complexity—careful detailing is required around windows, doors, and at interfaces with uninsulated elements.
Where external solutions aren't viable (listed buildings, conservation areas), internal approaches include:
Internal solutions are less effective than external strategies but provide improvement when other options are constrained.
Thermal bridges often coincide with air leakage paths. Sealing and vapour control work hand-in-hand with thermal mitigation. Gaps and voids allow convective heat loss that compounds conductive bridging.
Design stage: Commission detailed thermal modelling and bridge calculations. Specify insulation products and installation methods that address identified bridges. Build this into contract drawings and specifications—don't leave it to site interpretation.
Procurement: Ensure suppliers understand thermal performance requirements. Thermally broken lintels, high-performance fixings, and insulation materials cost more but are essential for compliant retrofit.
Site supervision: Thermal bridges are often created during installation through poor workmanship. Inspect cavity trays, insulation placement, frame installation, and sealant application. Common failures include incomplete insulation packing, gaps at floor-to-wall junctions, and misaligned thermal breaks.
Commissioning: Thermal imaging post-completion can verify that designed mitigation has been properly executed. This evidence supports compliance claims.
PAS2035 expects design intent to be realised on site. Document thermal bridge calculations, retain product datasheets, photograph installations, and use thermal imaging to verify completion. This evidence supports your compliance narrative and protects against performance disputes.
Thermal bridging is often invisible to clients but critical to retrofit success. Clear communication about why thermal breaks matter—and what happens without them—helps secure buy-in for the additional specification and cost they require.
Purpose-built retrofit coordination software — document generation, compliance auditing and project management.
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