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Thermal Bridge Risks in Retrofit
Common thermal bridge mistakes in retrofit projects, with a focus on PAS 2035 requirements, junction detailing, and the fRsi 0.75 criterion for controlling condensation risk.
THERMAL BRIDGING
7/28/20262 min read
Common Thermal Bridge Mistakes in Retrofits
Thermal bridging remains one of the most frequent causes of underperformance in retrofit projects. A dwelling may be upgraded with additional insulation, replacement windows, and improved airtightness, yet still exhibit localised heat loss, lower internal surface temperatures, and elevated condensation risk if junctions are not designed and executed properly.
In retrofit work, the principal issue is often not the primary insulation layer itself, but the discontinuities within the thermal envelope. Window reveals, floor edges, roof-to-wall junctions, ground-bearing interfaces, and service penetrations represent typical geometric and material weak points, particularly where the existing construction was never intended to operate as a continuous insulated system.
PAS 2035 requires retrofit design details to eliminate thermal bypass and minimise thermal bridging at junctions, corners, and edges of insulation layers. Where a standard accepted detail is not adopted, the junction should be justified by calculation, and the resulting internal surface temperature factor (fRsi) should not fall below 0.75.
Why Retrofit Thermal Bridges Matter
The significance of thermal bridges in retrofit is not limited to incremental heat loss. Their more critical effect is often the reduction in internal surface temperature at vulnerable junctions, which can create conditions conducive to surface condensation, mould growth, and occupant discomfort even where the overall fabric U-values appear satisfactory.
This is particularly relevant in retrofit because the existing building fabric typically contains multiple legacy junctions, material changes, and construction tolerances that do not align with modern thermal envelope expectations. If these interfaces are not addressed as part of the thermal upgrade strategy, the performance of the improved fabric will remain constrained by the weakest details.
Common Mistakes
A frequent mistake is to focus on the main insulation layer while leaving reveals and junctions insufficiently addressed. In practice, openings are usually the weak points where thermal continuity is broken, and surface temperature performance is most affected.
Another common issue is insufficient attention to structural penetrations and junction geometry during design. Floor slabs and lintels can introduce significant linear thermal bridging unless the insulation layer is maintained continuously or the detail is specifically designed to interrupt the heat flow path.
A further issue is relying on generic details without verifying junction performance for the actual construction. Retrofit buildings vary substantially in substrate, exposure, orientation, and workmanship tolerance, so a detail that appears acceptable in principle may still fail to meet the required surface temperature criteria when assessed properly.
How To Avoid It
The most effective approach is to assess thermal bridge risk early, before the insulation specification is fixed. Junctions should be reviewed alongside the architectural and construction detailing, rather than left as a site-level resolution after the main fabric measures have been selected.
For each project, it is useful to identify where the insulation line terminates, where the airtightness line is interrupted, and which junctions are likely to produce the lowest internal surface temperatures. That process helps prioritise the details that are most likely to govern condensation risk and overall fabric performance.
Where the geometry is complex or the risk is uncertain, thermal bridge calculation and hygrothermal assessment can be used to demonstrate compliance and quantify the impact of the detail. This is especially important in retrofit situations involving moisture-sensitive assemblies, exposed locations, or heritage constraints.
Practical Takeaway
The essential point is that retrofit performance is governed by continuity. If the insulation layer is improved but the junctions, penetrations, and interfaces are not resolved, the result can still be a cold, uncomfortable, and condensation-prone building.
A robust retrofit should therefore do more than improve headline U-values. It should produce a continuous thermal envelope, maintain airtightness continuity, and demonstrate junction performance with appropriate evidence. That is the practical intent behind PAS 2035: not just better materials, but better detailing, better coordination, and better performance assessment.
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