Whole‐House Thermal Bridge Modelling for Accurate Retrofit Energy Prediction

ABSTRACT This study investigates thermal‐bridging heat loss in domestic retrofit projects, a factor that can significantly affect energy‐performance predictions. Commonly, retrofit energy assessments rely on Standard Assessment Procedure methodology's default values for thermal‐bridging heat losses since calculating bespoke Ψ values using numerical modelling techniques can be complex and resource intensive. This study compares the impact of adopting default values versus calculated Ψ values for a case‐study home across pre‐ and multiple post‐retrofit scenarios, including External Wall Insulation (EWI), Internal Wall Insulation (IWI), Building Regulations and EnerPHit standards delivered through piecemeal and whole‐house approaches. This work employs finite‐element numerical modelling that is PAS 2035 compliant for a single 1930s solid‐wall UK dwelling. Whole‐house approaches reduced heat transfer coefficient by up to 63% (EWI) and 61% (IWI) compared with baseline, while potentially mitigating surface condensation risks. Thermal‐bridging contribution decreased to 16% (17.5 W/K) for EWI and 11% (12.7 W/K) for IWI, specifically due to these seven additional PAS 2035 compliant junction detailing measures applied on top of the basic fabric retrofits (loft, walls, and floor). The study demonstrates that default values are often not representative, especially for homes with fabric insulation retrofits. This has implications for reliance on default inputs in energy‐performance predictions and the accuracy of policy modelling frameworks. The findings suggest that a more comprehensive and representative database of thermal‐bridging Ψ values for common construction and PAS 2035 compliant retrofit scenarios would support more reliable energy‐performance predictions.