Decarbonization Assessment of External Wall Systems: Thermal Transmittance and Cradle-to-Gate Embodied Carbon Comparison of Conventional and Modular Configurations

Improving the environmental performance of ageing residential buildings requires design strategies that consider both thermal performance and material-related embodied impacts. This study provides early-stage, wall-level comparative evidence on the decarbonization potential of alternative external wall assemblies. A representative Hungarian “Kádár” Cube house is used as a reference to evaluate three configurations under identical geometric and climatic conditions: (i) the original uninsulated masonry wall, (ii) a masonry wall retrofitted with an External Thermal Insulation Composite System (ETICS), and (iii) a modular wall system based on Structural Insulated Panels (SIPs) applied in a morphology preserving reconstruction scenario. The analysis combines steady-state thermal transmittance (U-value) calculations with a cradle-to-gate (A1–A3) embodied-carbon assessment. Results show that both ETICS and SIP solutions substantially reduce heat loss compared to the baseline wall. The SIP configuration achieves the lowest U-value and cradle-to-gate embodied carbon due to its lightweight structure and reduced material mass. As the study focuses exclusively on wall-level A1–A3 impacts and steady-state thermal indicators, the findings support early design-stage decision-making rather than full building-level decarbonization modelling. The results highlight the importance of jointly considering thermal-transmittance performance and embodied impacts when comparing retrofit and modular reconstruction options for ageing residential buildings.