Sensitivity of Product-Stage Global Warming Potential to Declared and Design Thermal Conductivity in Sustainable Retrofit Design

Thermal modernization of existing buildings is an important part of sustainability-oriented retrofit practice because it can reduce operational energy demand, but its environmental effect depends partly on the insulation material selected and on the thermal assumptions used in design. This study examines how the use of declared thermal conductivity (λ decl ) and design conductivity (λ design ) affects the required insulation thickness and the A1–A3 global warming potential (GWP) of alternative insulation materials for an attic ceiling separating heated space from an unheated ventilated attic in a multi-family building. This study supports product-stage sustainability assessment; it does not constitute a comparison of the full life cycle climate effect of the selected material groups. The thickness needed to achieve U target = 0.15 W/(m 2 ·K) was determined for scenarios based on λ decl , temperature-corrected λ design , and a moisture sensitivity analysis for cellulose. Environmental assessment was based on European EN 15804+A2-compliant EPDs, with separate reporting of GWP fossil and GWP biogenic . In the analyzed case, differences between material groups were driven mainly by EPD data, whereas conversion from declared to design thermal properties had a smaller, but not negligible, effect. This effect became more important for moisture-sensitive materials. The results show that sustainability-oriented environmental comparisons based only on declared thermal conductivity may be misleading when functionally equivalent solutions are compared. In the analyzed case, the transition from λ decl to temperature-corrected λ design resulted in only a small change in the required insulation thickness and the corresponding GWP result. At the same time, the scenario-based sensitivity analysis for cellulose insulation and the variability of data reported in the EPDs indicate that moisture-related assumptions and the selection of input data may be of greater importance. The results show that, when interpreting the environmental performance of insulation solutions in sustainable retrofit design, consistency should be maintained between the adopted functional unit and the method used to define the thermal properties of the material after installation in the building envelope.