A comparative assessment of optimum and critical insulation thicknesses for external building walls in European climates

Abstract

The aim of this study is to investigate optimum and critical insulation thicknesses of external building walls in selected European climates using a life-cycle cost approach. Two widely used insulation materials, expanded polystyrene (EPS) and stone wool (SW), are evaluated for five representative provinces (Helsinki, Stockholm, Warsaw, Berlin, and Paris) for natural gas heating conditions. Optimum insulation thicknesses are found to depend strongly on both heating degree-days and fuel prices, varying from 17.3 to 33.3 cm for EPS and from 13.2 to 25.8 cm for SW. Application of optimum insulation reduces annual heating costs by about 90% and shortens payback periods (PPs) to below 1 year in most of the investigated cases, particularly for EPS due to its lower cost. The associated avoided CO2 emissions reach up to 46 kgCO2-eq/(m2⋅year) in cold climates. In contrast, critical insulation thicknesses determined to meet prescribed maximum heat loss coefficients provide thinner solutions with shorter PPs but lower long-term economic efficiency. In addition, this study introduces simple dimensionless relationships based on insulation and wall thermal resistance ratios to enable rapid estimation of energy, cost, and emission savings without repeated climate-specific calculations. The results provide practical guidance for cost-effective and low-carbon external wall insulation design for diverse European climatic conditions.