Abstract
Accurately assessing heat loss in historic masonry buildings is essential for improving energy efficiency, yet discrepancies between theoretical and measured thermal transmittance (U-values) remain a persistent challenge. This study investigates the thermal performance of red porphyry masonry walls in South Tyrol through a combination of in situ U-value measurements, laboratory analysis, and computational modelling. Over three winter seasons (2022–2024), U-values were measured on 10 historic buildings, while theoretical values were derived from material properties obtained through laboratory testing of porphyry samples from quarries and ruins, as well as historic lime mortars and plasters. A detailed stratigraphic analysis of traditional masonry typologies provided key insights into stone-to-mortar ratios and internal wall composition. By comparing calculated and in situ values, this research identifies a systematic correction factor that can improve predictive accuracy. The findings emphasize the need for an integrated approach that combines non-invasive measurement techniques with targeted material analysis to enhance the reliability of U-value estimations.