Thermal transmittance is often considered a constant value in building design and assessment of building performance. However, the thermal transmittance of the building envelope changes as a result of aging and environmental parameters. To accurately evaluate the energy performance of existing buildings, it is important to define the degradation of the thermal performance of the entire wall composition. This study proposes a simple method of calculating the thermal transmittance of a building's envelope, accounting for age and environmental conditions, by applying a correction coefficient to the initial thermal transmittance rather than conducting in-situ measurements. In-situ measurements of wall thermal transmittance were conducted in 40 older buildings using visual observation and the heat flow meter method. By examining the environmental factors that affect deterioration in the performance of insulation in an exterior wall through linear regression analysis, the study found that the correction factor has a high correlation with the type of exterior wall finish, color, surface finishing, pollution source, sea exposure, and the presence or absence of adjacent buildings. Three scenarios are proposed to quantify the correction factor, with the goal of obtaining an optimized model that can predict the thermal transmittance of building envelopes. These scenarios are reviewed based on ISO 15686 and regression analysis, and the scenario with the correction factor that best estimated the measured value is adopted. The thermal transmittance of the walls of older buildings, as estimated by the simple method proposed in this study, shows the best match with the on-site measured results.
