In the context of remodeling old buildings, enhancing insulation performance in the exterior skin necessitates an accurate assessment of a wall’s thermal performance. The conventional method for determining the thermal transmittance (U-value) of a wall is the heat flow meter (HFM) as outlined in the ISO 9869-1. However, this measurement is susceptible to errors influenced by indoor and outdoor environmental conditions and the wall’s material composition. This study evaluates the U-value of an internally insulated wall, specifically constructed for this purpose, utilizing both the average and dynamic methodologies of an HFM. Furthermore, it introduces a novel estimation method: the extended average method (EXAM). The effectiveness of this proposed method is ascertained by comparing the accuracy and convergence of the U-value estimations with those derived from existing methodologies. Additionally, the study explores the limitations of the HFM by analyzing the heat flow traversing the interior of a wall. The findings revealed that the EXAM method enhanced the precision of U-value estimation in all scenarios. Particularly, in walls with superior insulation, the HFM tended to underestimate the heat flow observed indoors, leading to negative errors. The EXAM method, incorporating considerations of both insulation and structural materials, offers an accurate in situ measurement of the U-value relative to the HFM.

Accurate and rapid in situ measurements of the thermal resistance (R-value) of building envelopes are necessary for assessing planned performance and identifying appropriate retrofitting strategies. Although there are several approaches for in situ R-value estimation, the average method of ISO 9869-1 based on the heat flow meter method is the most widely used. However, discrepancies between theoretical and in situ R-values are frequently reported in many studies that employ this method. This study aimed to investigate the cause of this discrepancy in estimating in situ R-values of highly insulated building walls using the average method of ISO 9869-1 by conducting long-term experiments. This study was made possible due to a specially constructed test wall wherein more sensors were installed than are required by the ISO 9869-1 standard. The findings showed that discrepancies between heat fluxes on the internal surface and heat passing through the wall is the main cause of the error in in situ R-value estimation. Measurement results from winter showed that deviation from the theoretical R-value was 9.12% for the average method and 0.6% for the extended average method, determined by additionally using the temperature and heat flux inside the wall.

Many studies have sought to overcome the two main limitations of the average method of ISO 9869-1—its long test duration and low accuracy. These studies reported that the reliability of the results is dependent on the temperature differences. This consensus was based on the results of studies that measured a few cases with specific temperature differences, and the convergence criteria utilized for the representative thermal transmittance (U-value) were rarely examined. This study analyzed the influence of the average temperature difference and test duration on the convergence characteristics and accuracy of U-value estimation using the average method. Data from a north-facing exterior wall with a theoretical U-value of 0.145 W/m²·K were measured between June 2022 and May 2023. The influences of different measurement conditions were analyzed for cases divided based on four measurement periods and 11 average air temperature differences. The findings show that an accurate U-value can be obtained from 7 days’ worth of measurement data with an average temperature difference of 10 °C or higher and that the improvement in accuracy is minimal under stricter conditions. To achieve a convergence probability of over 90% for temperature differences of 10 °C or greater, the second and third criteria required measurement periods of 7 and 15 days respectively.