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This study proposes the air–surface temperature ratio (ASTR) method as an in situ measurement method to rapidly and accurately measure wall U-values in existing houses. Herein, the wall U-values were measured in situ applying the heat flow meter (HFM) method of ISO 9869-1 and the ASTR method. The results obtained using the HFM and ASTR methods were compared, and the relative error rate and accuracy of the measurements were analyzed. The aging rates of the wall U-values were compared and analyzed by comparing them with the wall U-values before and after the installation of retrofit insulation. Subsequently, the ASTR method was used to analyze the U-value measurement error rates according to the number of measurement days (one day to seven days). In addition, this method calculated the appropriate measurement period required to satisfy the measurement conditions. As a result, the mean relative measurement errors rates of the HFM and ASTR methods were ±3.21%. The short-term (one day) and long-term (seven days or longer) measurement results indicated the average error rates as approximately ±2.63%. These results were included in the tolerance range. Therefore, it was determined that the ASTR method can rapidly and accurately measure wall U-values.
Kim et al. (2018) studied this question.