A method for quantitatively evaluating the impact of defects on wall U-value using infrared thermal imaging

The presence of walls’ thermal defects can lead to large deviations between predicted and actual values of energy consumption. Infrared images have been used to compare the differences in U-value (heat transfer coefficient) and heat flow between defect and normal building walls. However, the extent to which different defects affect the U-value of building walls remains to be investigated. This study proposes a method for evaluating the impact of various defects on the U-value of building wall envelopes. Infrared images were acquired for a building in Beijing, China to demonstrate the method. First, the accuracy of the method for measuring heat flow through building walls was validated using the heat flow meter method. Second, the K-means method was applied to classify wall pixel temperatures and to identify defects of building walls. The area and boundary of defects were got. Then, the validated infrared method was used to calculate the U-value for different defects. Finally, the difference in heat transfer coefficients $(\mathrm{\Delta }\overline{U_{\mathrm{s}\mathrm{d}}})$ was used to evaluate the impact of various defects on the building wall. The results indicated that variations in the area and temperature of defects contributed differently to the wall’s U-value. The maximum difference between the U-value of the repaired defect and the unrepaired wall is 0.42 W/(m²·K). The method proposed in this study quantitatively assesses the extent to which defects affect the U-value of a building wall, providing reference data for making decisions regarding wall renovation.