Estimating In-Situ R-Value of Highly Insulated Building Walls Based on the Measurement of Temperature and Heat Flux Inside the Wall

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.