The in-situ thermal resistance (R-value) measurement of building walls is essential for the accurate assessment of the thermal performance of an envelope and is lately a subject of increasing attention. The present study examines the four available standardized methods: for the R-value measurement as described in two international standards: ISO 9869 and ASTM C1155. The required measuring period and the variability of the results of each method are examined by measuring the thermal resistance of three representative walls (drywall, rubble and brick) at different measuring conditions in terms of surface temperature difference and direction of heat flow (stable or alternating during the day). It is concluded that the two most commonly used methods, the Average and the Summation-method, require high temperature difference between the indoor and outdoor surfaces of the tested wall in order to provide R-values in short measuring period with low variability. On the contrary, the required measuring period for the other two methods, the Dynamic and the Sum of Least Square method, appears to be independent of the measuring conditions. The resulting values have low variability as long as the direction of heat flow is stable during the measurements. (C) 2017 Elsevier B.V. All rights reserved.