Abstract Since 2009, the Construction Technologies Institute of the National Research Council of Italy has been developing a laboratory process for the evaluation and certification of the thermal performances of growing media for green roofs. The results of the first two experimental phases have highlighted the significant difference between the thermal resistance values of growing media and those of natural occurring soils, depending on both the composition and the degree of compaction and internal moisture content. This study aims to complete the research study by demonstrating and quantifying the error made in the standard calculation of the thermal resistance of a green roof stratigraphy if uncorrected thermal resistance values are chosen for growing medium and drainage layer . The experimentation is based on the comparison, under the same configuration and system boundaries, of the thermal performances of an extensive green roof configuration according to two different calculation methods as provided for in standards EN ISO 6946 and ISO 9869. The results underline that the potential discrepancy between calculated and monitored thermal resistance values can exceed 40% if no appropriate and specific inputs are assumed for growing media and drainage layer. Instead, if correct values are used, the calculation discrepancy can be reduced to 11,8% and this could be useful to designers to solve problems during standard calculations. This calls for further testing and for a reference standard for calculating the thermal resistance of growing media for green roofs and drainage layer that have become all the more urgent.