A large amount of heat loss through building envelopes takes place via inhomogeneous components such as windows, doors, and thermal bridges. This loss can be approximated by measuring in lab the actual thermal transmittance of these components with the use of a hot box. The calibration and experimental procedures can be performed, taking into account three standards for calibrating hot boxes: the European EN ISO 8990; the American ASTM C1363-05; and the Russian GOST 26602.1-99. An experimental setup for testing the accuracy of these standards has recently been created at the University of Perugia; after a measurement campaign for the validation of the test rig, the differences of the approaches were evaluated. Results showed that although the EN ISO 8990 and ASTM C1363-05 are similar in terms of procedures definition, methodology of thermal transmittance calculation, and level of uncertainty, the GOST 26602.1-99 differs from the others since it adds individual measurements of the thermal characteristics of each sample component. The analysis highlighted that the ideal procedure should include the Russian method to define the thermal behavior of each component under analysis, with a contemporary validation of the global results to be performed with one of the other two approaches.