This standard establishes the principles and criteria to be complied with for the determination of the laboratory steady-state heat transfer properties of masonry walls in a hot box by means of a heat flow meter mounted on one face of the masonry wall to be tested (i.e. the test specimen). It describes the apparatus, measurement technique and necessary data reporting. It does not, however, specify a particular apparatus design since requirements vary particularly in terms of size, and also to a lesser extent in terms of operating conditions.