The document applies to the determination of the 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 wall to be tested. It describes the apparatus, measurement technique and necessary data reporting. The property that is measured is the surface-to-surface thermal resistance of the specimen. From these measurements the thermal resistance for application in buildings are derived. The thermal transmittance of a masonry wall can then be calculated from this value with standardized surface coefficents.#,,#