This part of ISO 9869 describes the heat flow meter method for the measurement of the thermal transmission properties of plane building components, primarily consisting of opaque layers perpendicular to the heat flow and having no significant lateral heat flow. The properties which can be measured are: a) the thermal resistance, R, and thermal conductance, Λ, from surface to surface; b) the total thermal resistance, RT, and transmittance from environment to environment, U, if the environmental temperatures of both environments are well defined. The heat flow meter measurement method is also suitable for components consisting of quasi homogeneous layers perpendicular to the heat flow, provided that the dimensions of any inhomogeneity in close proximity to the heat flow meter (HFM) is much smaller than its lateral dimensions and are not thermal bridges which can be detected by infrared thermography (see 6.1.1). This part of ISO 9869 describes the apparatus to be used, the calibration procedure for the apparatus, the installation and the measurement procedures, the analysis of the data, including the correction of systematic errors and the reporting format. NOTE 1 It is not intended as a high precision method replacing the laboratory instruments such as hot boxes that are specified in ISO 8990:1994. NOTE 2 For other components, an average thermal transmittance may be obtained using a calorimeter or by averaging the results of several heat flow meter measurements. NOTE 3 In building with large heat capacities, the average thermal transmittance of a component can be obtained by measurement over an extended period, or the apparent transmittance of the part can be estimated by a dynamic analysis of its thermal absorption response (see Annex B).