Describes the heat flowmeter method for the measurement of the thermal transmission properties (thermal resistance and thermal conductance from surface to surface, total thermal resistance and transmittance from environment to environment) of plane building components, primarily consisting of opaque layers perpendicular to the heat flow and having no significant lateral heat flow. In detail, 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.