Method for the determination of a metric for a thermal transmittance of a building element, comprising the steps: - Emission of microwave signals in a multiplicity of frequencies onto a particular building element, - Detection of microwave signals that are reflected at the particular building element, and at transitions between layers in the particular building element, - Determination of a number of layers in the particular building element from the reflected microwave signals, in particular by determination of the propagation time of the reflected microwave signals, - Electromagnetic modelling of the particular building element in a computer model, wherein the computer model has a layered structure of the particular building element, with the determined number of layers, and comprises particular layer thicknesses and particular complex permittivities of the layers as variable parameters, - Specification of initial values for the variable parameters of the computer model, - Iterative customisation of the computer model by repeated variation of the values of the variable parameters, and by selection of a set of values of the variable parameters on the basis of a match between i) a reflection characteristic of the computer model, determined analytically or by simulation, and ii) an actual reflection characteristic, determined from a ratio between the emitted and the detected reflected microwave signals, - For each of the layers modelled in the computer model with the selected set of variable parameters: determination of a particular thermal conductivity of the particular layer on the basis of a first predetermined data set, wherein the first predetermined data set indicates a relationship between complex permittivities and thermal conductivities, and - Determination of the metric for the heat transfer of the particular building element from the determined layer thicknesses and thermal conductivities of the particular layers. - Emission of microwave signals in a multiplicity of frequencies onto a particular building element, - Detection of microwave signals that are reflected at the particular building element, and at transitions between layers in the particular building element, - Determination of a number of layers in the particular building element from the reflected microwave signals, in particular by determination of the propagation time of the reflected microwave signals, - Electromagnetic modelling of the particular building element in a computer model, wherein the computer model has a layered structure of the particular building element, with the determined number of layers, and comprises particular layer thicknesses and particular complex permittivities of the layers as variable parameters, - Specification of initial values for the variable parameters of the computer model, - Iterative customisation of the computer model by repeated variation of the values of the variable parameters, and by selection of a set of values of the variable parameters on the basis of a match between i) a reflection characteristic of the computer model, determined analytically or by simulation, and ii) an actual reflection characteristic, determined from a ratio between the emitted and the detected reflected microwave signals, i) a reflection characteristic of the computer model, determined analytically or by simulation, and ii) an actual reflection characteristic, determined from a ratio between the emitted and the detected reflected microwave signals, - For each of the layers modelled in the computer model with the selected set of variable parameters: determination of a particular thermal conductivity of the particular layer on the basis of a first predetermined data set, wherein the first predetermined data set indicates a relationship between complex permittivities and thermal conductivities, and - Determination of the metric for the heat transfer of the particular building element from the determined layer thicknesses and thermal conductivities of the particular layers.