The dynamic thermal behaviour of the buildings can be represented by thermal networks. These oriented weighted graphs are graphical representations of the set of equations of heat transfer by conduction, convection, radiation and advection. Techniques for linearizing the models, especially the radiative heat exchange, are shown. Being linear (or linearized) models, the thermal networks can be described by a set of matrices and vectors. An original method for assembling the thermal networks by merging specified nodes is introduced. The advantage of assembling over coupling is that the obtained model represents a single set of equations which can be analysed. Then, it is shown how to transform thermal networks into state-space representation.