http://www.ibpsa.org/proceedings/BS1991/BS91_300_307.pdf ; International audience ; This paper presents a new technique for the thermal systems modelling. This method, based on the structural modularity of a thermal system such as a building belongs to the modal methods family. It especially allows to obtain accurate enough reduced thermal models of large system. In this paper, the main advantages of this method are specified (recurrent, .). Its funtamentals are given and the results are illustrated by two examples (three layers wall and bizone building).