Description of multilayer walls by means of equivalent homogeneous models

The accurate characterization of building envelopes represents the first step to realize a model of built structures, upon which is possible to work for the design of appropriate retrofit measures. External building walls, indeed, are one of the most significant paths of heat transfer between outer and inner environment and, as such, their control is crucial to go in the direction of energy saving. Simple models that capture the essential thermal characteristics of building walls, which are usually layered structures, would be desirable. A homogeneous wall that can be equivalent, under appropriate circumstances, to a given multilayer structure would provide an example of such a simple model. Homogeneous walls that are completely equivalent to generic multilayer walls are in general not achievable. However, we show in this work that this is possible in particular cases, i.e. if one focuses only on temperature or on heat flux behavior. It is shown that, in such situations, both an exact and an approximate solution for equivalent thermophysical properties can be devised. The evaluation of the level of approximation introduced is presented. In addition, the significance and the implications of the introduced equivalent models is discussed both regarding their exact and their approximate formulations.