Stationary and transient heat conduction in multilayer non-homogeneous stratigraphy

摘要

The tendency to speed up the building process and reduce its capital costs is bringing to the increased use of semi-finished products in constructions. One frequent characteristic of those elements is the use of non-homogeneous layers of materials, possibly affecting their thermal properties. In addition to this, the gradual increase in the thickness of insulation layers in buildings is also shifting the focus from the heating loads to the cooling loads, especially for hot climates such as southern Europe, raising the necessity to evaluate the transient properties of construction elements, alongside with the most common stationary properties. The aim of this paper is to correctly evaluate both stationary and transient heat conduction properties of strongly non-homogeneous multi-layered constructions and evaluate the impact of the non-homogeneities. The evaluation of those properties are based on the definitions detailed in the EN ISO 13786 directive, for dynamic thermal characteristics, and in the EN ISO 6946 directive, for stationary thermal characteristics. The stationary thermal transmittance is the parameter considered for the stationary analyses; for transient analyses transient thermal transmittance, time shift and attenuation factor are evaluated. The methodology provided inside the respective directives is applied in order to estimate the parameters. Following, a finite elements model is implemented in the COMSOL Multiphysics software and the same properties are calculated through a finite element analysis performed based on the conditions detailed in the physical definitions of those properties in the respective directives. Each parameter assessed based on the respective directives, EN ISO 13786 and EN ISO 6946, is then compared with the results of the finite elements analyses. Based on those comparisons the impact of the non-homogeneities in the construction for the calculation of its thermal properties are evaluated and conclusions on their relevance in the identification of the thermal properties are given, also the ability of describing non-homogeneous constructions through the directive’s methodology is discussed.