The present paper thematically fits in the current concern for reducing energy consumption to ensure normal thermal comfort in residential and tertiary building heated spaces. Windows are an important area of the building envelope through which heat dissipates from the interior space to the exterior. As it is known the new insulating window types significantly reduce energy dissipation by both transmission and infiltration. This paper aims to establish the theoretical correlations that exist between different constructive functional parameters of insulating windows, such as gas or gases located between two or three window sheets and low-ε films to be applied on interior faces of the transparent elements. The theoretical work procedures as well as the tabulated and graphical results regarding the thermal resistances for various insulated window structures are presented. The results are analyzed in comparison with the values contained in the C107 regulation for building heat calculations.