Silica Aerogel is material with a special structure consisting of interconnected solid particles of SiO2 forming the skeleton that enclose nanopores filled with confined air and occupying more than 90% of the volume. It is characterized by a thermal conductivity that can reach lower values than any other material. Our aim was to explain the causes of the super-insulation of this material with the use of a calculating method of conductive heat transfer flux in an aerogel structure and determine the equivalent thermal conductivity. For this purpose, numerical specific software was developed to generate random structure of silica aerogel with predefined concentration of solid particles and properties of both skeleton and confined air. Calculation of the conductivity at any point in the confined gas domain shows variable values as a function of the pore size and the location of the point in the pore. A new developed numerical method was used to calculate the equivalent thermal conductivity of the whole structure. Concentration of solid particles has proved not to be the only parameter in which determine the thermal conductivity of silica aerogel and the influence of tortuosity has been demonstrated. A correlation linking thermal conductivity to both concentration of solid particles and tortuosity of the material was suggested.