The present work deals with the determination of a mathematical correlation for conductivity in the fully water-saturated state in terms of dry-basis conductivity and porosity. In the mathematical model, the material microstructure is taken into account in a multiscale percolation system and the macroscopical conductivity is obtained with a renormalization technique. The model is presented and the obtained correlation is tested for some porous building materials . To conclude, we show how porosity can affect thermal conductivity.