In the present work, numerical thermal analysis is used in order to optimize the lightweight hollow block design for internal floors with respect to the energy saving, by the finite element method (FEM). From an initial block design with 0.57 m x 0.25 m x 0.20 m constant external dimensions, other five different configurations were built varying the number the vertical and horizontal intermediate bulkheads. Besides, five different compositions of lightweight concrete have been taken into account, giving place to sixty different configurations of the floors, thirty per each heat flow direction: upward and downward heat flows. Based on the non-linear thermal analysis of the different configurations, it is possible to choose the best candidate block from the CTE rule requirements. Mathematically, the non-linearity is due to the radiation boundary condition inside the inner recesses of the blocks. Also, the temperature distribution and thermal characteristic values of the floors, both for downward and upward heat flows, are provided. From the numerical results, we can conclude that the main variables in the thermal performance are: the number of the horizontal intermediate bulkheads and the material conductivities. Therefore, increasing the number of horizontal intermediate bulkheads and decreasing the material conductivities, the best thermal efficiency is obtained. Optimization of the floors is carried out from the finite element analysis by means of the average mass overall thermal efficiency and the equivalent thermal conductivity. In order to select the appropriate floor satisfying the CTE rule requirements, detailed instructions are given. Finally, conclusions of this work are exposed. (C) 2009 Elsevier B.V. All rights reserved.