The aim of this current and innovative work is the numerical thermal analysis of multi-holed lightweight concrete blocks for external and non-habitable floors by the finite element method (FEM). Twelve different block designs with the same external dimensions 0.57 x 0.45 x 0.20 m were built varying the number of the horizontal intermediate bulkheads, from 3 to 12. Besides, five different compositions of the lightweight aggregate concrete (LWAC) and five different bulk temperatures have been taken into account, giving place to a total of 600 different floor configurations, 300 cases per each heat flow direction: upward and downward heat flows. A nonlinear thermal problem is solved for all cases analysed and then, it is possible to choose the best candidate block from the standard rule requirements. Mathematically, the nonlinearity due to the radiation boundary condition inside the inner recesses of the blocks is tackled by the matrix radiation method. Once the nonlinear thermal problem is solved, the temperature distribution is obtained and the thermal characteristic values of the floors, both for downward and upward heat flows, are calculated. From the numerical results, we can conclude that the main variables in the thermal performance are the total number of recesses and the material conductivities. Therefore, increasing the number of horizontal intermediate bulkheads and decreasing the material conductivities, the best thermal efficiency is obtained. The selection of the best candidate block of external floors and floors in contact with non-habitable spaces is carried out through the following parameters: the average mass overall thermal efficiency and the equivalent thermal conductivity. Finally, detailed instructions are provided in order to select the appropriate floor satisfying the standard rule requirements and conclusions of this work are exposed. (C) 2010 Elsevier Ltd. All rights reserved.