This study seeks to improve the geometrical distribution of bricks to optimise the equivalent thermal transmittance of a wall built of Termoarcilla (R) ECO 29 voided clay bricks, using calculations according to Spanish UNE [1], AENOR [10], European EN [6-9] and international ISO [11,121 standards. The objective is to study improvements in the wall's thermal conductivity, always remembering the limitations imposed by the manufacturing process. Simulations are made using a finite elements application [14]. It is concluded that, within the possibilities allowed by the manufacturing process, with non-rectangular voids the heat flux has to cross a higher number of voids, which improves its thermal properties. A rhomboid layout of voids with the longer diagonal at right angles to the heat flux is the best internal void layout. If the internal perforations are also extended to the end of the tongue and groove, the direct thermal bridge in this type of brick is broken. Finally, a 290 mm wide brick with 25 rows is obtained with the geometrical properties described above which shows an improvement of almost 16% over the original ECO 29 brick, performing well in all climatic areas of Spain, with a full-bed mortar joint 30 mm thick. (C) 2011 Elsevier B.V. All rights reserved.