One of the most important factors in the thermal behavior of exterior walls is the thermal conductivity of the clay in their bricks. The higher the percentage of lightening additives incorporated, the better the thermal behavior of the bricks. But as their thermal characteristics are improved the mechanical properties of bricks deteriorate. The goal is to study the influence of paper pulp as a lightening additive on the thermal and mechanical properties of the fired clay, and to improve the thermal properties as far as possible without the mechanical properties falling below the required levels. This involved the preparation of test specimens that were then subjected to the corresponding assays of thermal conductivity and mechanical strength. It is found that around 15% of paper pulp can be added without the brick's mechanical properties ceasing to comply with current legislation. This amount of paper pulp has improved conductivity properties by 39.69% compared to the clay without additives, recording a minimum conductivity value at 10 degrees C of 0.45 W/m-K. This decrease in the conductivity of the clay results in a 16% improvement in equivalent thermal transmittance for brick walls made in the same way. (C) 2013 Elsevier B.V. All rights reserved.