The aim of this work is to study the possibility of improving the thermal transmittance of a single-leaf wall made with lightened clay bricks. A first aspect is to study the possibility of lightening the clay by using additives, reducing the clay's conductivity without decreasing its resistive ability. The clay's conductivity has been decreased by up to 40% through the use of a suitable proportion of additives, with no reduction in the brick's compressive strength. This drop in conductivity (40%) represents a reduction of 20% in the wall's thermal transmittance. The internal and external geometry of the bricks has been studied by the finite element method (FEM) for two types of clay, decreasing brick conductivity by 35%. Different wall assemblies with these bricks have been studied, obtaining a 24% improvement with a thin horizontal joint assembly. To quantify the savings made, a standard building has been studied, which has shown that decreasing the wall's thermal transmittance by 43% would save 7.9% on the annual heating demand, according to calculations made with specialized energy efficiency rating software.