Energy consumption in residential buildings soared enormously during the pandemic. To construct highly efficient residence, materials are one of factors that need to be considered due to their impact of climate change, power consumption and operational cost. It was found that power plants produced various types of waste products that can be utilized in many applications. For instance, Cenosphere produced from power plants could promote thermal properties of mortar brick to a certain extent. The aim of this study is to evaluate the effects of Cenosphere as partial sand replacement on thermal properties of mortar and buildings’ energy efficiency at different percentages (0%, 5%, 10%, 15% and 20%). Both thermal conductivity and specific heat capacity were detected by using Fox 50 instrumentation while energy efficiency was determined by using Autodesk Revit and Green Building Studio (GBS) software. The findings of thermal properties show that the replacement of sand with 20% Cenosphere as partial sand replacement have significantly reduces thermal conductivity while increasing the specific heat capacity of mortar. This study revealed that the k value of mortar bricks have reduced as low as 0.62 W/m.K and as high as 932 J/kg.K for specific heat capacity due to incorporation of Cenosphere and mixtures at 20%. On the other hand, the EUI of 5% Cenosphere has reduced 2.13 kWh/m2 or 1.4% lower than control mix. The energy saving measure largely influenced by the composition of Cenosphere as compared to the buildings’ orientations.