Energy efficiency in the construction industry is crucial to reducing increased energy consumption. A significant portion of the energy is consumed in residential buildings. Thermal properties of the materials used in the building envelope can reduce the energy consumed in the buildings and thus contribute to the building economy. For this purpose, in the study, structural lightweight concretes (SLWC) with a lower thermal conductivity than normal weight concrete (NWC) were produced and energy efficiency and life cycle costs were compared between these concretes on a 1 + 1 reference flat. The compressive strength, unit weights and thermal conductivity coefficients of SLWCs and NWC were determined experimentally. Heating and cooling energy consumption and life cycle costs for the flat were calculated using the DesignBuilder simulation program according to the different concrete types produced. The results indicate that the thermal conductivity coefficients of all SLWCs produced were about 37-45 % lower than those of NWC. All mixes of the SLWCs provided energy saving by about 18-25 % compared to the NWC and two SLWCs reduced the life cycle cost by 4 %. In addition, the results showed that the best SLWC about energy was not the best SLWC about life cycle cost.