Aerated autoclaved concrete is used as one of basic materials of partition constructions. Its characteristics contribute for energy consumption decrease of construction work. Bulk density and humidity of aerated autoclaved concrete (AA concrete) are significantly influencing its thermal conductivity and other thermo-technical characteristics. This article shows the results of measurements of thermal conductivity of AA concrete (based on fly ash) at different bulk density and variable humidity of the material. The ISOMET 2104 a portable measuring device was used in this experiment. Acquired results demonstrate functional relationships between thermal conductivity of AA concrete, bulk density and humidity. The reliability and accuracy of the method of measuring and the measuring device was also vindicated.