Abstract The effect of composting temperature, phase and substrate composition on thermal conductivity of the composted substrate was analyzed. Thermal conductivity was affected by all of the above parameters. Temperature was a significant factor when the difference in the temperature range exceeded 5 °C. Significant differences were observed in substrates where the content of sewage sludge differed by 20% (between 50% and 30%). Thermal conductivity values differed significantly between the phases of the composting process. Thermal conductivity of composted material was determined at 0.37 ± 0.10 in phase I, 0.39 ± 0.072 in phase II, 0.23 ± 0.03 in phase III, and 0.29 ± 0.04 W/mK in phase IV. In the first phase of the process, substrate temperature reached 20 °C, and it was determined at 60 °C, 50 °C and 25 °C in the following stages of composting. At 20 °C, thermal conductivity varied in the range of 0.15–0.37 W/mK.