A model to calculate effective thermal properties (heat conductivity and temperature diffusivity) of substrates is proposed. Obtained theoretical results are in good agreement with experimental data on the effective heat conductivity of the substrate Balkanin. The contributions of vapor diffusion to the effective thermal conductivity of a substrate is analyzed by comparing the heat conductivity at temperatures between 15-45°C.The results of the effective thermal properties studies are used to evaluate the results obtained from the Greenhouse-2 plant growth experiment on the Mir Space Station. The temperature variations in the range of 25-35°C observed during the Greenhouse-2 experiment significantly affected the effective heat conductivity of the substrate used in this experiment. The importance of correct interpretation of the data on moisture levels obtained by the heat pulse moisture sensors used in the Greenhouse-2 experiment is shown.