Today the issue of energy saving is acute. The main sources of energy are radiant energy of the Sun, wind energy, energy of moving water. Therefore, the issue of solving alternative energy sources is relevant. The article aims to solve the problem by using low-potential heat of the soil mass by means of energy-efficient building constructions - foundations. It is necessary to know the thermal characteristics of soils for this. At the moment, methods for determining the thermophysical properties of inert materials with subsequent practical application in the field of construction have been widely studied, but no one of these methods takes into account the grain-size composition. Thus, the study of the connection between the thermal conductivity and the grain-size composition of the soil is important. The aim of the work is to Estimation of thermal conductivity of sandy soils based on grain-size composition. This article presents an analysis of the dependence of the thermal conductivity of the sandy soil of its grain-size composition. The matrix of experiment planning is made; the methodology and technological sequence of the experiment were tested. Statistical processing of the obtained experimental data was carried out. Based on a series of test experiments, it was concluded that there are two factors competing in its thermal conductivity: an increase in λ due to an increase in the degree of pore filling and a decrease in total heat conductivity due to a decrease in the degree of pore filling. These results suggest that grain-size composition has an impact on the thermal conductivity of the sandy soil. During the experiment, the dependence of the thermal conductivity of sandy soils on their grain-size composition was experimentally established.