Thermal insulation materials are widely used in engineering practice to reduce the depth of seasonal freezing. The effective thermal conductivity of the material is the main criterion for predicting the freezing depth of the structure base and determining the required thickness of the thermal insulation layer. However, the effective thermal conductivity of granular thermal insulation materials can significantly depend on the seasonal temperature, hydrological conditions of the soil and the degree of water content of the material. In this regard, calculating the effective thermal conductivity of granular thermal insulation materials in natural conditions is an urgent scientific and practical task. Granular foam-glass ceramic with a bulk density of 250 kg/m3 was used in the study. To solve the problem, we employed an experimental set to simulate the natural conditions of heat transfer in a horizontal layer, which makes it possible to change the magnitude and direction of the temperature gradient. It was established that the magnitude and direction of the temperature gradient have no significant influence on the layer of granules with a size of 5–10 mm. A predictive calculation of the temperature fields of the roadbed using experimental values depending on the water content degree was carried out. It was found that the depth of freezing of the roadbed covered with a 20 cm layer of foam-glass ceramic with effective thermal conductivity of 0.075, 0.111 and 0.138 W/(m·°C), respectively, is 12, 3.8 and 3 times lower than without the thermal insulation layer. A graphical interpretation of the temperature field in the form of -2°C isotherms shows that there is a dangerous zone of intense frost heaving with a depth of 58 cm forming at the roadbed without thermal insulation. Complete absence of zones of intense frost heaving in the roadbed covered with granular foam-glass ceramic was confirmed.