The article deals with the research of the thermal cracking resistance of massive concrete and reinforced foundation slabs of buildings and structures in the building period. The article examines the results of the analysis of the thermal stress state of a massive foundation slab with a fixed thickness of thermal insulation as well as the results of changing the minimum thickness of the insulation on a surface, providing the cracking resistance of the structures on different plate heights, with and without taking into account the hardening temperature influence on the concrete modulus of the deformation. The article authors determined that the solution of the problem of definition the thermal stress state of the massive foundation slab in the building period without the hardening temperature influence on the modulus of deformation may cause a significant distortion of the real diagram of the thermal stresses and elongation deformations in the structures body. It was indicated that the calculation error essentially depends on the height of the foundation slab. Additionally it was established that in case the slab height exceeds 1.25 m the problem should be solved in a strict setting, which would allow to minimize the insulation layer.