In the article the thermal stress state of the reinforced concrete foundation of a nuclear power plant during the building period are analyzed. The calculation results of thermal stressed state in massive foundation slabs with and without concrete creep is given. The article also provides a comparative selection of insulation to ensure crack resistance of the foundation plate with and without creep. Authors found that calculation of the problem at elastic definition leads to substantial over the tensile stresses and the elongation deformations on the surface of the slab for the point of time to create maximum of heat dissipation. Not taking into account concrete creep deformation in the problems of crack control for concrete and reinforced concrete massive structures in the building period leads to substantial increasing of required thickness of heat insulation.