The modeling of heat transfer in materials containing exothermic components must take into consideration the presence of heat sources in the Fourier–Kirchhoff equation. The aim of this investigation was the identification of real and effective thermophysical parameters of the insulating–exothermic materials used as riser sleeves containing these exothermic heat sources. The experiments of steel pouring into the mould, containing different insulating and exothermic sleeves were carried out, using thermocouples measurement systems (thermal analysis of casting–mould system). Then the thermophysical coefficients of these materials were calculated using inverse problem solution. The worked time–dependent formula of exothermic reaction heat (heating yield in W/m3) was called heat source function. The paper presents the basis and the practical expression of heat source by different functions, its justification and the results of simulations using these functions. The numerical system Calcosoft and its Inverse Solution procedure were applied.