A theoretical equation of the thermal resistance and adiathermic power of a coated fabric with nanocomposites is established. Samples, whose properties are analyzed by these equations, were constructed from commercial resins and Tunisian natural clay. Adiathermic power was determined by using a PASOD device measuring the necessary voltage to maintain a temperature difference between the inside and the outside of the fabric equals to 20°C. The mathematical equation, which modelling the coated fabric thermal behavior, contributes to analyze and explain the measured adiathermic power results by inserting the thermal resistance values. It was found that increasing quantity of clay enhance significantly the thermal insulation of a 400 g m-2 Sergey fabric 100% cotton. The calculated values show that the thermal resistances are in a good agreement with the measured adiathermic power values, which generally increases when the fabric becomes more resistant to the going through heat flow.