Thermal insulation characterization of polymers and other materials is an important requirement for present and future aerospace missions, as well as for the home construction industry. Some dispersed composites and layered samples have been successfully characterized by the thermal‐diffusivity flash technique, while the interpretation of these and other experiments for other systems remains problematical. One refinement for layered‐sample data reduction is investigated here. An exponentially decaying spatial penetration of radiant energy into the sample is accounted for. Under the conditions chosen for numerical evaluation, an 18% reduction in the back‐face rise timet1/2is predicted for a case in which only 5% as much radiant energy is deposited on the midplane as on the front surface. It is suggested how the use of the entireV(t) response function might lead to experimental methods of estimating the penetration depth of the radiant energy for particular samples.