Some authors use the symbol e to denote the thermal responsivity, although it usuage along with an exponential becomes difficult. The SI units for thermal effusivity are W s / ( m 2 K ) , or, equivalently, J / ( m 2 K s ) . Thermal effusivity is a good approximation for the material's thermal inertia for a semi-infinite rigid body where heat transfer is dominated by the diffusive process of conduction only.