Abstract: The problem of the theoretical derivation of a parameterization for the eddy diffusivity in decaying turbulence is addressed. This derivation makes use of the dynamical equation for the energy spectrum density and the classical statistical diffusion theory. The starting point is the Heisenberg's elementary decaying turbulence theory. The main assumption is related to the identification of a frequency, lying in the inertial subrange, characterising the inertial energy transfer among eddies of different size. The resulting eddy diffusivity parameterization is then applied to the decay of convective turbulence in the Residual Layer. Besides the intrinsic scientific interest, this topic has relevance for mesoscale transport and diffusion simulations. The resulting expression for the eddy diffusivity cannot be solved analytically. For this reason an algebraic approximated formulation, giving nearly the same results as the exact expression, is also proposed.