A closure for the effective relaxation time of the Boltzmann-BGK kinetic equation for fluid turbulence is presented, based on a double-averaging procedure over both kinetic and turbulent fluctuations. The resulting effective relaxation time appears to agree with values obtained via a renormalisation group treatment of the Navier-Stokes equation only at low values of k/T, the ratio of turbulent kinetic energy to fluid temperature. For k/T > 0.1, the kinetic treatment delivers a significantly longer effective relaxation time.