  In this paper we employ renormalized viscosity and thermal diffusivity to
construct a subgrid-scale model for large eddy simulation (LES) of turbulent
thermal convection. For LES, we add $\nu_\mathrm{ren} \propto \Pi_u^{1/3}
(\pi/\Delta)^{-1/3}$ to the kinematic viscosity; here $\Pi_u$ is the turbulent
kinetic energy flux, and $\Delta$ is the grid spacing. In our model, the
turbulent Prandtl number is unity. We performed LES of turbulent thermal
convection on a $128^3$ grid and compare the results with direct numerical
simulation (DNS) on a $512^3$ grid. There is a good agreement between the LES
and DNS results on the evolution of kinetic energy and entropy, spectra and
fluxes of velocity and temperature fields, and the isosurfaces of temperature.
We also show the capability of our LES to simulate thermal convection at very
high Rayleigh numbers and exhibit some results for $\mathrm{Ra=10^{18}}$.
