Lattice Boltzmann method (LBM) is employed to investigate natural convection inside porous medium enclosures at high Rayleigh numbers. Volume averaged porous medium model is coupled with the lattice Boltzmann formulation of the momentum and energy equations for fluid flow. A parallel implementation of the single relaxation time LBM is used, which allows the porous medium modified Rayleigh number Ram to be as high as 108. Heat transfer results in the form of the enclosure averaged Nusselt number Nu are obtained for higher modified Rayleigh numbers 104 ≤ Ram ≤ 108. The Nu values are compared with values in the absence of the form drag term. The form drag due to the porous medium is found to influence Nu considerably. The effect of the form drag on Nu is studied by using a form drag modified Rayleigh number RaC (extended from Ram). Utilizing the results for Nu in the high Ram range, a correlation is proposed between Nu and RaC for Darcy numbers 10-6 ≤ Da ≤ 10-2, encompassing the non-Darcy flow regime. © 2011 American Society of Mechanical Engineers.