I already have Durock installed around the face of my sealed combustion DXV45 fireplace (NOT extending into non-combustible areas). Now I am told today by Mendota that durock should not be used as it can't deal with the heat above the unit, and I should use Hardibacker. Durock is listed as non-combustible... but I also see it has a 225 deg temp limit... and my FP can get to 250 or higher above top vent (according to specs). Is this all BS, and durock is fine, or am I better off swapping out at least the piece above the FP with Hardi?