We study penetrative convection of a fluid confined between two horizontal plates, the temperatures of which are such that a temperature of maximum density lies between them. The range of Rayleigh numbers studied is Ra = [10(6),10(8)] and the Prandtl numbers are Pr = 1 and 11.6. An evolution equation for the growth of the convecting region is obtained through an integral energy balance. We identify a new nondimensional parameter, Lambda, which is the ratio of temperature difference between the stable and unstable regions of the flow; larger values of Lambda denote increased stability of the upper stable layer. We study the effects of Lambda on the flow field using well-resolved lattice Boltzmann simulations and show that the characteristics of the flow depend sensitively upon it. For the range Lambda = [0.01,4], we find that for a fixed Ra the Nusselt number, Nu, increases with decreasing Lambda. We also investigate the effects of Lambda on the vertical variation of convective heat flux and the Brunt-Vaisala frequency. Our results clearly indicate that in the limit Lambda -> 0 the problem reduces to that of the classical Rayleigh-Benard convection.