The present paper deals with a detailed numerical analysis of natural convective heat transfer in a rectangular enclosure subjected to heating from one side. The analysis is done independently for two different fluids: water (Pr = 7) and air (Pr = 0.7) by considering the Rayleigh number variation between 10³ and 4×10⁵ to cover both asymptotic and early laminar convection regimes. The solution of the governing equations (continuity, momentum, and energy) is obtained using a finite difference scheme with the Boussinesq approximation in MATLAB. The validation of the numerical method is successfully carried out with the help of benchmark solutions of De Vahl Davis and highly accurate solutions of Wan et al. After that, an obstacle (baffle) of small thickness is fixed at the mid-height of the cavity without hindering the flow, and the effect of the same on the overall heat transfer rate is analyzed. Finally, a low-density flap is attached to the upper edge of the obstacle to allow one-directional flow. Such a configuration allows designing a one-way thermal insulator; convective heat flow can be easily allowed from one side, while it is efficiently restricted from the opposite side when the valve is shut. These findings suggest that the presence of the baffle alone has no significant impact on the heat transfer process, which is reduced only by 2% for air and 0.2% for water due to the use of the baffle alone. In comparison, the heat transfer process is decreased by up to 95% for air and 92% for water with the help of the baffle-valve system.