The thermal performance of a building refers to the process of modeling the energy transfer between a building and its surroundings. The objective of this work is to develop a new correlation to estimate the number of Nusselt predictions, to facilitate the design of the walls of buildings based on a numerical simulation with a computational fluid dynamics software which can be coupled after with the Lagrange polynomial interpolation method for high Rayleigh number. For this purpose, a building is modeled as a collection of basic elements (walls, rooms, etc.). Moreover, we developed a FORTRAN program to control the equation of high order. This method is for predicting exchange coefficient and estimating Nusselt number of convection to optimize the design of walls in buildings. This method was performed via the simulation and theoretical case.