In this work a numerical study of the thermal behavior of a multilayer building wall (3 and 5 layers) is presented. The external side of the wall is subjected to the local atmospheric conditions of Algeria. The finite differences method is used to solve the transient heat transfer equations through the building wall, which is submitted to a solar heat flux, and a convective heat transfer with the environment. A sinusoidal forms of the external temperature and the solar flux were used to approach the measured data of Ouargla city (Algeria) (attitude 31°57' N, longitude: 5° 20'E, altitude 123-315 m), during the summer and winter seasons. An economic study is presented and a solution for a good thermal insulation at a lower cost is proposed.