The impact of thermal properties of different building wall materials on time lag (TL) and decrement factor (DF) have been explored in the current study. Before the construction of a building, the selection of wall material is essential. The investigation carried out by using numerical model using Finite Difference implicit backward difference method under convection boundary condition has been developed. A computer program in Fortran has been developed to solve the one-dimensional transient heat problem. The varying solar radiation and outdoor air temperature are periodic boundary conditions at the external surface of the building wall. The constant design room temperature is the boundary conditions at the internal surface of the building wall. The effect of material properties and the external convective coefficient on TL and DF have been investigated. The results show that the thermal properties of selected four wall materials significantly affect TL and DF. From the results, it is concluding that by choosing the higher thermal mass wall materials, we get a higher value of TL and a lower value of DF. Results also show the variation of indoor and outdoor surface temperature. The insignificant influence of the outdoor convective heat transfer coefficient on the TL and DF has also been observed. So the result achieved is helpful for designing the building envelope.