Heat Transfer behaviour through light building envelope wall can be managed using Phase Change Material (PCM) to enhance latent thermal mass with minimal burden on wall structure. By absorbing heat during the day and releasing it at night, PCM incorporated wall can reduce diurnal maximum inner wall surface temperature and decrease cooling load for air conditioning system. The research numerically investigated thermal behaviour daily cycle of a lightweight metal-sheet building envelope with fiberglass insulator incorporated with varied amount of PCM layer under Bangkok weather condition. Implicit Finite Difference Method using Crank-Nicolson scheme was applied for numerical calculation. The result indicates that the amount of PCM used influents the effectiveness of wall temperature reduction and how latent heat storage is activated per weight of implemented PCM.