This paper investigates the effect of emissivity on the thermal performance of a double wall façade with a closed cavity (DWF). A transient simulation was performed under real conditions of solar radiation and ambient temperature in Rabat. The governing equations are solved using finite element method with COMSOL Multiphysics considering air flow of the cavity and heat transfer by conduction, convection, and radiation. The results show that reducing the emissivity of a DWF from 0.9 to 0.2 will decrease the thermal transmittance of the overall wall by 72%, therefore it leads to an important energy save in the cooling load of building by 24%.