The purpose of this research is to assess thermal performance and energy saving of a residential building in the hot semi-arid climate of Marrakech (Morocco). The studied house is built as usual in Marrakech without any thermal insulation except for its external walls, facing East and West, which are double walls with a 5 cm air gap in between (“cavity wall” technique). The cavity wall effective thermal conductivity was carefully calculated taking into account both radiation and convection heat transfers. Experimental results, obtained from winter and summer monitoring of the house, show well dampening of air temperature, thanks to its thermal inertia. However, this temperature remained outside the standard thermal comfort zone leading to large cooling/heating load. Simulation results indicate that the cavity wall contributes to an overall reduction of 13% and 5% of the house heating and cooling loads respectively. Moreover, the addition of XPS roof thermal insulation significantly enhances the heating and cooling energy savings to 26% and 40% respectively.