The present work is part of research project titled 'Modeling, Simulation, Theoretical and Experimental Thermal Studies of Ghardaia Local Climate. Effect of Thermal Insulation'. The main objective of the current work was to determine the temperatures of the building in question with or without thermal insulation. This paper presents experimental and theoretical studies of two rooms thermal behavior. These rooms are two parts of an apartment building located in semi arid area (Ghardaïa). A mathematical model describing the thermal behavior of these rooms in question was developed and elaborated. These studies allowed also room internal temperature evolution profile to be determined. Through numerical simulation it has been found that the applied insulation layer reduced the losses of winter and maintained an appropriate temperature. It was found that the theoretically found results were consistent to an acceptable level with those found experimentally.