The building and spaces shape have a great impact on indoor temperature. Well-designed sun control and shading devices can dramatically affect indoor temperature. The thermal performance of interior spaces in hot arid desert is highly influenced by various passive design techniques, e.g. space dimensions, facade colors, fenestration ratio, glazing type; and vertical and horizontal shading devices. Simulation tools play an important role in taking decision during early design phase that could help in improving the thermal performance of buildings. The aim of this paper is to present the effect of vertical louvers length on the thermal performance of residential buildings in Egypt. To achieve this aim of the research, first, the climatic analysis of New Assiut City is introduced followed by identification for the prevailing residential patterns within the city and the selected residential model. Second, the role of the used simulation tool in enhancing the designs is highlighted. Finally, a simulation has been run for the selected residential model in New Assiut City over the four cardinal orientations. The results of the study showed that the vertical louvers with a protrusion of 38 cm or more result in a decrease of 2 °C in indoor temperature in all four orientation. However, for the northern façade it is noticed that any protrusion more than 38 cm had only a slight effect on the indoor temperature.