The state seeks to achieve the principles of sustainability contained in the 2030 Sustainable Development Strategy related to the rationalization of energy consumption, so the environmental dimension is one of the most important elements that must be taken into account in the design of buildings, especially in the hot desert regions, because of its great importance in achieving thermal comfort for building users and reducing the use of cooling, air conditioning and heating. The final roof of the building is considered one of the important components of the building envelope, which plays a major role in protecting the internal spaces from the external environment with harsh climatic conditions. Vault is one of architectural elements that contribute to improving the thermal performance within roofing with. The research assumes that the change in the formal elements of the vault (shape- proportions- dimensions) leads to an improvement in its thermal performance, especially in hot desert regions, The study aims to improve the thermal performance of roofed spaces in vaults in hot desert regions of Egypt by changing its formal elements, the research adopts the inductive method by studying the climatic features of the Egyptian hot desert region, the concept of thermal comfort, methods of evaluating them inside building, definition of the vault and its different forms and thermal characteristics, and the field monitoring methodology of the vaults in Egypt (their forms – their locations), through the field surveys of Egyptian cities, and finally an applied analytical approach using a simulation program for the elements of the vault, The results showed some of the necessary design criteria that must be followed when using the vault as a cover for the spaces in the buildings built in the hot desert regions of Egypt.