Today, the world suffers from rapid climate change such as; high temperatures, Cooling Loads, and less thermal comfort in Buildings that affect the energy efficiency of buildings and the health of citizens. The problem is increasing cooling load values in buildings and energy consumption during the summer season to make thermal comfort in Spaces. The research discussed two parts of theoretical studies on the importance of using retrofitting techniques that are considered suitable solutions in buildings to reduce energy consumption. Therefore, the research aims to use retrofitting techniques to improve and reduce cooling loads and develop proposals for possible solutions to achieve energy efficiency through the second part of using the simulation program to reach the appropriate results to reduce the energy consumption of cooling loads. It is applied by selecting a project of an existing residential building in Egypt and using a simulation program such as; Design Builder in three Phases that can be compared with the base case with three cases through various techniques and parameters of suitable materials to study and calculate cooling load from the high value to low value. The simulation result shows the maximum cooling load value of 4.53 KWh/m2 in the Base case without any insulation materials or retrofitting techniques, therefore retrofitting techniques were used to reach a minimum cooling load of 2.56 KWh/m2 in the third case. So the best result is the third case of minimum cooling loads that achieves energy efficiency to make existing buildings sustainable.