## 5 Discussion  
This study provides valuable insights into the potential of various facade retrofitting strategies for residential buildings in Saudi Arabia, with a primary focus on energy savings. While deep retrofits can achieve substantial energy reductions, their cost-efficiency must be carefully weighed by comparing energy savings to the cost of upgrading the building envelope. The building envelope, especially facades, plays a critical role in this regard, as it constitutes the largest portion of a building's external surface area. In extreme climates, retrofitting facades is expected to significantly reduce electricity costs. The investigation of external wall insulation strategies revealed that all eight examined options could reduce heat gains by 50% to 75%. This reduction in heat gain is directly linked to the decrease in the U-value of the proposed strategies, which subsequently lowered cooling energy demand. Similarly, upgrading windows proved to be an effective method for reducing cooling loads. The study assessed the impact of different glazing and shading device retrofit strategies, further emphasizing the importance of window and shading improvements in reducing heat gains.  
The presented analysis confirms the need for a holistic approach when evaluating retrofitting strategies. By comparing different elements of the building envelope against multiple objectives, such as energy savings and cost efficiency, an optimization process was conducted, testing 224 different facade compositions. The results highlighted that using ETICS for external wall insulation, louvers for shading, and low-e coated windows with Argon gas insulation was the most effective combination. However, it was evident that most of the energy savings stemmed from the external wall insulation, considering the relatively low window-to-wall ratio (about 10%) in the examined building. Additionally, reducing SHGC had a greater impact on cooling energy reduction than reducing U-value, owing to the intense solar radiation in the studied hot climate.  
Several limitations of the study should be noted. First, the study focused on the climatic conditions of the Eastern Province of Saudi Arabia, and its findings may not be directly applicable to other regions. Second, the study was centered on residential buildings, and other building types may allow for different retrofitting strategies due to varying design requirements, including HVAC systems. Future research should explore different climatic regions in Saudi Arabia and examine other building types. Moreover, further research is encouraged to investigate additional factors that contribute to facade retrofit efficiency, such as building orientation and window-to-wall ratio. Expanding the economic analysis by conducting life-cycle cost assessments and sensitivity analyses would also be beneficial. In addition, future studies should consider the impact of retrofitting strategies on occupants' comfort, including visual and thermal comfort. Finally, it is recommended that the environmental impact of retrofit materials be examined, the integration of renewable energy systems be explored, and the emerging smart technologies in the construction industry in this regard be investigated.