## 2 Literature Review  
Building retrofitting, also known as building renovation, involves modifying existing structures to enhance their performance after they have been constructed and used [15]. One of the primary benefits of retrofitting is energy savings, which directly translates into economic gains [16]. Retrofit projects can be categorized into several types. These include behavioral interventions aimed at improving building operations and maintenance, which can lead to up to 15% energy savings; minor retrofits, which can achieve 15-45% savings; and deep or major retrofits, which can result in 45-60% or more energy savings [17]. While deep retrofits offer higher energy savings, their cost-effectiveness must be carefully assessed by comparing the energy savings to the costs associated with upgrading the building envelope and mechanical systems, considering the financial and technical risks involved [18; 19]. Another benefit of retrofitting is the enhancement of indoor thermal comfort and Indoor Air Quality (IAQ), achieved with lower energy consumption [20; 21]. The building envelope plays a crucial role in retrofitting, as facades make up the majority of the external surface area through which heat transfer occurs. Windows, a key component of facades, are among the weakest elements in terms of heat resistance [22]. Retrofitting the building envelope involves several strategies, including replacing windows, installing new seals to reduce air leakage, and lowering the U-value by adding insulation to roofs, external walls, and floors [23]. Adding both internal and external insulation is a common approach to improving the thermal performance of external walls by minimizing thermal bridges and insulating construction joints. Internal insulation involves adding materials such as cork, fiberglass, and rockwood, which can be applied in various forms, including spray-in-place, rolls, or rigid boards. External insulation, on the other hand, encompasses several retrofittingtechniques, such as the External Thermal Insulation Composite System (ETICS), ventilated facades, and prefabricated panels. External cladding not only increases insulation efficiency and reduces heat absorption but also enhances the building's appearance [24; 25]. ETICS is a widely used method for retrofitting external walls, where an insulating layer is applied to the facade, followed by a finishing layer or decorative cladding [26]. Another system, the ventilated facade, consists of an external facade attached to an internal one with a frame structure, incorporating thermal insulation and an air gap for natural ventilation. Various cladding materials can be used as the finishing layer [27]. Prefabricated wall panels provide a time-efficient and minimally disruptive solution for upgrading existing buildings. These panels typically feature a modular timber frame filled with mineral wool for insulation [28]. Window retrofitting is also essential for improving buildings' energy efficiency. Several solutions can be implemented, including double or triple glazing, Argon gas filling, and low-emissivity (low-e) glass [24; 29]. The choice of frame material also plays a critical role, as it can enhance airtightness and allow for thermal insulation, such as polyurethane foam injection into aluminum profiles [30]. Smart glass is another advanced option for window retrofitting, with the ability to automatically adjust to control glare and solar gain. It can be categorized into passive types, like thermochromic glass, and active types, such as electrochromic glass [31]. Electrochromic glazing, for instance, can change tint when a low-voltage DC current (up to 10 V) is applied, providing control over light transmission and heat [32]. Additionally, installing shading devices such as vertical fins and overhangs further reduces heat gains, enhancing overall energy performance [33]. Several studies in the literature highlight the role of building retrofitting in achieving energy savings.