Performance, endurance, and aesthetic value losses occur due to environment, human, and time factors in windows, which is a significant component of buildings in terms of function and aesthetics. The maintenance planning of windows should be conducted on time during the usage phase as well as accurately implementing the decisions taken at the design stage so that the windows can serve for a long time. If maintenance and repairs are not performed on time, directly or indirectly, deteriorations that will decrease the performance of buildings and products’ lifespan will occur. These issues will affect the performance and aesthetic properties of windows and facades. A systematic approach that ensures effective use of timely accurate identification, analysis, and intervention regarding the malfunctions encountered in windows during occupancy is presented. The proposed model that will be used by experts is aimed to prevent erroneous maintenance practices, protect user comfort, reduce usage costs, and contribute to the national economy by conserving resources via a decision support model for the deterioration of windows. The decision support model that is developed consists of three main stages, namely condition assessment, determination of possible intervention alternatives, and preparation of the condition report and inventory, and a support database is created to provide data for these stages. The first stage of the model, also known as the condition assessment stage, consists of three phases: data collection, condition analysis, and assessment. General information about the building and window are recorded during the data collection phase. In the second phase, i.e., condition analysis, the deterioration intensity and the area of disturbance will be determined after detecting malfunctions in windows. In the condition assessment phase, deterioration’s severity is determined using NEN 2767 method, while deterioration’s scores in the component are calculated by using the cardinal scale method. The second stage of the model, which is known as the determination of possible intervention alternatives, consists of four steps: possible intervention alternatives, intervention priority scores, intervention scores, and appropriate intervention alternatives. Possible intervention alternatives are prepared according to intervention lists created in the support databases. In determining the intervention priority scores (the second step), intervention priority score was given by evaluating the effects of deterioration on the three parameters, i.e., aesthetics, performance, and strength, on a 5-point scale according to the type of deterioration and the component in which it is located. In the last step of determining the possible intervention alternatives, intervention type and intervention procedures are selected according to the intervention score. The third stage of the model, called as the preparation of the status report and inventory, involves information on the current state of the window, the diagnostic methods used, and possible intervention alternatives. In the identification of deterioration in windows, on-site non-destructive diagnostic methods were examined within the scope of the model. In the study, it is decided to use internal and external observations and to make crack measurements and alignment checks when necessary. The proposed model is limited to windows only, although it could be used with all facade components. The model was applied to six windows in total by choosing two windows from three frame types in an education building in Istanbul, where wooden, PVC, and aluminum framed windows are used. The data of the aluminum-framed window were shared within the scope of the article. It is observed that physical effects and usage errors affect the service life and performance of the window in terms of the effects of environmental and human-induced factors on windows’ deterioration. The conclusion was that deteriorations in the examined aluminum-framed window affect the aesthetic, thermal, air, and water performances of windows. In conclusion, an expert can obtain results of condition assessment swiftly, completely, and numerically by filling out the checklists in the program prepared during data collection and analysis stages if the proposed model is used. The model is designed to create an inventory by recording the status of buildings’ components that require maintenance such as windows during usage. This should provide data for studies on the types and causes of deterioration in building elements. The developed proposal of the “decision support model”, should raise awareness of decision-makers, manufacturers, and designers for renovation, to protect user comfort, to reduce value losses in facade and window systems, to prevent performance losses resulting from erroneous interventions, and to protect national resources.