# Design of the Refurbishment of Historic Buildings with a Cost-Optimal Methodology: A Case Study
### Nzeb  
The standard commonly used in Europe regarding this topic is ISO 50001 [3], focused on improving the energy performance of buildings including energy efficiency, use and consumption. This norm establishes the use of an energy management system within existing buildings. This system is founded on the premises of finding potential energy savings with the application of improvement measures and monitoring such improvements from the building's baseline. Hence, the key element is to obtain real indicators of the building and compare them with the reference values. Using the energy savings as benchmark, the selection of the set of measures (catalog defined for the building) should be analyzed.  
The developed methodology must be completed by studying all possible improvement measures that make up that catalog. For this, the most used are cost-optimal procedures as proposed in European regulations [4]. In this line, there are many works that have in common the use of the optimal cost procedure [5; 6; 7; 8]. The main objective of these studies was the evaluation of the interest of every measure and their combinations, allowing finding the optimal alternative by taking into account different constraints: economic optimum, maximum extra cost or maximum payback period.  
Another point of view of the procedure to obtain NZEB appears in the standard [9]. This standard defines the first three stages to achieve net zero energy buildings. The first stage requires knowing the real needs of the building and acting on them (step 1: energy needs). Once characterized, the need to analyze the systems of that building arises (step 2: energy consumption). In addition, finally, overcoming these two points becomes imperative to analyze the possibility of producing the energy consumed by that building (step 3: renewable energies) or even producing more and exporting the surpluses. In addition to the aforementioned challenges, there is an intrinsic complexity of acting on existing buildings (rehabilitation).  
If the standard in [9] is analyzed, it may be understood that stages 1 and 2 are linked to the difficulty of accurately characterizing the real energetic situation of the building. Related to this, there are many research studies that base their results on the estimation of the needs of the buildings, for example studies on the implementation of renewable energies. The majority of these works show the need to carry out a realistic modeling of the building, both geometrically and energetically. [PERSON] et al. [10; 11; 12] developed different approaches to model buildings with the available geographical data: the complete study of photovoltaic potentials with 3D district modeling [10], thermal loads simulation [11] and shading analyses for studies related to solar resources, also defining operation strategies [12]. Presently, studies on the estimation of PV potentials are relevant and interesting, as shown by [PERSON] et al. [13].  
The roadmap design to obtain an optimal rehabilitation project was previously defined by life cycle studies, appearing in research and norms developed from several studies [14; 15; 16]. These studies can be classified according to the procedure implemented for the evaluation of the energy performance: detailed procedures with commercial or own simulation tools, and simplified procedures.