## 3 Case Study: Apartent Building MedZEB Renovation  
### Apartent Building  
This section presents the current state of an apartment building by describing its structure, envelope and building systems specifications. The building, which has been studied and simulated in detail, is located in an area of urban expansion in the City of Castellon de la Plana (Spain). This area is characterized by a high-density urbanism dating back to the 1970's and 1980's. The case study (Figure 3) is an apartment building built in 1979. In its current configuration, the building has seven floors above ground level and a basement, with a total built area of 3698.14 m\({}^{2}\). It has a trapezoidal floor plan, covers a total length of 48.2 m in the southwest facade (front elevation), and 48.6 m in the northeast (back elevation), while the envelope wall facing southeast measures 16.7 m and the one facing northwest 12 m (Figure 4). The envelope for the front and back elevations is a non-insulated exposed-brick system with an air cavity and an internal brick-lined wall rendered with plaster. The internal partitions are made of hollow brick rendered with plaster on both sides. The roofing system is standard for the buildings of this period, namely a non-insulated, non-ventilated roof finished with ceramic tiles and waterproofed with bituminous membranes. The windows and other facade openings have steel or aluminium frames and single glazing with very low levels of airtightness and thermal performance. Therefore, roof and facades have no insulation and, even if thermal bridges represent a high part of the thermal losses, the building envelope itself (including the openings with single glazing) represents an important part of the thermal losses too. The whole building is supported by a reinforced concrete structure. Internally, its layout is divided into three entrance halls, two of which give access to 12 residential units, and the other one to 11. In the basement there is a shared parking lot connecting the three blocks and the ground floor has space for five retail units. This kind of apartment building has no heating, cooling or mechanical ventilation systems. The domestic hot water is provided by natural gas/butane boilers. The climate of Castellon could be described by a winter severity climatic index of 1.24, and a summer severity climatic index of 0.62 according to the methodology proposed by [PERSON] et al. [32] and [PERSON] et al. [33]. The building components considered to estimate the present building's energy performance are the envelope (facades, roof, ground floor slabs), thermal bridges, airtightness, and ventilation air flow rates, as well as the internal partitions, glazing system, heating, cooling, domestic hot water production and ventilation system. Their main geometrical and thermal properties are gathered in Table A1 in Appendix A. Key issues identified prior to the renovation were significant thermal bridging situations, high heating and cooling demand and a general need for maintenance. These factors opened a window of opportunity for upgrading to a higher quality/performance level instead of renovating to the original situation [34; 35]. Indeed, the objective of the HAPPEN project is to trigger energy savings of 60%. In this case, the building envelope is characterized by (i) one type of opaque exterior walls, referred to as the facade, which also includes its openings; and (ii) two different types of horizontal elements, the external one referred to as the roof, and the one referred to as the slab, which separates the retail units on the ground floor from the apartments on the first floor. Six slabs creating the different levels are added to determine the thermal zones and enclosures in the energy simulation (Figure 5). Figure 4: Main floor plan (**left**) and section (**right**) of the building in its current state. Figure 3: Panoramic view of the building—Main facade is oriented to the southwest according to urban plot. Source: [31].