## 2 Methods
### Methodology  
The carbon footprint throughout the building's life cycle was quantified by the environmental indicator known as Global Warming Potential (GWP). For the purposes of this study, the operational and embodied GWP in the building's Life Cycle Assessment (LCA) was calculated. The methodology employed for calculating the total GWP of the building is outlined in Figure 2. This indicator enables us to evaluate the building as a Zero-Emission Building (ZEB)-type. To assess the carbon footprint impact, the \"cradle-to-grave\" methodology, as defined in EN 15978 standard, is employed. Depending on the life cycle phase being analyzed, a distinction is made between embodied carbon and operational carbon. Figure 3 outlines the life cycle phases according to this standard and illustrates the concepts related to embodied and operational carbon. Figure 3: Description of the LCA Phases according to EN 15978 (“Modules analyzed). Figure 2: Methodology used in the calculation of total GWP. This study has been focused on the modules marked (*) (Figure 3), as they account for over 80% of carbon emissions, as indicated in the introduction section. Stages A1-A3 + A4 quantify embodied carbon, while stage B6 refers to operational carbon. (a) Operational Global Warming Potential (GWP). The operational GWP, defined by module B6 of the UNE-EN 15978 standard, evaluates Greenhouse Gas (GHG) emissions associated with energy consumption during the building's operational phase. This includes the energy required to maintain indoor comfort conditions, encompassing heating, cooling, and lighting systems. In the section below, Table 1 provides data on the final energy consumption of the nZEB building for 2022, as calculated using DesignBuilder software V7 [47]. By applying standardized weighting factors to the energy consumption figures, these values were converted into equivalent kgCO2 emissions, enabling a comprehensive assessment of the building's carbon footprint during its operational phase. Furthermore, Table 2 presents the calculated operational GWP for the building across various projected lifespans: 50, 60, and 70 years. The results are expressed as kgCO2 equivalents per m2 of usable interior floor area, offering a normalized metric to evaluate the building's environmental performance over its life cycle. This approach ensures consistency and comparability with global sustainability benchmarks, highlighting the building's contribution to reducing GHG emissions [48; 49]. Initial embodied carbon represents the Greenhouse Gas (GHG) emissions generated during the production phases of a building's life cycle, corresponding to stages A1-A3 as defined by the UNE-EN 15978 standard. These emissions occur prior to the building's operational phase and include those associated with the extraction and processing of raw materials, the manufacturing of construction products, and the construction processes. Additionally, stage A4, which accounts for the transportation of materials to the construction site, is integrated into the analysis. The calculation methodology for embodied carbon is grounded in a comprehensive inventory of construction materials, utilizing Environmental Product Declarations (EPDs) as the primary source of data. EPDs, developed in accordance with the ISO 14025 standard [50], provide detailed insights into the environmental impact of materials across their life cycle, enabling precise quantification of embodied GHG emissions. As highlighted in the building description, a significant proportion of the materials used in the project--63.8%--are recycled and characterized by low carbon emissions. This substantial use of recycled materials contributes to a marked reduction in the embodied \begin{table}
\begin{tabular}{c c c} \hline \hline  & **Final Consumption (kWh/Year)** \\ \hline Heating & 165,290 \\ Cooling & 9270 \\ Lighting & 33,020 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Energy consumptions (kWh/year).