## 3 Results
### Model Usefulness
#### 3.3.4 Number of Buildings and Total Floor Area per the Worst-Performing 43% of Buildings  
The 2024 EPBD recast establishes the identification of the worst-performing 43% of buildings, establishing the number of buildings and total floor area of the worst-performing 43% of residential buildings as mandatory indicators [2]. The European directive requires establishing the definition of worst-performing buildings. For this study, we hold that the NRPEC value is the most appropriate indicator to determine which buildings are the least efficient. With this model proposed, the 43% with a higher NRPEC per m\({}^{2}\) is identified, obtaining a total of 3,732,037 buildings with 5,748,002 dwellings which correspond to a built surface area of 612,877,691 m\({}^{2}\). The average NRPEC of this group of buildings is 303.46 kWh/(m\({}^{2}\cdot\)y), and the total NRPEC of 179,919,636,781 kWh/y. Analyzing the worst-performing 43% of buildings, we observed the following outlines:  
* Single-family houses are 92% of the worst-performing buildings. * The buildings located in climate zones D and E, the coldest, represent 80.10% of the buildings. * The buildings built before 1940 are 35.34%; between 1941-1960, 16.15%; between 1961 and 1980, 25.41%; and between 1981 and 2007, 22.62%. * The limit to be considered a building in the worst-performing 43% of buildings is an NRPEC of 253.22 kWh/(m\({}^{2}\)y). This model enables targeted identification of building zones with specific energy performance across municipalities, districts, regions, and climate zones. Given that NRPEC values vary significantly by climate zone, the model is able to isolate the lowest-performing 43% of buildings within each climate zone in Spain. This approach identifies not just the highest-consuming buildings, which are generally concentrated in colder zones, but those with the poorest energy performance relative to the energy demands of their respective climate zones. Table 5 outlines the NRPEC and GHG emissions thresholds that categorize a building within the lowest 43% of performers in each climate zone. These findings highlight the significance of the climatic zone, with the top 43% of energy consumption being 2.6 times higher in cold zones compared to warm zones, alongside emissions that are 2.3 times greater. This underscores the pressing need for targeted interventions in these areas, as they present the highest potential for energy savings. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Climate Zone** & **Limit Value kWh/m\({}^{2}\)y** & **GHG Limit kgCO\({}_{2}\)eq/m\({}^{2}\)y** \\ \hline Alpha 3 & 125.4 & 32.2 \\ A2 & 148.6 & 34.8 \\ A3 & 146.9 & 28.7 \\ A4 & 153.4 & 29.5 \\ B3 & 194.1 & 42.8 \\ B4 & 192.1 & 38.0 \\ C1 & 250.0 & 56.0 \\ C2 & 229.3 & 49.4 \\ C3 & 220.1 & 45.5 \\ C4 & 230.8 & 47.2 \\ D1 & 293.7 & 67.4 \\ D2 & 284.7 & 63.4 \\ D3 & 278.8 & 60.5 \\ E1 & 328.3 & 74.3 \\ \hline \hline \end{tabular}
\end{table} 
Table 5: Limit value of the NRPEC to be considered a building in the worst-performing 43% of buildings in every climate zone in Spain. 3.5 Annual Operational Greenhouse Gas Emissions (kgCO\({}_{2}\)eq/m\({}^{2}\)y) per Building Type  
Residential buildings in Spain have an average NRPEC of 239.35 kWh/m\({}^{2}\)y and average GHG emissions of 52.04 kgCO\({}_{2}\)eq/m\({}^{2}\)y. The data is shown in Table 6, classifying the types of buildings according to the classification used by Spain's Long-Term Renovation Strategy (LTRS-ERESEE) [60,61]. This classification has also been used in previous studies [62, 63, 64, 65, 66, 67] and is structured based on the building type: single-family housing, multi-family blocks with three or fewer floors, and multi-family blocks with four or more floors, followed by the building's construction period. It is noteworthy that NRPEC and GHG emissions have significantly decreased over recent decades, primarily due to energy regulations introduced in 2008 and 2019 within the construction sector.