## 1 Introduction
### Carbon Footprint  
The CF is the most widely employed indicator within the LCA methodology. Its direct relationship with the GHG Control Protocols and its ease of application for environmental decision-making has been responsible for its success [16]. The main strength of CF is definitely its ability to be communicated to a broad audience [29], and it has gained popularity and helped raise public awareness in recent decades. Authors such as [PERSON] and [PERSON] [15] advocate greater transparency in the communication of the limitations and methodological choices associated with the use of CF.  
There are a wide variety of studies related to the determination of CF in construction. [PERSON] et al. [30] analysed the impact of CF throughout the life cycle of buildings by applying it both in new buildings and in rehabilitation. [PERSON] et al. [31] evaluate the embodied energy of 95 cases of residential buildings, mainly located in Europe. They use a cradle-to-site LCA analysis, that is, an analysis covering A1-A5 life-cycle phases, which correspond to manufacturing (A1-A3) and construction (A4-A5), according to the modules defined in the standard UNE-EN 15978 [32]. The results for total carbon emissions lie in the range of 348.5-6485 kg CO2 eq/m\({}^{2}\) during a 50-year service life. Embodied emissions vary between 128 and 1350 kg CO2 eq/m\({}^{2}\) and operational emissions between 97.5 and 6032 kg CO2 eq/m\({}^{2}\). Previous studies [33] resulted in embodied carbon emissions for residential buildings lying in the range 250-750 kg CO2 eq/m\({}^{2}\), which falls within the range calculated by [PERSON] et al. [31]. [PERSON] et al. [34] also evaluate the CF of different residential projects through an open tool. The results fit within the ranges given by [PERSON] et al. [31] and [PERSON] [33].  
[PERSON] et al. [35] analyse the embodied emissions generated by an educational building of 4013 m\({}^{2}\) GFA in Reykjavik (Iceland) and propose different construction systems: a first base scenario, formed of concrete reinforced components; a second scenario using optimised concrete; and a third scenario where the nonstructural elements of Scenario 2 are replaced by wooden walls. Using tools such as those provided by Simapro and GaBi, they obtained results for all LCA indicators for phases A1-A3 and for A4. The values obtained from climate change were 664 (base scenario), 672 (Scenario 1), and 562 kg CO2 eq/m\({}^{2}\) (Scenario 2).  
A more recent study [36] looked at more than 700 buildings in five European countries from the point of view of embodied emissions throughout the life cycle of such buildings. The life span considered was 50 years, and values were obtained for the complete life cycle, 400-800 kg CO2 eq/m\({}^{2}\) for residential buildings and 100-1200 kg CO2 eq/m\({}^{2}\) for nonresidential buildings. The researchers considered that of these embodied emissions, approximately 2/3 corresponded to the so-called upfront embodied carbon, that is, the associated embodied emission before the building is put into use, which, in the LCA methodology, would be equivalent to phases A1-A5.  
From these reference studies, it can be said that the construction phase of the building is the most environmentally intensive because it is concentrated over a short period of time (1-2 years). These impacts are diluted over time, although the decisions made during this stage greatly influence the remaining phases of the building's life cycle [37]. The study on buildings across the world by [PERSON] et al. [38], concluded that, of those that comply with energy-efficiency regulations, 20-25% of the GHG emissions of their life cycle are embodied emissions, while for high-energy-efficiency buildings, that percentage increases to 45-50%. Therefore, in nearly zero-energy buildings, as currently built in the EU, emissions from the construction phase represent a very high percentage of their emissions over their total life cycle [39], and hence it is necessary to focus more on reducing the embodied CF of the construction phase.