## 1 Introduction
Thus, it is clear that the energy efficiency and building physics of these temporary buildings are of crucial importance in order to ensure the comfort and health of the occupants, as well as to ensure minimum utility expenses to people who have lost their homes or places of work in the areas where energy infrastructure is often damaged. Regardless of the construction system used for the building envelope, including the Lightweight Steel Frame (LSF), it must be designed to ensure indoor environmental quality and very low energy consumption in all different climatic conditions in which it can be used. Furthermore, except in such extraordinary situations, there is an increasing demand on the market in general for prefabricated and modular construction of residential, public, and commercial buildings. Buildings that can provide comfort to occupants and very low energy consumption are known as Nearly Zero-Energy Buildings (NZEB), which are defined in [13] as buildings with very high energy performance. This \"nearly zero\" amount of energy needed for heating and cooling is to be covered to a very significant extent by the energy from renewable sources produced on-site or nearby. The commission's proposal to revise the directive from December 2021 [15] makes a step forward from the current NZEB to zero-emission buildings (ZEB), aligning the energy performance requirement for new buildings to the longer-term climate neutrality goal and \"energy efficiency first principle.\" According to the Commission's proposal, a ZEB is defined as a building with a very high energy performance, with the very low amount of energy still required fully covered by energy from renewable sources at the building, district, or community level [15]. All the above considered, the NZEB as well as future ZEB levels will force designers to rethink the constructive details so that their buildings will comply with the requirements,without increasing the construction costs over an optimum level, but at the same time reducing greenhouse gas emissions [16]. Regarding the emissions of the buildings, when performing a life cycle analysis (LCA) of a building, it is vital to evaluate embodied and operational emissions as well as primary energies. In order to reduce the environmental impact, it is essential to reduce both emissions and energies from production, operation, and end of life phases. This paper will focus only on the operational energy related to the heating and cooling energy needs and the impact of the thermal bridges caused by the LSF elements in the building envelope. Nevertheless, it needs to be emphasized here that one should not exaggerate the thickness of thermal insulation in order to minimize the operational energy of buildings. It was shown in [17] that there is an optimal insulation thickness, which takes into account operational as well as embodied energy combined with the economic parameters. Since LSF is not conventional, but is, rather, quite a scarce construction system to date in Croatia, the building physics and energy efficiency aspects have not been studied in depth considering southern European climate conditions. Thus, the main goal of this paper is to investigate whether the newly proposed LSF structure can meet high-performance requirements, such as the Croatian national requirements for NZEB, and to what extent thermal bridges, which are unavoidable due to the metal structure, affect the energy performance. In addition, this will show whether the newly proposed LSF module can be used for purposes including, but not limited to, temporary buildings, such as high-performance emergency housing units after natural disasters, which implies providing occupants with a high level of indoor thermal comfort, a healthy environment (moisture-free construction details), and very low energy consumption. The paper is structured as follows. Section 2 provides a review of the literature on the conventional LSF structures currently available on the construction market, identifying its advantages and disadvantages.