The purpose of this study was to analyze the indoor air quality, in particular, the CO₂ level, in a real environment, within 16 apartments located in the municipality of Zalău, Romania, in five recently renovated multifamily buildings in which families live under normal conditions. Long-term monitoring was undertaken for 4.5 months, during the heating season of 2022–2023. A representative microclimate perspective was analyzed: the median value of the time fraction, which exceeded the Indoor Environment Quality Category I (IEQcat I) threshold for adults’ bedrooms was 82.30%. For children’s bedrooms, it was 75.65%, while for living rooms, it was 58.78%. When considering IEQcat IV, it was identified that for the bedrooms, the time fraction for which the threshold values were exceeded was still significant (median value of 45.37% for adults’ bedrooms and 50.14% for children’s bedrooms). Even if the indoor thermal comfort conditions increased, it was found that for almost half of the time, the bedrooms did not provide a health-safe indoor environment for the occupants due to inadequate ventilation, exceeding the health threshold values. Thus, it was identified that the ventilation of buildings is a key issue within the renovation process and, currently, is not properly undertaken for the mass renovation of existing multifamily residential buildings.

A holistic approach with high level of replicability and financial feasibility for the public existing fund of buildings is undertaken, in order to transform existent public buildings, that are serving communities, into nearly Zero Energy Buildings. Finding large scale solutions from both technical and financial point of view, to reduce energy consumptions and CO₂ emissions for the existent building fund represents at this point a major challenge for all stakeholders. Several existent educational units (kindergartens, schools, high schools) from a city in Romania were analysed, in order to improve their energy efficiency and to transform the typical existent educational unit into nZEB. A framework package of technical solutions is proposed and the effect of its implementation is analysed, in terms of energy consumptions and CO₂ emissions reductions. Economic analysis is undertaken, to identify the feasibility of the nZEB concept implementation. An alternative approach is analysed for public authorities, in order to transform groups of existent buildings, which are serving communities, into nZEB.

This study developed a calibrated, data-supported energy simulation model for the Arrivals Terminal of Cluj-Napoca International Airport (Romania), addressing challenges in modelling complex building typologies. The objective is to improve the accuracy of predicting energy savings and CO₂ emission reductions, supporting renovation and decarbonization strategies aligned with the 2050 targets. The hourly multizone simulations over one year integrated measured operational data, building documentation, and two types of climate datasets (AMY and TMY). The calibration methodology introduces a “Miscellaneous equipment” variable, representing unmonitored indoor electricity consumption, which is incorporated as an internal heat gain in the thermal balance. Validation against real energy measurements showed high agreement (AMY-based RMSE: 3.13 kWh/m²·yr for thermal energy and 1.57 kWh/m²·yr for electricity; relative errors: 2.3% and 0.5%, respectively). The results demonstrate that calibrated modelling reduces the performance gap and provides a robust alternative to standard design-condition energy assessments, which are inadequate for airport terminals but mandatory for several countries, including Romania. The developed model enhances predictive reliability and can guide energy efficiency measures and investment decisions for similar complex buildings.