This study looks at how reversible windows can minimize cooling and heating energy use while maintaining thermal comfort in buildings in Sweden and Spain. The growing need for energy-efficient structures and the potential advantages of reversible windows served as the driving forces behind this study. The approach taken in this study is a comparison of energy use of buildings where reversible windows were installed. The windowpanes had properties such that different solar heat transmittance coefficients were attained at the two states of the windows, being altered in spring and autumn. The windows were tested in two different buildings, one in each country, using IDA-ICE software in order to calculate the energy consumption both in winter and cooling, focusing in heating in winter and cooling in summer in order to know if there was a real energy saving compared to the base model used.The major findings indicate that reversible windows significantly reduce cooling and heating energy usage while maintaining thermal comfort levels that are on par with those of traditional windows in buildings. The effectiveness of reversible windows may, however, fluctuate throughout nations, depending on elements including climate, building design, and user behavior. Among themain findings of the study, the ones to be mentioned would be the total energy use reduction in the whole year that add up to 15 % in Sweden and 21 % in Spain and that most of the energy savings came in Summer for both Spain and Sweden. Additionally, the study also made findings regarding the role that awnings play in energy usage, determining that they may be truly significant in terms of reducing electricity bills. The awnings helped reduced the amount of energy used for both climates, in Madrid, the reduction was of 8 % , and in Stockholm it was of 4 % , these findings lead us to believe that the awnings are better suited to warmer climates. This study also investigates the economic viability of this novel kind of windows. The idea was to calculate the extra cost that could be associated to the installation of these newer reversible windows technologies by use of the NPV method to calculate the present value of the future energy savings to know if the money investment of the reversible windows is profitable or not. When comparing the reversible window to the base window both of which share the same U-value an extra 19 % of the base window installation cost could be associated to the new installation of reversible windows (for a 30-year period) for Stockholm, and a 25 % for Madrid. Changing already existing 2-pane windows of ordinary, less insulating quality for the reversible windows, the total energy use is reduced by about 50 % in Spain and in Sweden, but it seems that is just economical for the Swedish model, regarding the well-insulated buildings. An additional conclusion is that, compared to investing in reversible windows on all façades, an investment in reversible windows only on the Southern façade and base windows on the rest becomes a bit more economical, with 25 % allowed extra cost for the reversible windows in Stockholm climate and 27 % in Madrid; however clearly less energy is saved.According to the study’s findings, reversible windows have proven to be useful tools to reduce energy usage in both climates and locations. Meaning that they may play an important role to reduce the impact of the energy crisis that affects the current situation.Reversible windows can lower a building's energy costs and carbon footprint, which can have good economic and environmental effects. For architects, builders, and legislators interested in promoting energy-efficient buildings, the study's findings may be helpful. Reversible windows are highlighted in this research as a viable solution to lower energy use and enhance thermal comfort in buildings, particularly in areas with large temperature and solar radiation changes.