The objective of this research is the environmental assessment of two possible construction solutions that improve the energy efficiency of existing buildings: replacement of entire windows with new, more efficient ones, and the use of solar control window film applied to current panes. A quantitative evaluation is proposed that aids in decision making to minimize the consumption of natural resources and energy incorporated in the materials, as well as their environmental impact, as measured by equivalent CO2 emissions. As an assessment model, the methodology of the ARDITEC research group at the University of Seville was adapted, in collaboration with the Higher Technical Institute at the University of Lisbon, and was applied to real projects in Portugal. The model has a budget structure in which each resource is assigned coefficients for the calculation of its incorporated energy and carbon footprint, in its manufacture and implementation. The study of two office buildings with a high percentage of glazed surface in facades is presented. The results obtained show 70% lower emissions if windows are improved with films compared to complete window replacement.