This paper proposes to consider the impact of window replacement, and in particular its glazing, on the energy savings and CO2 emissions’ reduction of the building stock. Based on a quantitative report commissioned by Glass for Europe to TNO (2018), the paper presents the quantities of energy and CO2 that can be saved by 2030 and 2050 using adequate glazing depending on the building type and location. Four different scenarios are analysed for the entire EU and per countries: maximum savings and three different renovation rates. The paper provides answers to questions such as: How much energy savings can be made (heating and cooling) by changing outdated windows in a 1%, 2% or 3% renovation rate of the old building stock? How much savings can be made if all windows were replaced in the old building stock by energy efficient glazing, and new constructions equipped with highly energy efficient glazing? Based on these findings, the paper will reflect on the current renovation trajectories and policies, and how public policies and other initiatives can support higher renovation rates in the building sector and promote the installation of energy efficient glazing.