Windows impact energy efficiency of buildings depending on what type of glass it is and what functions it has. For example gas content is a significant factor in improving the energy efficiency of insulating glass and is thus an essential part of global energy saving efforts.
Energy-savings within the building ans glass industry
It is known that buildings cover around 40 % of the energy consumption in the world and around 30 % of the primary energy in the buildings goes to heating and cooling. Also, it has been estimated that windows cover around 40% of the energy loss of buildings. That alone should be a reason to focus on the quality of windows, especially IGUs as they are installed often to enhance the thermal insulation which greatly impact to energy savings. Therefore you can state that windows impact greatly to the energy efficiency of buildings.