We call it Lucky Window Insulation because, like gold, it has been here all the time. Those who discover it feel lucky to have it.
Add-on secondary glazing to stop noise and unnecessary heat-loss or heat-gain is something you can have done. It it is cheaper than new double glazing and easier to install. The thermal insulation performance can be as high as it is for new double glazing and even higher for noise insulation. This is because it is often possible to use heavier glass and achieve wider spacings than is usual for double glazing units.
Adding secondary glazing to existing windows and doors (double glazing them) improves their insulation value. There are several systems that can be used. The choice often depends on the type of windows you have and whether thermal (heat), or audio (noise) insulation is your primary focus. Each of the secondary glazing systems available can give you greatly improved thermal and audio performance.