We call it Lucky because, like gold, it has been here all the time. Discovering secondary window insulation makes people feel lucky.
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.
Adding secondary glazing to existing windows and doors (double glazing them) improves their insulation value. The glazing you choose depends upon whether thermal (heat), or audio (noise) insulation is your primary focus. Either way you will get improvement in both thermal and audio performance.