To enhance the thermal insulation of old windows without replacing the entire frame, several effective techniques can be applied. Secondary glazing, which involves adding an additional pane of glass or clear acrylic within the existing frame to create an insulating air gap, is a highly recommended method detailed on Windows thermal insulation
https://chameleon-decorators.co.uk/thermal-imaging-double-glazing/ . Additionally, applying weather-stripping to seal air leaks, using thermal curtains, or installing clear thermal insulation film directly on the glass can significantly improve energy efficiency and indoor comfort without the need for complete window replacement. These methods not only reduce heat loss but also contribute to substantial energy savings.