Secondary glazing puts a second layer of glazing on the room side of an existing window, trapping a pocket of still air that does the insulating. It will never quite match a sealed double-glazed unit, but it costs a fraction as much, keeps your original windows, and gets you most of the way there.
How it earns its place
The enemy at a window is the cold pane and the air movement around it. Add an inner layer and you create a buffer of trapped air, which slows heat loss, kills the chilly downdraught, and cuts condensation on the original glass. As a bonus it noticeably dampens outside noise, which is why it is popular on busy roads and near railways quite apart from any energy benefit.
From a few pounds to a proper fit
At the cheap end, shrink-fit insulating film taped over the frame and warmed tight with a hairdryer costs very little and works surprisingly well for a winter, though it is not pretty and goes on and off seasonally. Magnetic or clip-in acrylic panels are a tidy middle ground you can make yourself. At the top end, slim aluminium-framed secondary units are fitted permanently inside the reveal and open and close like a proper window. The right choice depends on budget and how it needs to look.
Made for period homes
Secondary glazing comes into its own where replacement is impossible or unwanted. Listed buildings and conservation areas often forbid swapping original windows, and owners of period houses frequently want to keep handsome old sashes. Because secondary glazing sits discreetly behind the existing window and changes nothing outside, it sidesteps all of that while still warming the room.
Seal it well to get the benefit
The trapped air only insulates if it stays trapped, so a good seal around the secondary layer matters more than the glazing itself. A loose film or a gappy panel leaks the benefit away. Combine it with draught-proofing of the main window and heavy curtains at night, and a cold single-glazed room becomes genuinely comfortable for very little outlay.