How secondary glazing works
The inner unit sits within the window reveal, leaving a gap between it and the original glass. That air gap and the extra seal around the inner frame are what cut draughts and outside noise. Because the original window is untouched, the appearance from the street is preserved, including old glass, glazing bars and painted timber.
Inner units can be made to slide horizontally, slide vertically, hinge open or lift out, depending on the window behind and how you need to reach it. Choosing an opening style that lets you open the original window, clean both panes and escape in an emergency matters as much as the thermal benefit.
Useful for listed and conservation homes
In listed buildings and some conservation areas, replacing original windows may be restricted. Secondary glazing is often seen as a less intrusive improvement because it is fitted inside and can usually be removed later without damaging the historic window. Check with your local planning authority before committing, as consent requirements vary from property to property.
Even where replacement is allowed, some owners prefer to keep handmade sashes or leaded lights and improve comfort another way. Our guide to secondary glazing vs replacement windows compares the two approaches in more detail, including where each one tends to suit best.
Secondary glazing installation
Fitting starts with careful measuring of each reveal, because old openings are rarely square. The inner frame is fixed to the reveal or to a sub-frame, and any gaps are sealed so the unit performs as intended. Window boards, shutters, blinds and curtain tracks need to be considered, since the new frame takes up some depth.
Before fitting, make sure the original windows are in reasonable condition. Secondary glazing does not fix rotten timber or a sash that will not close, and trapping a leaking frame behind a second pane can make matters worse. Any repairs needed to the outer window are best identified at the survey.
Secondary glazing cost and what affects it
The number and size of windows, the opening style, glass choice and how straightforward each reveal is all affect the price. Arched heads, bays and very tall sashes involve more work than a standard rectangular opening. Acoustic or toughened glass, where it is wanted, also changes the specification.
When comparing quotes, check the frame colour, how each unit opens, the glass specified and whether making good around the reveal is included. A written quote that lists each window separately makes like-for-like comparison easier, and helps you decide whether to treat every room or start with the noisiest or coldest ones.
Choosing the best secondary glazing for your windows
The best secondary glazing for one home may be wrong for another. A bedroom on a busy road may benefit most from an acoustic specification, while a draughty sash in a sitting room may simply need a well-sealed sliding unit that is easy to open. Slim sightlines help the inner frame line up with existing glazing bars.
Fairway Window Systems Ltd offers a free home survey and written quote, so you can talk through each window before deciding. If full replacement turns out to be the better option, our sash windows and windows pages explain the alternatives.
Frequently asked questions
What is involved in fitting secondary glazing?
Each reveal is measured, then a slim inner frame is made and fixed inside the existing window and sealed. The original window stays in place.
How long does secondary glazing installation take?
It depends on the number of windows and how straightforward the reveals are. Measuring comes first, then the units are made to size and fitted. The timing is set out with the quote.
What affects secondary glazing cost?
Window size and number, opening style, glass specification and the shape of each reveal. Arched or very tall openings involve more work.
Is secondary glazing suitable for listed buildings?
It is often considered because it is fitted inside and can usually be removed later. Check consent requirements with your local planning authority first.