Secondary Glazing for Sash Windows: Costs, Types and Is It Worth It?

Secondary glazing is a slim second window fitted on the inside of your existing sash windows. It keeps the original windows untouched while cutting draughts, heat loss and traffic noise, and it is usually far cheaper than replacement. It is also one of the few upgrades that conservation officers and listed building officers regularly accept. This guide covers what it costs, which type suits sash windows best, and when it is worth it.

What is secondary glazing?

Secondary glazing is an independent glazed panel or frame fitted inside the window reveal, a short distance behind your existing window. The air gap between the two panes acts as insulation and a sound buffer. Your original sash window stays exactly as it is, both inside the frame and from the street.

How much does secondary glazing cost?

Typical 2026 UK prices:

Option	Typical cost
Professionally made and fitted secondary glazing	£180 to £400 per window
Large, arched or acoustic secondary glazing	Higher, quoted per window
DIY magnetic or clip-on panels	Much cheaper, but less durable and less effective
Draught-proofing an existing sash (London)	£250 to £450 per window

For comparison, a new timber double glazed sash window typically costs £1,200 to £3,500. See our sash window replacement cost guide for a full breakdown.

The best types of secondary glazing for sash windows

Vertical sliding

The most popular choice for sash windows. The secondary panels slide up and down, matching the sash behind, and the meeting rail can line up with your existing one so it almost disappears from outside.

Horizontal sliding

The panels slide side to side. These work well on wide windows and are easy to open for ventilation and cleaning.

Hinged

The panels open inwards like a casement. They give full access to the sash behind, which is useful if you open your windows often.

Lift-out and fixed panels

Lift-out panels are removed in summer and refitted in winter. Fixed panels give the best insulation but are only suitable for windows you never need to open, and you must keep a route for escape and ventilation.

DIY kits

Magnetic panels and clip-on acrylic sheets are cheap and easy to fit. They can help in a rented home or on a tight budget, but they are less airtight, can scratch or yellow, and do little for noise.

What are the benefits?

• Less heat loss: the trapped air gap greatly reduces heat escaping through the window.
• No more draughts: a well-sealed secondary unit stops cold air leaking through gaps around the sash.
• Noise reduction: secondary glazing is one of the best ways to cut traffic noise. A wider gap and thicker or acoustic glass give the best results, so mention noise when you ask for quotes. See how to soundproof sash windows.
• Keeps original windows: your period sashes stay untouched, which protects the character and value of your home.
• Less condensation: a well-fitted unit can reduce condensation on the original glass.
• Extra security: a second locked window makes break-ins harder.

Secondary glazing vs double glazing

	Secondary glazing	Double glazed sash windows
Typical cost per window	£180 to £400	£1,200 to £3,500
Keeps original windows	Yes	No
Noise reduction	Very good, especially with a wide gap	Good
Planning and listed building approval	Usually straightforward	Often needed and can be refused
Appearance inside	Visible second frame	Single clean window

If your sashes are sound, secondary glazing plus draught-proofing is often the best value. If they are badly rotten, replacement may make more sense. Read sash window repair or replacement to help you decide.

Do you need permission?

Secondary glazing is fitted inside the home, so it usually does not need planning permission, even in a conservation area. For listed buildings, removable secondary glazing is often the preferred way to improve original windows, but listed building consent may still be needed depending on the fixings and the council. Always check before you start. Our guides to planning permission and double glazing in conservation areas explain the rules.

Is secondary glazing worth it?

It is usually worth it if:

• your original sash windows are in reasonable condition
• you live in a conservation area or a listed building
• traffic noise is a problem
• you want warmer rooms without the cost of new windows

It is less worthwhile if your windows are badly rotten and need replacing anyway, or if you dislike the look of a second frame inside the room.

Tips for choosing a supplier

• Ask for slim aluminium frames painted to match your interior.
• Line up the meeting rail with your sash so it is hidden from outside.
• Ask about acoustic glass if noise is a concern.
• Check how the panels open for cleaning and ventilation.
• Ask whether they will draught-proof the original sashes at the same time.

Find secondary glazing and sash window specialists near you and compare quotes.

Frequently asked questions

How much does secondary glazing cost for sash windows?

Professionally made and fitted secondary glazing typically costs £180 to £400 per window in the UK. Large, arched or acoustic units cost more.

Is secondary glazing better than double glazing?

It depends. Secondary glazing is cheaper, keeps your original windows and is usually better for noise. Double glazed sashes give a cleaner look and are better if your windows need replacing anyway.

Can you open sash windows with secondary glazing?

Yes. Sliding and hinged secondary glazing open so you can reach and open the sash behind for ventilation and cleaning.

Does secondary glazing need planning permission?

Usually not, because it is fitted inside. Listed buildings may need listed building consent, so check with your council first.

Does secondary glazing stop condensation?

A well-sealed unit usually reduces condensation on the original glass, but good ventilation is still important.