Secondary glazing is a second, slim window fitted inside your existing sash window. It keeps the original window untouched, cuts heat loss and noise, and can be removed later without a trace.
How it works
A slim aluminium or timber frame is fixed inside the window reveal, with its own glass. The air gap between the original window and the secondary pane provides insulation. For sash windows, the secondary units usually slide vertically to match, or horizontally, or lift out. Many have slim frames aligned with the meeting rail, so they are hard to notice from outside.
Benefits
- Warmer rooms. Combined with a draught-proofed sash, secondary glazing can perform about as well as double glazing for heat loss.
- Much quieter. A wide gap (100mm or more) and acoustic glass make it one of the best ways to cut traffic, rail and aircraft noise, often better than double glazing.
- Keeps the original window. Nothing changes outside, which is why it is generally accepted on listed buildings.
- Reversible. It can be removed without damage.
- Less condensation on the original glass when properly installed.
Drawbacks
- Two windows to open for ventilation or cleaning.
- It can sit awkwardly with internal shutters, window seats or very shallow reveals.
- Your original sash still needs maintaining, and it works best if the sash is overhauled and draught-proofed first.
Cost
Secondary glazing typically starts at around £500 per window, with the price depending on size, style (sliding, hinged or lift-out) and glass. Acoustic and low-emissivity glass costs more. See our sash window cost guide.
Listed buildings and conservation areas
Historic England’s 2024 advice on energy efficiency in historic buildings says secondary glazing is generally acceptable on historic windows. You may still need listed building consent for the fixings, so check with your council’s conservation officer. In conservation areas, secondary glazing does not usually need planning permission because it is internal.
Secondary glazing or double glazing?
| Secondary glazing | Double-glazed sashes | |
|---|---|---|
| Keeps original sash | Yes | No (unless slim or vacuum units) |
| Noise reduction | Excellent | Good |
| Heat loss | Good to very good | Very good |
| Listed buildings | Generally accepted | Case by case |
| Reversible | Yes | No |
Read our guide to double glazed sash windows to compare.
