Secondary glazing vs double glazing for sash windows
Which one actually suits your window — on noise, on heat, on cost, and on what a conservation officer will accept.
Secondary glazing is a slim second window fitted behind the original sash. Slim double glazing replaces the single pane inside the original sash with a sealed unit. Secondary glazing wins on noise and on listed buildings; slim double glazing wins on looks from inside and is £980 + VAT per window. Draught-proofing at £280 + VAT usually makes more difference than either.
The short version
Noise
Secondary glazing. The wide air gap is what blocks sound, and no sealed unit can match it.
Appearance from inside
Slim double glazing. Nothing extra sits in the reveal and nothing to slide back.
Listed buildings
Secondary glazing, in most cases. The original sash, glass and joinery stay untouched.
Cost
Slim double glazing is published at £980 + VAT per window. Secondary glazing is quoted per opening after a survey.
Heat
Close. Slim units are better in the glass; secondary covers the whole opening. Draught-proofing first changes more than both.
Keeping the original glass
Secondary glazing. If your window still has old cylinder or crown glass, do not let anyone take it out.
How we decide at the survey
- Is the property listed, or in a conservation area with an Article 4 direction? That usually settles it.
- Is the complaint noise or cold? Noise points to secondary glazing.
- Is the glass original? If it is, it stays.
- Is the sash rebate deep enough for a slim unit without weakening the sash?
- Is the window being repaired anyway? If the sash is out on the bench, slim glazing costs less to do at the same time.
- Has it been draught-proofed? If not, that is the first £280 + VAT to spend.
Common questions
Which is warmer, secondary glazing or slim double glazing?
In practice they are close. A sealed slim double glazed unit has the better U-value in the glass, but secondary glazing covers the whole opening including the gaps around the sash, so the room often feels similar. Draught-proofing the sash matters more than either.
Which is quieter?
Secondary glazing, clearly. Noise reduction depends on the width of the air gap, and a secondary unit sits 50–100mm off the original glass where a sealed unit has a cavity of a few millimetres.
Which is allowed on a listed building?
Secondary glazing, usually, because nothing is removed from or cut into the original window. Slim double glazing means routing out the sash rebate, which many conservation officers will not accept.
Can I do both?
Yes, and on a noisy main road it is the best result available: draught-proof and slim-glaze the sash, then add secondary glazing behind it.