Secondary vs double glazing: which one fits your London property?
They are not competing products, whatever the salesman told you. They solve different problems, and in a lot of London homes the cheaper one is also the better one. Here is how we decide at survey.
One replaces the window, one sits behind it
Double glazing replaces your window. Two panes in a sealed unit, argon between them, fitted in a new frame. The old window goes in a skip.
Secondary glazing leaves your window exactly where it is and adds an independent second window on the inside, usually slim aluminium, sliding or hinged so you can still open the original behind it. Nothing on the outside of the building changes. That single fact drives most of what follows.
If this is your problem, secondary glazing wins outright
This surprises people, so it is worth being blunt about. For traffic noise, secondary glazing usually beats double glazing, and it is not close.
Sound reduction depends mostly on the distance between the panes. A sealed double glazed unit has a cavity of around 16 to 20mm, which is optimised for heat, not sound. Secondary glazing sits 100mm or more from the original glass, and that air gap does far more against low frequency traffic rumble. Add different glass thicknesses front and back so the two panes do not resonate together, and the difference against a busy road is obvious the moment you close it.
If you live on a main road in Camden, Islington or along a Haringey through route and noise is the reason you are reading this, secondary glazing is very likely your answer.
Double glazing wins, but by less than you think
For pure thermal performance a modern A-rated sealed unit in a new frame beats secondary glazing. No argument.
But the gap narrows once you account for the whole window. A lot of heat leaves a period property around the frame and through gaps in an old sash, not through the glass itself. Secondary glazing seals that entire perimeter, which is why in practice it often delivers more of the comfort improvement than the raw numbers suggest. It is also reversible, which matters if you are renting or plan to restore later.
The deciding factor in a lot of London
This is where the choice usually gets made for you. Secondary glazing is fitted internally and changes nothing on the elevation, so it rarely needs planning permission and is normally acceptable even on listed buildings. Double glazing replaces the visible window, so in a conservation area or on a listed property it needs consent, and consent is often refused.
Large parts of Islington, Camden and Hampstead Garden Suburb in Barnet fall into exactly this trap. People pay a deposit for replacement windows, then discover the address will not permit them. If your property is listed or in a conservation area, read conservation area glazing before you commit to anything.
The whole comparison in one table
| Secondary glazing | Double glazing | |
|---|---|---|
| Traffic noise | Better, a 100mm+ gap beats a 20mm cavity | Good, but not built for low frequency sound |
| Heat loss | Good, and seals the whole perimeter | Better on the glass itself |
| Planning consent | Rarely needed, internal only | Needed in conservation areas, often refused |
| Listed buildings | Usually acceptable | Usually refused |
| Cost per opening | Lower | Higher |
| Reversible | Yes, comes out without trace | No, the original window is gone |
| Look from outside | Unchanged | Changed, for better or worse |
| Cleaning | Two windows to clean, and the cavity between | One window |
| Best for | Period, listed, conservation area, noisy roads, rentals | Postwar and interwar stock, anywhere replacement is permitted |
Three questions at survey
Is the property listed or in a conservation area? If yes, secondary glazing is usually the only route that will proceed without a fight, and often the only one that will be approved at all.
Is the problem noise or cold? If it is noise, secondary glazing, almost regardless of the building. If it is cold and you are free to replace, double glazing.
Are the existing windows worth keeping? A sound original sash with good joinery is worth preserving, and secondary glazing lets you keep it. A rotten frame or a failed eighties replacement is not worth protecting, and replacement is the honest answer.
If you are still not sure, that is what the free survey is for. We will tell you which of the two your property actually needs, including when the cheaper option is the right one. Call 07399 253464 or book a survey.