Secondary glazing vs double glazing
Two options that sound similar and do quite different things. Which one is right depends on your building, your priority and often your conservation officer.
If you are looking at improving the windows in a period property or a noisy house, you have two main options. They sound similar but they do quite different things.
The simple version
Double glazing is one window with two panes sealed inside a single frame, with an inert gas filling. You replace the existing window entirely.
Secondary glazing is two separate windows: your existing window stays in place, and a new slim frame is fitted just inside it, with an air gap of usually 50–150mm between the two.
That gap is the crucial difference, and it explains almost everything else. A sealed double-glazed unit has a cavity of 16mm or so, optimised for heat. A secondary system has a cavity ten times larger, which is far better at stopping sound and slightly worse at stopping heat.
When to choose double glazing
- The existing windows are tired uPVC or aluminium that you do not mind replacing.
- You are not in a conservation area or listed property.
- You want a single set of seals to clean and maintain.
- Your priority is thermal performance with the best modern figures (U-value 1.0–1.4).
When to choose secondary glazing
- Your home is listed or in a conservation area where original windows must stay.
- You have original sash or casement windows you want to keep.
- Your priority is acoustic improvement (secondary glazing usually beats double for noise).
- Budget is a factor, secondary glazing is generally less than half the cost of full replacement.
Performance compared
| Metric | Single glazing | Secondary + single | Modern double |
|---|---|---|---|
| U-value (W/m²K) | ~5.0 | ~1.8 | 1.0–1.4 |
| Acoustic reduction | ~25 dB | 35–45 dB | 30–35 dB |
| Visual impact outside | None | None | Visible new frame |
| Listed building OK? | - | Yes (usually) | Rarely |
Why secondary wins on noise
Sound insulation depends on three things: mass, the width of the air gap, and the absence of a rigid connection between the two leaves. A sealed double-glazed unit fails on two of those. Its cavity is narrow, and the two panes are rigidly linked by the spacer bar around the edge, which lets vibration bridge straight across.
Secondary glazing has a 50mm to 150mm gap and no rigid coupling at all, because the two windows are separate structures. Add different glass thicknesses in each leaf, so they resonate at different frequencies, and the improvement is substantial. Anyone living on a main road, near the A27, or under a flight path will notice the difference far more than a change in U-value.
If noise is the main problem, secondary glazing is usually the better answer even in a property where replacement would be permitted.
Listed buildings and conservation areas
Sussex has a great deal of protected housing stock, and Steyning, Storrington, central Brighton and much of Hove are heavily represented.
The general position: replacing original windows in a listed building needs listed building consent, and it is frequently refused where the existing windows are original or of historic interest. Doing it without consent is a criminal offence, not merely a planning breach, and it can surface awkwardly during conveyancing years later.
Secondary glazing is usually acceptable to conservation officers because it is fitted internally, does not alter the external appearance, and is reversible. That reversibility is the key word. It can be removed and leave the original window exactly as it was.
Consent is still sometimes required, particularly for internally listed interiors, so check with your local authority conservation team before ordering. We are happy to supply drawings and product detail for an application. Nothing here is legal advice, and your own council has the final say.
Living with each option
Secondary glazing has real drawbacks and it is fair to state them. There are two sets of glass to clean, and the inner faces need the secondary unit opened to reach. On a sash window, you operate two things instead of one. Cheaper systems can look like exactly what they are. And condensation can form in the cavity if the outer window is draughty and the inner one is well sealed, which is the opposite of what people expect. The fix is to ventilate the cavity slightly and seal the inner leaf well, not the other way round.
Done properly, with slim sightlines aligned to the existing glazing bars, it is close to invisible from inside the room and completely invisible from the street.
Cost
Secondary glazing is generally less than half the cost of full window replacement, and on a period property the gap widens because sympathetic replacement timber windows are expensive to make. We do not publish fixed prices because window sizes and configurations vary enormously. Every quote follows a survey and is itemised.
Which we would recommend
If your building is listed or in a conservation area, secondary glazing is usually the only realistic route and a good one. If noise is your main complaint, secondary glazing wins regardless of building status. If your windows are already modern uPVC and you simply want them warmer, replacement double glazing is the better answer and we would tell you so. Our secondary glazing service page has more on how we fit it.
Want a quote? Visit our secondary glazing service page or request a free quote.
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