The short answer
- Secondary glazing when the original windows and glass should stay: listed buildings and surviving historic glass. With a wide gap it is also the best answer to traffic noise, and it can be taken out again.
- Slim double or vacuum glazing when the sashes are being replaced or reglazed anyway and the conservation officer accepts it.
- In a listed building, either can need listed building consent. Ask the council first.
Side by side
| Slim double or vacuum glazing | Secondary glazing | |
|---|---|---|
| What it is | A thin sealed unit in the sash: about 10–14 mm, vacuum about 6–9 mm | A second, slim window fitted inside the existing one |
| Original window | Reglazed or replaced | Kept as it is |
| From outside | Close to single glass; a double reflection can show | Unchanged |
| Noise | Some improvement | Best, with a gap of 100 mm or more |
| Everyday use | One window to open and clean | Two windows to open and clean |
1. Slim double and vacuum glazing
A slim double glazed unit is thin enough to fit the shallow rebate of a sash, sometimes with a slightly deeper rebate in a new sash. It has a low-emissivity coating and a gas fill. Vacuum glazing is thinner still and usually insulates better. Both make the sash heavier, so the weights or balances have to be matched to the new glass.
On sashes with several panes, conservation officers usually prefer true glazing bars, with a separate unit in each pane, to bars stuck on a single sheet of glass.
2. Secondary glazing
Secondary glazing is an independent slim frame fixed inside the window reveal. It can slide, hinge or lift out, and it leaves the original window and its glass untouched. The gap between the two decides what it does best: a narrower gap for heat, 100 mm or more for noise.
To avoid condensation, the inner window should seal better than the outer one, so warm, moist room air doesn’t reach the cold outer glass. A little ventilation of the gap through the outer window helps.
3. Planning and consent
On an unlisted house, internal secondary glazing doesn’t need planning permission. Replacement sashes of similar appearance are usually permitted development, unless an Article 4 direction removes that right. In a listed building, anything that affects its character needs listed building consent, and that can include reglazing and secondary glazing. Secondary glazing is often the option conservation officers prefer, because it can be removed. See our conservation areas guide.
4. Building Regulations
When you replace whole windows, they have to meet Approved Document L: a U-value of 1.4 W/m²K or better, or Window Energy Rating band B. For historic buildings, building control can accept a different approach where meeting the limit would harm the building’s character. Agree it before the work starts.
Our sash windows
We supply timber sash windows with slim double or vacuum glazing, with the proportions, horns and glazing bars conservation officers look for.
Questions
Is vacuum glazing better than slim double glazing?
It is thinner and usually insulates better, so it suits slender sashes. It also usually costs more. Both need the sash weights or balances matched to the new glass.
Does secondary glazing need planning permission?
Not on an unlisted house: it is internal. In a listed building it can need listed building consent, so ask the council’s conservation officer first.
Which is better for traffic noise?
Secondary glazing with a gap of 100 mm or more between the two windows. Slim double glazing helps a little, but the gap does most of the work.
Can I fit slim double glazing in a listed building?
Sometimes, decided case by case. It is more likely to be accepted where the historic glass has already been lost. Where original glass survives, expect to keep it and add secondary glazing instead.
Sources
The official documents this guide is based on. Check the current edition before you order.
- Traditional windows: their care, repair and upgrading (Historic England)
- What permission might I need to make changes to my older home? (Historic England)
- Listed building consent (Historic England)
- Doors and windows: planning permission (Planning Portal)
- Approved Document L: conservation of fuel and power (GOV.UK)