Secondary glazing is a separate, slim glazed frame fitted on the room side of an existing window, creating an insulating air gap without altering the original. It reduces draughts, heat loss and noise, and because it can usually be removed again, it is widely used in period homes, listed buildings and conservation areas where replacing the original windows would not be acceptable.
What is secondary glazing and how does it work?
Secondary glazing is an independent internal window installed on the room side of an existing primary window, usually within the window reveal. The Glass and Glazing Federation describes it as a separate unit with its own frame, sealed around the edges, so that together with the original window it forms a double layer of glazing.
The benefit comes from the gap between the two layers and from the seal of the new inner frame. The air trapped between the panes slows heat loss, the seal cuts draughts through gaps around the old sashes or casements, and the second layer of glass blocks more outside sound. The original window stays in place and keeps working as before.
How does secondary glazing reduce heat loss and draughts?
Older windows can become draughty as timber shrinks, joints weaken and sashes wear, and single glass loses heat readily. Historic England's guidance on secondary glazing explains that a well-sealed secondary unit reduces air leakage and heat loss through the window, and that a low-emissivity coating on the secondary glass, facing outwards, improves performance further. The coating needs to be kept clean to work as intended.
Secondary glazing is also recognised in building regulations guidance. In England, Approved Document L says that where replacement windows cannot meet the normal energy limits because the building's character must be kept, single glazing can be supplemented with low-emissivity secondary glazing. That makes it a practical upgrade where new windows are not an option.
How much can secondary glazing reduce noise?
Secondary glazing is often more effective against noise than replacing single glazing with standard double glazing. The reason is the gap: a standard sealed unit has a narrow cavity, whereas secondary glazing can leave a much wider space between the panes, which reduces the transfer of sound from one pane to the other.
Historic England's guidance says a cavity of 100mm or more decouples the two panes, with little extra improvement beyond about 200mm. The Glass and Glazing Federation likewise suggests a gap of at least 100mm, or laminated sound-reducing glass, for better noise reduction. Acoustic laminated glass in the secondary unit, good seals and lining the reveals with sound-absorbing material all help further.
For heat loss alone, a narrower gap is more efficient, so the right design depends on whether noise or warmth is the priority. If both matter, the survey should agree the gap, glass and seals that balance them for each window.
- Wider cavity between the original and secondary pane
- Acoustic laminated glass in the secondary unit
- Well-sealed secondary frame
- Sound-absorbing lining to the window reveal
Why is secondary glazing used in period and listed buildings?
Historic windows often contribute to the character and significance of a building through their design, materials and old glass. Replacing them can harm that character, and in listed buildings it usually requires consent. Secondary glazing leaves the original windows in place and unaltered, and Historic England notes that in most cases a careful installation is easily reversible, with no loss of historic fabric.
The Energy Saving Trust describes secondary glazing as a non-intrusive way of insulating historic windows from the inside and a suitable option where listed building restrictions apply. The Glass and Glazing Federation notes it can be the only effective option in some older properties, including where an Article 4 Direction or planning rules prevent external changes to the primary windows.
Do you need consent for secondary glazing?
It depends on the building and the nation. In England, Historic England explains that listed building consent is needed for changes that might affect a listed building's character, and that listing covers the whole building, inside and out. In Wales, Cadw says consent may be needed for even relatively minor work, because historical fixtures and details inside are protected as well as outside. Applications go to the local planning authority.
In Scotland, Historic Environment Scotland's Managing Change guidance on windows sets out the principles for altering windows of historic buildings, and is used in decisions on listed building consent within the Scottish planning system. In Northern Ireland, listing also covers the whole building, inside and out, and the Department for Communities' Historic Environment Division is a statutory consultee when councils decide listed building consent applications.
Because secondary glazing is fitted inside, it is often acceptable where replacement is not, but whether consent is needed depends on how it is fixed and the significance of the interior. Ask the local authority's conservation officer before work starts. In a conservation area that is not listed, internal secondary glazing does not change the outside appearance.
Can secondary glazing cause condensation?
It can if the design is wrong, and it is usually avoidable. Condensation forms in the gap when warm, moist room air leaks into the cavity and meets the cold outer pane. Historic England's guidance is that the secondary unit should be well sealed, while the original window should not be sealed, so that the cavity has some ventilation to the outside air.
Keeping the secondary unit closed in cold weather also reduces the risk, and fixed panels can include small vents. Do not add draught-proofing to the outer window if you are fitting secondary glazing behind it. Normal room ventilation still matters: in England, the government's ventilation guide for existing homes advises keeping trickle vents open and unblocked, even in winter.
What types of secondary glazing are available?
Historic England's guidance describes several types. Lift-out panels suit windows that are fixed or seldom opened. Hinged casements give full access for cleaning and opening the original window. Horizontal sliding units run on glides or rollers, and vertical sliding units suit sash windows, usually with the upper sash innermost. Lightweight removable panels, such as magnetic acrylic systems, are also available for simple seasonal use.
Good design keeps the secondary glazing discreet. Historic England recommends placing any divisions in the secondary glazing behind the meeting rails or glazing bars of the original window, keeping the glazed area at least as large as the existing one, and considering colour-matched frames and low-reflection glass so the installation is less noticeable from outside.
What safety points should you check?
Secondary glazing in a low-level window or near a door sits in the same zone people can fall against, so it should be safety glass in those positions, such as toughened or laminated glass marked to the relevant standard. Rules on critical locations differ in detail between England, Wales, Scotland and Northern Ireland, and the guidance in England, Wales and Scotland all covers glazing within 800mm of floor level.
If a window is needed as a means of escape from a room, the secondary unit should open easily so the escape route is not blocked. Hinged and sliding units also need restrictors or stays where opening panels could be a hazard. Our glaziers survey each window, list the type, glass and opening arrangement on the written quote, and confirm guarantee terms in writing with your quote.
Frequently asked questions
Is secondary glazing as good as double glazing?
It depends on the goal. For noise, a wide gap makes secondary glazing very effective, often more so than standard double glazing. For heat loss, modern double glazing in a new frame usually performs better, but low-emissivity secondary glazing on an existing window gives a worthwhile improvement and keeps the original window in place.
Does secondary glazing need planning permission?
Secondary glazing is fitted inside, so it does not normally change the outside of the building. Listed buildings are different: consent may be needed for works affecting their character inside or out, in every UK nation. Check with your local planning authority or council before fitting secondary glazing to a listed building.
Can I still open my original windows?
Yes, with the right type. Hinged and sliding secondary units open to give access to the original window for ventilation and cleaning, while lift-out panels can be removed. Vertical sliding units are designed to work with sash windows. The opening arrangement for each window is agreed at survey and shown on the written quote.
How do I stop condensation between the panes?
Make sure the secondary unit is the better-sealed of the two layers, and leave the original window unsealed so the gap can ventilate slightly to the outside. Keep the secondary unit closed in cold weather and keep normal room ventilation working. If condensation persists, the seals on the secondary unit should be checked.
Can secondary glazing be removed later?
Usually, yes. Historic England notes that carefully designed and installed secondary glazing is in most cases easily reversible, leaving the original window unaltered. Fixings into historic joinery or plaster should be kept to a minimum and agreed in advance for listed buildings. Lift-out and magnetic systems are the simplest to remove seasonally.
Sources
- Energy Efficiency and Historic Buildings: Secondary glazing for windows – Historic England
- Listed buildings – Historic England
- Listed building consent – Cadw
- Managing Change in the Historic Environment: Windows – Historic Environment Scotland
- Listed buildings – Department for Communities NI
- What is secondary glazing? – Glass and Glazing Federation
- Energy efficient windows and doors – Energy Saving Trust
- Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments – GOV.UK
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