Secondary glazing is a slim, separate glazed panel fitted on the inside of your existing windows, leaving the original windows in place. It is one of the most effective ways to cut heat loss, draughts and outside noise in period homes, listed buildings and properties in conservation areas, where replacing the windows may not be allowed or desirable. Inspired Glazing surveys, supplies and installs secondary glazing for homes around Windsor, Maidenhead and across Berkshire, Buckinghamshire, Surrey, west Kent and north West Sussex. This page explains how it works, what the rules say and how it compares with replacement double glazing.
How secondary glazing works
A secondary unit is fitted within the window reveal, a short distance behind the existing window. The gap between the two panes acts as an insulating air space, and the seals on the secondary unit stop draughts. Because nothing is removed from the original window, the installation is usually reversible: if the panels are taken out, the historic window is still there, unaltered. Secondary glazing panels are typically made with slim aluminium frames so they are hardly noticeable from inside, and they can be colour matched to your existing windows or decoration.
Types of secondary glazing
- Vertical sliding: panels slide up and down, ideal behind sash windows because the meeting rail can line up with the original.
- Horizontal sliding: panels slide side to side, suited to wider windows and easy to open for cleaning and ventilation.
- Hinged: panels open like a casement, useful for casement windows and where you need full access to the original window.
- Lift-out or fixed: for windows you rarely open, such as stairwells and landings, giving the neatest appearance and best seal.
Secondary glazing for listed buildings
Historic England’s guidance states that installing secondary glazing does not generally need listed building consent, because the original windows are left in place. There are exceptions. Consent may be needed if the installation would damage window frames, panelling, shutters or other historic features, for example by cutting into mouldings or fixing through decorative joinery. Historic England also notes that the total replacement of a window in a listed building is likely to require listed building consent, which is why secondary glazing is so often the preferred route.
Good practice, following Historic England’s advice, includes:
- making the secondary glazing no smaller than the existing glazed area;
- placing divisions behind the meeting rails or glazing bars so they are hidden from outside;
- fixing to a timber subframe or the reveal rather than to original joinery, wherever possible;
- keeping any working shutters free to close.
Always speak to your local council’s conservation officer before work starts on a listed building, because they make the final decision for your property.
Conservation areas
In a conservation area, replacing windows on an unlisted house may need planning permission where an Article 4 Direction has removed permitted development rights. Secondary glazing is fitted internally and does not change the outside appearance, so in most cases it does not need planning permission. Our guide to planning permission for replacement windows explains the rules for replacements in more detail.
Thermal benefits
Old single-glazed timber windows lose a lot of heat. Historic England’s research quotes a typical single-glazed timber window at around 4.8 W/m²K. Adding secondary glazing reduces this to about 2.5 W/m²K, and using low-emissivity glass in the secondary unit can take it below 2.0 W/m²K. Historic England reports that low-E secondary glazing can cut heat lost through the whole window by over 60%. Draught-sealed secondary panels also stop the cold air that leaks around loose sashes, which is often as noticeable as the heat lost through the glass itself.
Acoustic benefits
Secondary glazing is one of the best ways to reduce noise from roads, railways and flight paths, which matters across much of the Thames Valley and around Heathrow. The wide air gap between the original window and the secondary unit is what makes the difference. Historic England notes that an unsealed single-glazed window may reduce noise by only 18 to 25 dBA, while a secondary air space of 100mm or more can achieve up to around 45 dBA, with little extra gain beyond 200mm. Thicker or acoustic laminated glass in the secondary panel, and acoustic lining of the window reveals, improve performance further. If you are comparing options for a noisy home, see our sound reducing windows page.
Avoiding condensation
Condensation between the panes is the most common worry with secondary glazing, and good design prevents it. Historic England recommends keeping the secondary unit well sealed while leaving the original window unsealed, so any moisture in the gap can ventilate to the outside. In older homes, which rely on some natural air movement, it is also worth checking that rooms with open fires or stoves still have enough ventilation after installation.
Secondary glazing vs replacement double glazing
Both reduce heat loss and noise. The right choice depends on your building and what you want to achieve.
- Choose secondary glazing if your home is listed, the original windows are historic or in good condition, you are in a conservation area with an Article 4 Direction, you rent or own a flat where external changes are restricted, or noise is your main concern.
- Choose replacement windows if the existing frames are rotten or failing, you want windows that are easier to open and maintain, or you want to change the style or colour. Options include timber-look sash windows, UPVC windows and slim aluminium windows. Our guide to double vs triple glazing explains the glass choices.
Secondary glazing is not a substitute for repairing a rotten frame or a broken sash cord, so we will point out any obvious repairs needed on the original windows at survey.
Glass options
- Low-emissivity glass for the best thermal performance;
- Acoustic laminated glass for noise reduction;
- Toughened or laminated safety glass where panels sit low or close to doors, in line with Approved Document K;
- Obscure glass for bathrooms.
What affects the cost of secondary glazing?
- the number and size of windows;
- the type of opening, from fixed to vertical sliding;
- the glass specification, with acoustic laminates costing more;
- frame colour;
- whether timber subframes are needed in uneven or deep reveals;
- access, for example stairwells or upper floors.
How it works
- We survey each window, measure the reveals and check for shutters, mouldings and anything that limits fixing positions.
- We recommend the right type and glass for each room and give you a written quote.
- Our own team fits the panels carefully, with minimal disturbance to the original windows and decoration.
- We show you how to open, clean and, where relevant, remove the panels.
Book a free survey
If you want a warmer, quieter home without replacing your original windows, we will survey your windows and give you honest advice and a clear written quote. We work from our base at Fifield, near Windsor, and cover Berkshire, Buckinghamshire, Surrey, west Kent and north West Sussex. Call 01753 965 005, message us on WhatsApp on 07934 152 343, or request a free quote online and we will arrange a time to visit.
Frequently asked questions
Does secondary glazing need listed building consent?
Historic England says installing secondary glazing does not generally need listed building consent, because the original windows stay in place. Consent may be needed if the work would damage frames, panelling, shutters or other historic features. Always check with your local council's conservation officer before installing secondary glazing in a listed building.
Do I need planning permission for secondary glazing?
In most cases, no. Secondary glazing is fitted inside the house and does not change the outside appearance, so it is not usually treated as development needing planning permission, even in conservation areas. Listed buildings are a separate matter under listed building consent, so speak to your council if your home is listed.
How much noise does secondary glazing reduce?
It depends on the air gap and glass. Historic England notes an unsealed single-glazed window may cut noise by only 18 to 25 dBA, while a secondary air space of 100mm or more can achieve up to around 45 dBA. Acoustic laminated glass and lining the reveals improve results further, with little extra gain beyond a 200mm gap.
Is secondary glazing as good as double glazing?
For heat, low-E secondary glazing can bring a single-glazed window close to modern standards, and for noise it often outperforms standard double glazing because of the wider air gap. Replacement windows are the better choice when frames are rotten or you want a new style. Secondary glazing suits listed buildings and homes keeping original windows.
Will secondary glazing cause condensation?
Well-designed secondary glazing should not. The secondary unit is sealed to the room while the original window is left unsealed, so moisture in the gap can escape outside. Keeping rooms reasonably ventilated, especially kitchens and bathrooms, also helps. We check for existing damp or ventilation issues at survey and advise on them.