Secondary Glazing: Systems, Performance and Listed Buildings

Secondary glazing is a second pane of glass or transparent material fitted on the inside of an existing window, in its own slim frame, leaving the original window untouched¹. It is an additional window unit that improves thermal and acoustic performance and gets round planning restraints that prohibit replacing the existing window². The original sash or casement still opens; the secondary unit slides, swings or lifts out in front of it.

On performance, manufacturers state that quality secondary glazing reduces heat loss through the window by up to 65 per cent and, when correctly fitted, cuts noise by up to 80 per cent, the equivalent of 54 decibels³. Independent advice is more measured: the Glass and Glazing Federation puts the change from single glazing with the addition of secondary glazing at approximately double the energy efficiency, while noting the seals are not as airtight as those on double glazing⁴. Expect around £104 a year off bills from secondary glazing, or £150 to £250 a year where it is combined with draught-proofing across all a home's windows⁵.

On cost, one manufacturer states that secondary glazing can be installed for around 25 per cent of the cost of double glazing, depending on size and style⁷, while the GGF puts a typical secondary unit at around half the price of a double glazing window, and roughly 50 per cent less on a supply-and-install basis⁴. Independent advice gives upwards of £300 for a single window and a range of £300 to £500 per window⁸.

What secondary glazing is and how it works

Secondary glazing is a pane of glass with a discreet frame installed on the inside of the existing window¹⁰. It can be added to a single, double or triple glazed primary window, though the case for it is strongest over single glazing⁷. The mechanism is the same trapped-air principle that makes a sealed double glazed unit work: double glazing traps air between two panes, creating an insulating barrier that reduces heat loss, noise and condensation¹¹. With secondary glazing the cavity is much wider and is formed by the room-side frame rather than a factory seal, giving a buffer zone where air cannot circulate¹⁰.

Because the unit sits behind the original window it also stops draughts at the window opening, which is a large part of the practical benefit in an older house¹². The GGF describes secondary glazing as a cost-effective way of improving heat retention and sound reduction, as well as providing some additional security, and notes it is low maintenance and does not cause the same level of disruption as fitting replacement primary double glazing¹³. The extra layer of glass or plastic creates a barrier that is more difficult for an intruder to break through¹⁴.

What it does not do is change the building. It is fitted without major construction work, and the installation does not alter the fabric of the building and is entirely reversible¹⁴. For a household, that matters twice over: it is the reason planners accept it in historic buildings, and it is the reason it can be removed or upgraded later without a second round of consent. Related comparisons are covered under secondary glazing vs double glazing and in the wider guide to double glazing.

Performance: up to 65% less heat loss, up to 80% less noise

The headline manufacturer figures are consistent across product lines: up to 65 per cent reduction in heat loss and up to 80 per cent noise reduction¹⁶. One maker's calculation, using 6mm toughened glass with a low-E coating, gives a fall from 5.7 W/m2K to 1.87 W/m2K, described as a 69 per cent decrease in heat loss through the window⁹. A fully sealed unit is claimed to reach U-values as low as 0.97¹⁸. The GGF records a single pane with secondary glazing, glazed with a slim double glazed low-E unit, at 0.9 W/m2K, adding that this is not often used because the cost is similar to standard double glazing⁴.

Independent framing is cooler. Around 10 per cent of the heat loss from a typical house is through the windows, which caps what any glazing measure can deliver⁵. The Planning Portal states that double glazing can cut heat loss through windows by half¹¹. The GGF's judgement is that secondary glazing approximately doubles the energy efficiency of single glazing, but that it is not the best energy-efficient glazing solution because the seals are not as airtight as those on double glazing⁴.

Acoustically the position is reversed. Makers state that secondary glazing significantly reduces noise entering a building, more so than double and even triple glazing in most instances¹⁹, and hinged units are described as reducing noise by up to 80 per cent and significantly outperforming double glazing²⁰. The key to blocking noise is to install the system 150mm to 200mm away from the primary window¹⁴. That sits awkwardly with the thermal advice from the same maker, which recommends fixing no greater than 80mm from the primary glazing⁹. A household chasing noise and a household chasing heat are being pointed at different cavity widths.

Horizontal sliding, vertical sliding, hinged, lift-out or fixed: choosing a system

The range covers horizontal sliding windows, vertical sliding windows, hinged units, lift-out panels and fixed secondary glazing¹⁹. The GGF's list adds top hung, side hung, double side hung and removable fixed panels fitted onto the back of doors¹³. Independent advice notes that semi-permanent units can be fitted sealed, lift-out or openable, on hinges or on vertical or horizontal slider tracks, so the original windows can open as normal⁸.

Secondary glazing is fitted to casement, sash, bay and tilt-and-turn windows¹⁹. Frames can be arched in-house to match the styles and types of primary glazing¹⁴. Every system is offered face-fixed or reveal-fixed¹⁸; the reveal option creates a gap of 100mm to 200mm between the glazing and the existing window¹⁸.

The choice is driven by the primary window and by how the room is used. A sash window cannot take a horizontal slider¹⁶. A window that is opened daily in summer argues for a hinged or vertical sliding unit rather than a fixed panel. A high or awkward window, opened rarely, suits a lift-out or fixed unit and costs less.

Frames, glass and specification

Frames are uPVC or aluminium, with over 200 RAL colours available on either¹⁸. Permanent systems described by the Energy Saving Trust use custom-built frames and glazing in wood or uPVC¹. Manufacturers fitting listed and heritage work favour slimline aluminium, stating that it blends with interior decor and does not change the appearance of the window⁷, with a current range offering narrower sightlines than its predecessor. Anglian's secondary glazing also uses aluminium frames²⁶.

Glass runs from 4mm to 8mm with thermal coatings, and up to 10.8mm laminate across one maker's range¹⁶. Case-study units use 4mm glass panels in slimline aluminium frames finished in white powder coating. For best performance one maker recommends 6mm toughened glass with a low-E coating⁹; low-E uses a transparent coating to insulate more effectively than plain toughened glass¹⁸. A system built with an 8mm air-filled cavity is offered with that low-E glazing²⁷.

Cavity width is the specification decision that matters most, and the guidance pulls two ways: no greater than 80mm from the primary glazing for thermal performance⁹, against 150mm to 200mm for acoustic performance¹⁴, with a general fitting cavity of 150mm to 200mm also quoted⁷. Regulatory drafting recognises the product type: the Welsh consultation version of Approved Document L, Volume 1, lists an alternative of 1.2 W/m2K centre pane or low-E secondary glazing for windows and roof windows²⁸. Hardware on one range is stated to meet British Standards regulations¹⁸. Wider glazing standards are set out under window energy ratings.

Cost: cheaper than replacement, but not cheap

Secondary glazing is generally more cost-effective and affordable than window replacement¹⁹, and the GGF describes it as a less expensive option to improve energy efficiency, on the condition that the existing windows are in excellent condition⁴. The per-window figures from independent sources are upwards of £300, depending on the size and complexity of the window and the type of opening⁸, and £300 to £500 per window⁶. Bespoke units are quoted individually: the cost depends entirely on the type of primary window with which it is being paired²⁴.

Temporary options sit in a different bracket entirely. A 3mm acrylic sheet of around 150 by 100cm costs upwards of £45 from DIY stores, and enough transparent shrink-fit film to cover a large bay window costs around £10 to £15⁸. Another guide prices film at £5 to £15 per window, and magnetic-strip temporary secondary glazing at £65 per square metre for materials²⁹. Those two figures, £65 per square metre for a DIY magnetic system and £300 to £500 for a fitted window, are not measuring the same product.

Against that, secondary glazing carries the standard 20 per cent rate: gov.uk's list of energy-saving products that qualify for the reduced rate explicitly excludes secondary or double glazing³². On savings, the modelled figures for whole-house double glazing give a sense of scale: A-rated double glazing replacing single glazing in an entirely single-glazed semi-detached gas heated property is modelled at £85 a year in Great Britain and £140 a year in Northern Ireland³³. Broader figures are in insulation and glazing costs.

Where secondary glazing beats replacement glazing

For listed buildings and those in conservation areas, secondary glazing is often the preferred option where the original windows cannot be replaced due to planning restrictions⁷. Some properties with listed status or in a conservation area are not permitted to use double-glazed windows at all, and in those homes it is frequently the only route to improved thermal performance¹⁹. The Energy Saving Trust calls it a non-intrusive way of insulating historic windows from the inside¹.

The reason it is accepted is that it does not replace the old window. It is installed without affecting the original windows and frames, helping to preserve the building's character³⁴, and a case study of an eighteenth-century Grade II listed cottage describes internal casement windows fitted inside the existing frames, without altering them at all³⁵. Brighton and Hove's heritage guidance sets a clear test:

That is the specification brief for a historic interior: slim frames, clear of shutter boxes, architraves and panelling, and not blocking the primary window's operation. In an unlisted conservation-area property the alternative route is slimline double glazing of 12 to 14mm fitted into the existing sashes or casements, which the same guidance treats as requiring no permission where there is no material change in appearance³⁶. Replacing single with double glazing might be acceptable in some locations but not in more sensitive ones, or where the traditional glazing bars are too narrow³⁷. More on this in insulating and glazing a listed building.

Planning permission, listed building consent and certificates of lawfulness

Two separate regimes apply, and they behave differently. Planning permission governs external appearance. Because secondary glazing is internal, one guide states plainly that secondary units fitted behind a sash window require no planning permission⁶, and permission may not be required where the installation does not alter the building's original features³⁴. Planning authorities are, in any event, much more likely to accept a secondary glazing installation than a replacement window³⁸.

Listed building consent is a separate control under the Planning (Listed Building and Conservation Areas) Act 1990³⁹, and it reaches inside the building. Hammersmith and Fulham state that listed building consent would also be required for the installation of replacement windows and any internal works such as the installation of secondary glazing⁴⁰. Other councils take a narrower view: Bristol advises that adding secondary glazing that does not involve the loss of original fabric can usually be covered by a certificate of lawfulness, while consent or a certificate is probably needed where new material is introduced or original fabric removed³⁷. Upgrading windows within listed properties needs permission from the local authority¹⁴, and a property in a conservation area, or listed Grade I or II, cannot change its glazing without permission¹⁷. Whether full double glazing can go into a listed building depends on the building and on how the installation would affect its character⁴¹.

• Unlisted house, not in a conservation area: internal secondary glazing makes no external change; permission is not the issue⁶
• Unlisted house in a conservation area: no material change in appearance means no permission³⁶
• Flat or maisonette, listed building or conservation area: planning permission or listed building consent may be required, especially where the external appearance changes⁴²
• Listed building: treatment varies by council, from full listed building consent for internal works⁴⁰ to a certificate of lawfulness where no original fabric is lost³⁷

Where an application is needed, it sets out the type of secondary glazing, the method of installation and other relevant details, and the planning authority reviews it and notifies its decision³⁴. See also planning permission for insulation and windows.

Installation: lead times, fitting and reversibility

Fitting is quick. One manufacturer states most installations are completed within the hour¹⁵, and that secondary glazing can be installed easily, often within just a few hours, with no major construction work¹⁹. Lead times of a few weeks are quoted from order¹⁹, with delivery on one listed-building range stated at 10 days from order¹⁷. Units are supplied either through local installers trained and accredited by the maker or delivered to the home¹⁶, and one listed-building product is described as easy to install, DIY or installed by a tradesman¹⁷.

Independent advice is less relaxed about DIY at the permanent end. Semi-permanent units are fixed by screwing into place or using a strong adhesive or sealant, and this type of glazing is not generally suitable for DIY⁸. Temporary film, by contrast, is installed with double-sided tape around the window frame and shrunk with a hairdryer⁸. Anglian distinguishes temporary units, easily removable using magnets or adhesive, from permanent units, professionally fitted with durable frames and installed and removed without damage to the building fabric²⁶.

Reversibility is the structural point. The installation does not change the fabric of the building and is entirely reversible¹⁴. Lift-out panels can be taken out of the frame for cleaning and maintenance¹⁴, and Historic Environment Scotland notes secondary glazing can be removed in the warmer months when it is not needed⁴³. Horizontal sliding sashes can be completely removed for cleaning the existing window¹⁶, and tilt-in vertical sliders allow the same²².

One warranty and certification point matters for householders. As secondary glazing is more specialised than double glazing, there is no central body that currently certifies these installations¹. That contrasts with replacement windows, which fall under the schemes described in replacement window rules. One horizontal sliding range carries a 10 year guarantee on all frames and glazing¹⁶.

Where secondary glazing falls short

The honest limits are set out by independent bodies rather than makers. Secondary glazing is not the best energy-efficient glazing solution, because the seals are not as airtight as those on double glazed units⁴. Which? puts it directly: it is not as effective as full double glazing, but it could still save some money on heating bills, and it is much cheaper to install⁴⁴. Where a window is beyond repair, the GGF's condition applies: secondary glazing only makes sense where the existing windows are in excellent condition⁴.

Funding is the second constraint. Secondary or double glazing does not qualify for the reduced VAT rate on energy-saving products, so the standard 20 per cent applies³². Ofgem's ECO4 measures guidance states the window glazing measure does not include windows with secondary glazing or external doors with double or secondary glazing, other than double glazed patio doors⁴⁵. Home Energy Scotland lists double, triple and secondary glazing at £8,000 per improvement on a loan basis with no grant available, and only for improving single glazing, not for replacing or improving existing double, triple or secondary glazing⁴⁶. Secondary glazing was an eligible secondary measure under the closed Green Homes Grant, alongside draught proofing and energy-efficient doors⁴⁷, and it appeared in the Scottish Home Energy Efficiency Equity Loan pilot as an eligible measure. It has also been fully subsidised in particular situations, such as under airport flight paths and near railways⁴. Current schemes are in insulation grants in Scotland and insulation grants in England; the closed ones are covered in closed insulation schemes.

Third, the assessment. RdSAP conventions record secondary glazing with single glazing as secondary glazing, normal or low-E if determinable, which is a distinct category from double glazing for EPC purposes. And fourth, some secondary glazing options are not allowed in listed buildings, so the choice of system is narrower than the catalogue suggests⁸.

Practical drawbacks follow from the design. Two sets of glass mean two sets of surfaces to clean and, on sealed units, the original window becomes harder to open. Shrink-fit film is temporary, has to be refitted every year, and removing it can damage the window's paintwork⁸. Where the cheapest plastic-sheet approach is used across a house, advice is to leave at least one window per room uninsulated or openable for ventilation. That point generalises: adding a second airtight layer at the window changes the ventilation balance of a room, which is the subject of condensation, damp and mould after insulation and home ventilation.

What it means for household energy independence

Secondary glazing reduces the amount of heat a house needs to buy, and it does so on the one part of the building envelope that is hardest to change in a protected property. Around 10 per cent of the heat loss from a typical house is through the windows⁵, so the measure works at the margin rather than transforming demand. The independent savings figures, around £104 a year, or £150 to £250 with draught-proofing across all windows, are real but modest against a whole-house bill⁵.

The dependence that remains is straightforward. The house still buys its heat from a grid or a gas supplier; fabric measures only shrink the bill. The product itself is a passive one: no app, no cloud service, no firmware, no ongoing supplier relationship after the fitting, and nothing that fails when a company does. That is a genuine advantage over metered or controlled measures. The counterweight is that there is no central certification body for these installations¹, so the guarantee depends on the individual firm continuing to trade, and reliance falls on the installer rather than on a scheme. Draught-proofing, loft insulation and the rest of the fabric sequence are set out in the insulation and glazing pillar and in planning a whole-house retrofit.