Secondary glazing vs double glazing compared

Secondary glazing is a second window unit fitted to the inside of an existing window. Double glazing replaces the window itself with a sealed unit holding two panes and a narrow air gap. For a household that cannot or will not replace its windows, secondary glazing is the cheaper route: Granada Glazing states it can be installed for around 25% of the cost of double glazing, depending on the size and style of the chosen system¹, while the Glass and Glazing Federation puts it at approximately 50% less on a supply and install basis². The two figures disagree and both are given here.

The performance case is narrower than the price case. Granada states its fixed secondary glazing achieves approximately 10% more thermal insulation than double glazing achieves³, and that it reduces noise by up to 80%³. Independent guidance is more cautious: Which? states that secondary glazing is not as effective as full double glazing, though it can still save money on heating bills and is much cheaper to install⁴. The Centre for Sustainable Energy puts permanent secondary glazing at upwards of £300 for a single window, depending on size, complexity and type of opening⁵.

The choice usually turns on the building rather than the budget. Where original windows cannot be replaced because of planning restrictions, secondary glazing is often the preferred option¹, and it can be a non-intrusive way of insulating historic windows from the inside⁶. Where replacement is permitted and the household wants the lowest long-run heat loss, double glazing is the stronger fabric measure. This page sets out what each does, what each costs, and where the limits fall.

Secondary glazing vs double glazing: the short answer

The two products solve different problems. Secondary glazing involves adding an additional internal window in addition to an existing single, double or triple glazed window¹. Double glazing works by trapping air between two panes of glass, creating an insulating barrier that reduces heat loss, noise and condensation¹¹. One replaces the window; the other adds to it.

The Glass and Glazing Federation describes secondary glazing as an additional glazing system, that is a window unit, fitted to the inside of an existing window, improving thermal efficiency and acoustic performance and overcoming planning restraints which prohibit replacing the existing window¹². That last clause is the heart of the case. Where replacement is barred, secondary glazing is not a compromise against nothing, it is the only glazing upgrade available.

Where replacement is allowed, the comparison is less favourable to secondary glazing. Double glazed windows provide twice the amount of insulation as single glazed windows¹³, and Which? states plainly that secondary glazing is not as effective as full double glazing⁴. The Glass and Glazing Federation notes that installing secondary glazing inside existing windows is a less expensive option to improve energy efficiency, but that the existing windows must be in excellent condition¹⁴. A rotten or draughty primary window is not a sound base for a second unit.

Secondary glazing is low maintenance and does not cause the same level of disruption as fitting replacement primary double glazing¹². Secondary window units usually have separate handles and open so that the primary windows can be cleaned and opened¹². That matters in practice: a second skin that cannot be opened turns a ventilated room into a sealed one.

What secondary glazing is and how it works

Secondary glazing is a second window, not a second pane. It is fitted on the inside of an existing window⁶, leaving the original frame, glass and external appearance untouched. The Centre for Sustainable Energy describes the semi-permanent versions as units that can be fitted to be sealed, lift-out or openable, on either hinges or vertical or horizontal slider tracks, so the original windows can open as normal⁵.

The insulation comes from the cavity, not the glass. Granada states that secondary glazing is often fitted with a 150mm to 200mm cavity from the primary window, and that the air-filled cavity acts as insulation¹. For comparison, the same source gives double glazed units a 14 to 20mm air gap between their two panes¹. The much deeper air space between a primary window and a secondary unit is why acoustic performance can be strong even where thermal performance is only comparable.

There is a security side effect. Granada states that by adding an extra layer of glass or plastic to an existing window, secondary glazing creates a barrier that can be more difficult for intruders to break through¹⁰. That is a maker's claim about its own product category rather than an independent test result, and it should be read as such.

At the cheapest end, the term covers much less. The National Energy Action guidance describes the cheapest type of secondary glazing as attaching some form of plastic sheeting stretched across the window, with acrylic sheets on magnetic edges or plastic carrying strips fixed with double-sided tape or hook-and-loop tape, reusable, with at least one window per room left uninsulated or openable for ventilation⁷. Temporary secondary glazing film over single glazed windows costs £5 to £15 per window⁸. That is a different product from a framed aluminium unit, and the performance figures quoted for the framed systems do not apply to it.

Cost: secondary glazing at around 25% of the price of double glazing

The headline comparison comes from Granada: secondary glazing can be installed for around 25% of the cost for double glazing windows, depending on the size and style of the chosen system¹. The Glass and Glazing Federation gives a different figure, describing secondary glazing as approximately 50% less on a supply and install basis². The two documents disagree, and the gap is wide enough that a household should treat both as indicative rather than as a quotation.

Independent pricing sits between the two claims. The Centre for Sustainable Energy states that secondary glazing for a single window will cost upwards of £300 depending on the size and complexity of the window and the type of opening⁵. USwitch gives £300 to £500 per window for a permanent install⁷, and the same range for secondary glazing per window in its Victorian house guidance⁸. A whole-house internal casement project in an independent case study came to around £10,000 for the whole house¹⁵.

Two things drive the spread. The first is the unit type: a fixed unit is simpler than a horizontal slider, and a slider is simpler than a lift-out or hinged unit. The second is the primary window. Granada states that secondary glazing is generally more cost-effective and affordable than window replacement¹⁰, but the Glass and Glazing Federation's condition that existing windows must be in excellent condition¹⁴ means that a household with failing primary windows may end up paying for both.

For context on the alternative, triple glazing is around 20% more expensive than double glazing⁶. Neither double, triple nor secondary glazing is available through the Scottish scheme that Home Energy Scotland lists, which records all three as not available¹⁶.

Thermal performance: up to 10% more energy saved than double glazing

The strongest thermal claim in the material is Granada's: its fixed secondary glazing achieves approximately 10% more than double glazing achieves³, and its listed buildings range is described as 10% more heat efficient than double glazing⁹. Both are maker claims about the maker's own products, and they sit against a more cautious independent position.

Which? states that secondary glazing is not as effective as full double glazing, but that it could still save money on heating bills and is much cheaper to install⁴. The Glass and Glazing Federation frames secondary glazing as an improvement on single glazing rather than a rival to a sealed unit: it is an alternative to replacing single glazed windows for those wanting energy efficient glazing¹⁷.

The official position is that secondary glazing works, within limits. Scottish Government guidance states that secondary glazing can provide an effective way to reduce heat loss without altering original windows, nor significantly reducing light levels or views¹⁸. That is the case for it in a traditional building: the heat loss falls, and the fabric and appearance survive.

The mechanism explains the range. A 150mm to 200mm cavity¹ is far deeper than the 14 to 20mm gap in a double glazed unit¹, but the outer pane of a secondary glazing installation is usually the original single glass, which is itself a poor insulator. The result is a system that performs well against single glazing and roughly comparably to double glazing, with the exact figure depending on the primary window, the unit type and how well the secondary frame is sealed.

Noise reduction: up to 80% less external noise

Acoustic performance is where secondary glazing has the clearest advantage. Granada states that its fixed secondary glazing reduces noise by up to 80%³, and gives the same figure for its listed buildings range⁹. Granada also states that secondary glazing can significantly reduce the amount of noise that enters a building, even more so than double and even triple glazing in most instances¹⁰.

That last claim is a maker's comparison against rival products and should be read as such. The physical reason it can hold is the cavity depth: a 150mm to 200mm air space¹ absorbs more sound than the 14 to 20mm gap in a sealed double glazed unit¹. A wide, well-sealed cavity between two panes of different thicknesses is a recognised acoustic arrangement, and secondary glazing creates one by default.

The practical conditions matter. A sealed or lift-out unit will perform better acoustically than an openable one, because an openable frame has more paths for sound to pass. The Centre for Sustainable Energy notes that units can be fitted sealed, lift-out or openable⁵, so the acoustic result depends on which is chosen. A unit left open for ventilation gives up most of its acoustic benefit while it is open.

For a household on a noisy road, this is the measure that most directly changes daily life, and it is the one where secondary glazing is most likely to outperform a replacement window of similar cost. For a household whose main concern is heat loss, the thermal case is closer and the independent evidence is more cautious.

Where secondary glazing fits: listed buildings, conservation areas and rentals

Planning status, not performance, decides most secondary glazing installations. Granada states that 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 restrictions¹. The Centre for Sustainable Energy describes it as a non-intrusive way of insulating historic windows from the inside⁶.

Councils set conditions rather than bans. Brighton and Hove council states that secondary glazing will be acceptable provided its framing is unobtrusive and it does not obscure internal features or interfere with the operation of shutters and windows¹⁹. Exeter council states that double glazing can sometimes be installed on a listed building, depending on the building and how the installation would affect its character²⁰. The Centre for Sustainable Energy warns that some secondary glazing options are not allowed in listed buildings, so consent should be checked first⁵.

Conservation areas add their own rules. Brighton and Hove states that on elevations facing a street or public open space, the original windows' material, style, pattern and joinery sections must be matched¹⁹. Home Energy Scotland advises that there may be planning restrictions on what can be installed depending on where you live, and that advice should come from the local council²¹. The Chartered Institute of Plumbing and Heating Engineering notes that some properties in the United Kingdom, such as those with a listed status or in a conservation area, are not permitted to use double glazed windows²².

Rentals are a different case again. Secondary glazing is reversible in a way that replacement windows are not, which matters where a leaseholder needs freeholder consent or a tenant needs a landlord's agreement. The material does not give a specific rule for rented homes, so the position depends on the tenancy and the freeholder.

Unit styles and glass options: sliders, hinged, lift-out and low-e

Granada lists the types of secondary glazing as horizontal sliding windows, vertical sliding windows, hinged secondary glazing units, lift-out, fixed secondary glazing and several more¹⁰. The Centre for Sustainable Energy describes the same range from the fitting side: units can be fitted to be sealed, lift-out or openable, on either hinges or vertical or horizontal slider tracks, so the original windows can open as normal⁵.

Glass choice is contested in the material. Granada's listed buildings page recommends 6.8mm acoustic glass with an air cavity of 150mm to 200mm glass to glass, and separately lists 6.4mm low-E glass⁹. The two recommendations are not reconciled in the source, so a household specifying a system should expect the choice to be made per window, with acoustic glass where noise is the priority and low-E where heat retention is.

On the glass versus acrylic question, one maker's support article states that although acrylic secondary glazing is lightweight and comparatively cheap to install, glass secondary glazing performs significantly better than acrylic, with better insulation and noise reduction, a crystal-clear appearance, easier cleaning and much longer life²³. Acrylic remains relevant where weight on the frame or cost rules out glass.

Granada's fixed range is available in several different styles as well as single or double-glazed aluminium profiles³. Its listed buildings range uses high-durability aluminium secondary glazing in over 200 RAL colours, with arched frames available⁹.

Granada Glazing: the UK's largest secondary glazing maker

Granada Glazing is the maker whose product pages carry most of the specification detail on this page. It states that it has over 35 years of experience installing the best quality secondary glazing in the industry¹⁰. Its listed buildings range is high-durability aluminium secondary glazing, available in over 200 RAL colours, with arched frames⁹.

Installation is offered two ways: local installers trained and accredited by Granada, or home delivery³. The company runs a helpline for homeowner product, service or installation queries³. Its published contact numbers differ between pages: 019094 99899 on its comparison guidance¹⁰ and 01909 499899 on its fixed secondary glazing page³. The two documents disagree and both are given here.

Warranty terms also differ by page. Granada's fixed secondary glazing page states a 10 year guarantee on all frames and glazing³. Its listed buildings page states a 5 year warranty on the fittings, frames and glass, described as a FENSA warranty of installers, with a 2 year warranty on the movable parts⁹. A buyer specifying a listed buildings system should establish which term applies before ordering.

For comparison, double glazing often comes with a 10 year, 20 year or sometimes lifetime warranty²⁴, and most new installations are guaranteed for 10 years²². Double glazed windows can last between 20 to 35 years if they are high-quality, installed properly and well maintained⁶, with a typical lifetime of around 20 years, varying from 10 to 35 years depending on product quality, installation and where the windows are situated²⁵.

What secondary glazing does for energy independence

Secondary glazing reduces the heat a home loses through its windows, which reduces the gas or electricity it buys to replace that heat. That is a real gain in independence, and it is bounded. The household still buys its heat from a supplier, still depends on the grid or the gas network, and still depends on the maker for frames, glass and moving parts. A secondary glazing unit is a manufactured product with a warranty and a supply chain behind it, not a self-sustaining system.

The dependence is smaller than for a replacement window in one respect: the original window stays, so the household is not tied to a single sealed unit that must be replaced whole if a seal fails. It is larger in another: the performance of the installation depends on the primary window, which the household does not control and may not be able to repair.

Where the building is listed or in a conservation area, secondary glazing is often the only glazing measure available at all¹, and it can be a non-intrusive way of insulating historic windows from the inside⁶. In that setting the independence gain is not a choice between two products; it is the difference between an upgrade and none.

For the wider fabric picture, double glazing covers sealed units and frames, triple glazing covers the higher-specification alternative, and secondary glazing covers the systems themselves. Window energy ratings explains how glazing performance is rated, listed buildings insulation and glazing covers consent in detail, and insulation and glazing costs sets these figures against the rest of the fabric. The pillar guide places all of it in the order a whole-house retrofit should follow.