Slimline double glazing, also sold as slim-profile, slim-cavity or heritage double glazing, exists for one reason. A standard sealed unit is too thick for the rebates and glazing bars of a Georgian or Victorian window, so fitting one often means new, fatter sashes or a new window, and the thing the planners were protecting goes in the skip. A slim unit is thin enough to go into the window you have. It is a conservation product first and an energy product second, and most of the confusion around it comes from selling it the other way round. This guide sets the makers' figures against the only in-place measurements a UK heritage body has published, and is plain about where the evidence runs out, which includes the price.
What counts as slimline double glazing
There is no legal definition, and the trade uses the word loosely. Historic England's window guidance describes conventional units as 22 to 28 mm thick overall and slim-profile units as 10 to 16 mm. Historic Environment Scotland describes standard cavities of around 13 to 20 mm and slim ones of 3 to 6 mm, with a narrower edge strip. The units Historic Scotland measured for its 2010 research ran from 8.2 to 16 mm. Bath and North East Somerset Council caps what it will accept at 12 mm; installers variously say 14 mm or less, 12 to 14 mm, or 12 to 18 mm. The useful habit is to ask for the build in millimetres: 4-4-4 means a 4 mm pane, a 4 mm cavity and a 4 mm pane, 12 mm overall.
| Glazing | Overall thickness | Gap between the panes | Source |
|---|---|---|---|
| Single glazing | 4 to 6 mm | None | Historic Scotland Technical Paper 9 |
| Vacuum glazing | About 6 to 12 mm | About 0.2 mm, air removed | HES; maker data sheets |
| Slimline (slim-profile) double glazing | 10 to 16 mm (HE); 8.2 to 16 mm measured (HES) | 3 to 6 mm (HES); makers use 3 to 8 mm | Historic England; HES |
| Conventional double glazing | 22 to 28 mm (HE); 20 to 25 mm (HES) | About 13 to 20 mm | Historic England; HES |
| Triple glazing | 35 to 45 mm | Two cavities | Historic Scotland Technical Paper 9 |
Sources: Historic England, Traditional Windows (HEAG039, 2017); Historic Environment Scotland, Managing Change: Windows; Historic Scotland Technical Paper 9 (September 2010). Research in lib/research/slimline-double-glazing-2026.md.
The gas matters more in a narrow cavity than a wide one. Historic Scotland's research found that argon gives only a marginal gain over air in a small gap, while krypton and xenon do better, which is why most slim units are krypton filled. Cornwall Glass's own figures for a unit with a 4 mm cavity show the step: 3.0 W/m²K with air, 2.6 with argon, 1.9 with krypton. The same research put the ideal gap for krypton at 11 mm, wider than any slim unit, and said plainly that the panes it tested were "not optimised for thermal performance", because thinness came first. The trade-off between the two gases is covered in argon vs krypton gas.
One naming trap. "Slimline aluminium" or "slim-profile aluminium" on a quote means a narrow metal frame, not a slim sealed unit. Both turn up in conservation areas and they are different products; this guide is about the glass.
What it achieves: the brochure figure and the measured one
Makers publish centre-pane U-values, the figure for the middle of a new pane of glass, and they agree with each other closely. For a krypton-filled unit, a 4 mm cavity is declared at about 1.9 W/m²K, a 6 mm cavity at about 1.5 and an 8 mm cavity at 1.1 to 1.2. Slimlite gives its original krypton range as 1.4 to 2.1 depending on the cavity. None of these is a whole-window figure, and none was measured in a sash.
| Unit (4 mm glass each side) | Overall thickness | Declared centre-pane U-value |
|---|---|---|
| 4 mm cavity, krypton | about 12 mm | about 1.9 W/m²K |
| 6 mm cavity, krypton | about 14 mm | about 1.5 W/m²K |
| 8 mm cavity, krypton | about 16 mm | 1.1 to 1.2 W/m²K |
| For comparison: 4 mm cavity with argon | about 12 mm | 2.6 W/m²K |
Source: maker data from Heritage Glass UK, CN Glass, Cornwall Glass (June 2025), UK Glass Centre and Histoglass. Declared centre-pane values for new units, not measurements in a window.
The only in-place test by a UK heritage body is Historic Scotland's Technical Paper 9 (September 2010), by Changeworks, Glasgow Caledonian University and Heriot-Watt University. It fitted ten systems into Georgian windows in listed buildings in Edinburgh and measured them. Centre-pane values ranged from 1.0 to 2.8 W/m²K against 5.4 for the single glazing, and "most systems achieved a U-value close to 2.0". Its verdict on the brochures: "Generally the manufacturer's specification tends to overestimate the performance of the glazing unit", with two exceptions, the vacuum unit and the Slenderglaze unit.
| System tested in 2010 | Build (mm) | Fill | Measured centre-pane U-value |
|---|---|---|---|
| Pilkington energiKare Legacy | 4-0.2-3 | Vacuum | 1.0 |
| Slenderglaze | 4-3.9-4 | Xenon and krypton | 1.7 |
| Sashworks (new sashes) | 4-8-4 | Argon | 2.0 |
| Slimlite (new sashes) | 3-3-3 | Xenon and krypton | 2.0 |
| Slimlite | 3-3-3 | Xenon and krypton | 2.3 (two units) |
| Histoglass | 3-4-4 | Krypton | 2.3 and 2.7 |
| Slimlite | 3-3-3 | Air | 2.8 |
| Supalite (new sashes) | 4-4.8-3 | Argon | 2.8 |
| Original single glazing | Single pane | None | 5.4 |
Source: Historic Scotland Technical Paper 9, Slim-profile double glazing: thermal performance and embodied energy (September 2010), Tables 1 and 2. These are 2010 products; current specifications may differ.
The whole window does worse than the glass, because frames, glazing bars and unit edges lose heat too. Technical Paper 9 calculated whole-window values of 1.9 to 3.4 W/m²K for the Georgian windows and 1.4 to 3.0 for Victorian ones, which have fewer glazing bars: cuts in heat loss of 35 to 63% and 41 to 73% against single glazing. That is a large improvement on a single-glazed sash and it is not modern double glazing: the site's figure for A-rated double glazing is 1.0 to 1.4, and a slim-unit quote should never borrow it. Historic Environment Scotland's later retrofit guide (November 2023) gives a double glazed pane in an existing sash a better figure, 1.3, without reconciling it with the 2010 measurements. Both are recorded here; we have not picked one.
The number on a slim unit's data sheet is a centre-pane figure for new glass. When a heritage body measured these units in real sashes, most came in near 2.0. Buy slimline to keep the windows, not to hit a U-value, and ask any seller who quotes 1.1 which cavity and which gas that figure is for.
Double Glazing Cost Calculator Team
Three ways to fit it
Historic England describes two routes for a historic window: where the glass is of no historic interest and the frame profiles are thick enough, "it may be possible to fit slim-profile or vacuum double glazing into existing sashes or casements with only minimal alterations", or new double glazed sashes can go into the existing frames. The third route, a whole new window, is treated differently by the Building Regulations, below.
- Reglaze the existing sashes. The rebates that hold the glass are cut deeper, by about 6 mm on Historic Environment Scotland's description, the units are bedded in, and the sashes get new cords and sometimes heavier weights to balance the extra glass. Historic Scotland's research found this the more sustainable route, because new sashes add embodied energy.
- New sashes in the original box frame. The frame, pulleys and linings stay; the sashes are remade to take the units. One Devon joinery firm prefers it, saying it is "best to make a new sash", and adds makeweights so it balances.
- A new window. A replica timber sash made to take slim units from the start. This is a new window for Part L purposes, which changes what it has to achieve.
No heritage body or standards body publishes a minimum rebate. Makers give their own: CN Glass fits an 11 mm unit, 3 mm glass outside and 4 mm inside, into a rebate of at least 17 mm; Histoglass recommends a 20 mm rebate and a 24 mm glazing bar for new frames; a Devon joiner says a slim unit fits within a 22 mm bar where a standard one needs 40 mm. Historic England's older guidance (2017) warned that multi-paned windows often cannot take even slim units without altering frames and bars, which makes the finely barred Georgian window the hardest case.
The details that make it pass or fail with a conservation officer are small. Advice Note 18 wants original glazing bars kept and says stuck-on bars "are unlikely to be acceptable". Historic Environment Scotland asks for true, through glazing bars (astragals, in Scotland) and says bars sandwiched between the panes alone "will be very unlikely to be acceptable"; it also suggests a white spacer can better match the bar, and Bath asks for a spacer the colour of the painted timber. And the putty: Historic England says standard linseed oil putty can damage the seals of a sealed unit, so a compatible glazing compound is needed. The spacer at the edge of the unit is explained in our warm edge spacer guide.
What slimline double glazing costs, and why the evidence is thin
No independent, dated source prices slim units specifically. The figures below are the best available, and each carries a caveat worth reading before you judge a quote against it.
| Route | Per window | Source and caveat |
|---|---|---|
| Double glazing fitted into existing sash frames | £650 to £1,300 | Checkatrade (July 2026). Does not say slim units, or whether the sashes are reglazed or remade. |
| Reglazing existing sashes, London | £650 to £1,300 plus VAT | Golden Ratio Sash Windows (2026 prices). One installer, standard argon units; the same range as Checkatrade, so not independent corroboration. |
| New double glazed sashes in the original box, London | £850 to £2,000 plus VAT | Golden Ratio Sash Windows (2026 prices). One installer. |
| Slim units in an average six-over-six sash | £600 to £800 | Bath and North East Somerset Council planning guidance. Undated. |
| New timber sash window with slim units | £1,330 to £2,850 | This site's timber heritage sash range; a new window, not a reglaze. |
| Secondary glazing, standard window | £300 to £500 | Checkatrade (July 2026). |
Sources as listed. Golden Ratio's full replacement double glazed sash starts from £2,400 plus VAT. Research in lib/research/slimline-double-glazing-2026.md.
On the premium over standard units, only sellers have published a figure: one London installer says heritage slim-bar units attract 15 to 25%, a Bristol supplier says 30 to 45% more per square metre than a standard 28 mm unit. Supply-only units are listed from £86.60 by one supplier, with no size given. Across four front sashes, the arithmetic on the figures above is roughly £2,600 to £5,200 to reglaze, £5,320 to £11,400 for new timber sash windows, and £1,200 to £2,000 for secondary glazing. The sash window calculator and the timber window calculator price new windows on your own count.
Keep the saving in proportion. Historic England's view is that windows may be a relatively small share of a building's heat loss, and that "improvements beyond repair and draught sealing may not be cost effective in either financial or carbon terms". The Energy Saving Trust puts the saving from A-rated double glazing across a whole single-glazed semi at around £85 a year in Great Britain, so a claim of hundreds of pounds a year from a few slim-glazed sashes should be treated as a sales figure. Draught-proofing a sash, £248 to £388 a window on Checkatrade's July 2026 figures, comes first; our sash windows guide covers it.
Listed buildings and conservation areas: Historic England in 2026
Historic England's position moved in its Advice Note 18, Adapting Historic Buildings for Energy and Carbon Efficiency, revised in February 2026. It is more permissive than the 2017 window guidance that is still online and marked as under review, and the two are not reconciled, so expect to meet both.
Installation of slim-profile or vacuum double-glazing within historic frames will generally be acceptable.
Historic England, Advice Note 18 (revised February 2026)
The same note sets three exceptions where such units "are unlikely to be acceptable": windows that retain historic glass of interest, windows of historic or architectural interest whose frames or glazing bars cannot support the units, and windows with leaded lights, which have their own routes in our guide to stained glass double glazing. And it says "listed building consent is likely to be required, unless replacing panes in modern windows". Historic England's web guidance (2023, updated 2024) is firmer: inserting double or triple glazing into historic windows "will usually require Listed Building Consent", and replacing windows always does. On whole windows, Advice Note 18 accepts replacing windows that add nothing to a building's interest, but says original windows that do "should not be replaced", although their panes could take slim or vacuum units in the existing frames. The cost side is on our listed building costs page.
Planning permission is a separate question from listed building consent. Advice Note 18 notes that houses in conservation areas without an Article 4 direction can change windows of similar appearance without planning permission, while flats, and houses covered by an Article 4 direction, generally need it. How that works street by street is in our guide to conservation area window rules, and what it does to the budget is on the conservation area cost page.
Scotland reached the same place earlier. Historic Environment Scotland says slim and vacuum units "have directly resulted in double-glazing becoming more generally acceptable in historic buildings", while retrofitting double glazing to a listed building "will be likely to require consent"; it is consulted on A and B listed cases, and the council decides Category C alone. Its 2013 follow-up paper records that the Edinburgh research led the city to change its planning guidelines for listed buildings in 2012 to allow slim-profile double glazing except where historically important glass survives, with consent still required first. The Edinburgh detail is on our sash window repair in Edinburgh page.
Building Regulations: reglazing is outside Part L, a new window is not
This is the part most slimline pages skip. Approved Document L (2021 edition, with 2023 amendments) paragraph 10.6 says: "Replacing glazing, or a window or door in its existing frame is not providing a controlled fitting. Such work does not need to meet the energy efficiency requirements." So reglazing existing sashes with slim units does not have to hit any U-value. Whether remade sashes in an original box frame count as "a window ... in its existing frame" is a reading to confirm with building control, not something to assume. FENSA's compliance guide likewise puts sash and glass-only work outside its scheme, so there is no FENSA certificate for a reglaze; our FENSA guide covers what the scheme does and does not certify.
A new window is a controlled fitting and has to reach 1.4 W/m²K or Window Energy Rating band B (Table 4.2). Where that cannot be done "because of the need to maintain the character of the building", paragraph 4.10 allows either a centre-pane U-value of no more than 1.2 or single glazing backed by low-emissivity secondary glazing. That is a different 1.2 from the new-build figure in our Part L guide. And paragraphs 0.8 and 0.9 let listed buildings, homes in conservation areas and scheduled monuments comply only as far as they can without unacceptably altering their character, with building control expected to take the conservation officer's advice.
Put the declared figures against those rules and the problem is plain. A slim unit declared at 1.4 to 2.1 at the centre of the glass meets neither 1.4 for the whole window nor 1.2 at the centre, unless it is the widest krypton build (declared 1.1 to 1.2) or a vacuum unit, or the listed or conservation-area exemption applies. So a new replica sash with slim units usually relies on that exemption, and it is worth asking the supplier in writing which route they are using. Approved Document L 2026, in force from 24 March 2027, keeps both paragraph 10.6 and the centre-pane 1.2 route.
Edge seals, lifespan and what the guarantee covers
A sealed unit works only while its edge seal keeps the gas in and the moisture out, and on slim units the seal is where the arguments are. The Glass and Glazing Federation's 2017 statement, the most recent we could find, puts the problem not in the narrow cavity but in the low sightline: one maker cut 5 mm warm edge spacer bars down to 3 mm and used as little as 2 mm of sealant to hide the edge of the unit behind a slim bar, and "there was a high failure rate of these units". A purpose-made 3 mm spacer followed, marketed with at least 5 mm of sealant behind it, which gives a sightline of 8 mm. The GGF said the reduced-sightline units being supplied did not comply with BS EN 1279, the sealed unit standard, but that most agencies took the view that "Heritage takes precedence", and it had referred one maker to Trading Standards. Historic Environment Scotland says units should be made to EN 1279. Makers' claims vary from full certification to one admitting it has none, so ask for the certificate and the sightline.
On life, Historic England puts the service life of slim-profile units at around 25 years, and of sealed units generally at 15 to 25, against 60 to 100 years or more for secondary glazing. The only re-measurement, Historic Scotland's Technical Paper 20 (2013), went back after two years and could draw no firm conclusion: most results sat within the margin of error, the median pointed to some decline in most gas-filled units, and one xenon and krypton unit's U-value rose by 19%. It expected gas-filled units to drift towards the performance of an air-filled one, and recommended periodic checks. When a unit does fail, the symptoms and the fix are in our blown double glazing guide.
| Unit | Published guarantee |
|---|---|
| Westcountry heritage units (Cornwall Glass, UK Glass Centre) | 2 years, subject to correct glazing installation |
| Slimlite, trade-supply units | 5 years against manufacturing failure |
| GS Haydon, slim units | 5 years |
| Histoglass HD11 | 7 years |
| Pane Relief, slim units | 10 years on edge-seal integrity |
| Slimlite, complete window systems | 10 years |
Sources: each maker's or fitter's own page, mostly undated. Service life: Historic England, Modifying Historic Windows (2023, updated 2024).
Read what a guarantee covers, not only how long it runs. Slimlite's, for example, covers manufacturing defects, sealed unit failures and internal condensation. Gas escaping shows first as a worse U-value and only later as misting, so a guarantee triggered by condensation may not cover lost performance. And weigh who is talking. Three of the top results for "slimline double glazing" sell vacuum glazing and argue slim units fail early. The loudest claim, that 30 to 50% start failing within five years, comes from a vacuum installer with no source given; another vacuum seller says performance degrades "often within just a few years", citing research that measured the units once. A Devon joiner who fits slim units says it has used them for about 15 years without large-scale failures, and guarantees them for five. Neither side has independent data beyond two years.
Slimline or vacuum glazing
They solve the same problem differently. A slimline unit holds gas in a cavity of a few millimetres behind a polymer edge seal. Vacuum glazing pumps the air out of a gap of about 0.2 mm and holds the panes apart with a grid of tiny pillars; in the units Historic Scotland tested, the panes were welded together at the edge rather than sealed with a polymer. It can be as thin as about 6 mm. In Historic Scotland's tests the vacuum unit was the best performer "by a significant margin", measured at 1.0 W/m²K at the centre of the glass, and on the 2013 re-measurement it showed no significant deterioration. Historic Environment Scotland also notes it is dearer than the other options, comes with a minimum pane size, and has pillars that some people notice close up. The products, the glass-only price evidence and the gap between its glass and window figures are in our vacuum glazing guide.
Slimline or secondary glazing
For heat, the evidence is split. Technical Paper 9 calculated whole-window values of 2.0 and 2.1 W/m²K for Georgian and Victorian windows with secondary glazing, better than every gas-filled slim system with one minor exception, though not as good as the vacuum unit. Historic Environment Scotland's 2023 retrofit guide ranks them the other way, 1.3 for a double glazed pane in the existing sash against 1.7 for secondary glazing. Advice Note 18 says secondary glazing "will generally be acceptable", that on multi-pane windows "it often outperforms double-glazing", and that listed building consent is unlikely to be needed for most of it. It has no seals to fail, is better for noise, and costs £300 to £500 fitted for a standard window (Checkatrade, July 2026); the full breakdown is on our secondary glazing costs page. The downside is a second window to open and clean behind. The head-to-head is in secondary vs double glazing.
Getting a heritage window priced
Slim glazing is a specialist job and the published prices are few, so the quotes you get are the real evidence. If you want double glazing, new sashes or replacement windows priced on your own openings, tell us what you have and Leads 2 Trade will arrange quotes from installers covering your postcode.
Frequently asked questions
A sealed double glazed unit thin enough to fit the rebate of a period sash or casement, so the original frame and glazing bars can stay. Historic England puts it at 10 to 16 mm thick overall against 22 to 28 mm for a conventional unit, and Historic Environment Scotland describes cavities of 3 to 6 mm. It is also sold as slim-profile, slim-cavity or heritage double glazing. There is no legal definition, so ask for the build in millimetres.
Nobody independent publishes a slim-specific price. Checkatrade (July 2026) puts fitting double glazing into existing sash frames at £650 to £1,300 per window, but does not say the units are slim. One London installer quotes the same £650 to £1,300 plus VAT for reglazing existing sashes and £850 to £2,000 plus VAT for new sashes in the old box frame. A new timber sash window with slim units falls in the £1,330 to £2,850 range for timber heritage sashes. Two sellers put the premium over standard units at 15 to 25% and 30 to 45% per square metre.
Usually, with consent. Historic England's Advice Note 18, revised in February 2026, says slim-profile or vacuum double glazing within historic frames will generally be acceptable, and that listed building consent is likely to be required unless the panes being replaced are in modern windows. The exceptions are windows with historic glass of interest, frames or glazing bars that cannot take the units, and leaded lights. In Scotland, Historic Environment Scotland says retrofitting double glazing will be likely to require consent.
Makers declare centre-pane values of about 1.9 W/m²K for a krypton unit with a 4 mm cavity, 1.5 for 6 mm and 1.1 to 1.2 for 8 mm. When Historic Scotland measured ten installed systems in Edinburgh sashes, the results ranged from 1.0 to 2.8, most close to 2.0, against 5.4 for the single glazing, and it found makers' figures generally overestimated performance. Calculated for the whole window, the Georgian sashes came out at 1.9 to 3.4.
Not if only the glass changes. Approved Document L paragraph 10.6 says replacing glazing, or a window in its existing frame, is not a controlled fitting and does not need to meet the energy efficiency requirements. A new window does: 1.4 W/m²K or band B, or where the building's character must be kept, a centre-pane U-value of 1.2 or low-emissivity secondary glazing. Listed and conservation-area homes need only comply as far as that would not unacceptably alter their character.
Historic England puts the service life of slim-profile units at around 25 years, and of sealed units generally at 15 to 25 years. The guarantees published by makers and fitters are shorter: 2, 5, 7 and 10 years. Some, such as Slimlite's, cover condensation between the panes, which appears late, rather than the loss of gas, which shows first as a higher U-value.
No. Slimline units hold air or an inert gas, usually krypton, in a cavity of a few millimetres behind an edge seal. Vacuum glazing has a gap of about 0.2 mm with the air pumped out and tiny pillars holding the panes apart, is thinner still, and was the best performer in Historic Scotland's tests, measured at 1.0 W/m²K at the centre of the glass.
On Historic Scotland's calculations, yes for heat, against gas-filled slim units: secondary glazing gave whole-window values of 2.0 and 2.1 W/m²K and beat the gas-filled slim systems, with one minor exception, though not the vacuum unit. A later Historic Environment Scotland guide gives the opposite ranking, so the evidence is split. Secondary glazing has no seals to fail, usually needs no listed building consent, and costs £300 to £500 fitted for a standard window (Checkatrade, July 2026).