London · 2026
Double Glazing in
London.
What is actually inside a double glazed unit
A sealed unit, properly an insulating glass unit, is not just two panes of glass. Six things determine how it performs, and quotes rarely spell out more than two of them.
The panes. Our standard build is 4mm toughened outer and inner glass. Thicker or laminated panes are specified where safety, security or acoustics require it. The cavity. 20mm on our standard unit, which is close to the optimum for argon; wider is not automatically better, because beyond about 20mm convection inside the cavity starts working against you. The gas. 90% argon fill, which conducts less heat than air. The coating. A soft-coat low-emissivity layer on the cavity face, microscopically thin and effectively invisible, which reflects long-wave heat back into the room. The spacer. A warm-edge spacer bar rather than aluminium, which is what stops the perimeter of the pane becoming the cold bridge that causes edge condensation. The seals. The primary and secondary seals hold the gas in and the moisture out, and they are what eventually fails on a unit that mists.
Add those together and our standard unit is 4-20-4, 28mm overall, reaching centre-pane U-values from 1.0 W/m²K, up to A+ rated as a whole window depending on the frame, with a sound reduction of 31 dB Rw and a 15-year guarantee against seal failure and misting.
Standard sealed unit specification
| Property | Standard | Upgrade available |
|---|---|---|
| Unit build | 28mm, 4-20-4 | 36mm triple, 4-12-4-12-4 |
| Glass | 4mm toughened | Laminated, toughened-laminated, acoustic |
| Cavity fill | 90% argon | Krypton in narrow cavities |
| Coating | Soft-coat Low-E on the cavity face | Solar control, self-cleaning |
| Spacer | Warm-edge | Colour-matched spacer |
| Centre-pane U-value | From 1.0 W/m²K | As low as 0.5 W/m²K (triple, Passive spec) |
| Whole-window U-value | From 1.1 W/m²K | Depends on frame system |
| Acoustic (Rw) | 31 dB | Acoustic laminated interlayers |
| Energy rating | Up to A+ | A++ with triple glazing |
| Guarantee | 15 years against seal failure | n/a |
Full detail on the units themselves is on our double glazed unit page and triple glazed unit page.
Unit thickness, and what fits what
People search for 28mm, 32mm and 36mm units because they are trying to work out what their frames will take. The overall thickness is simply the panes plus the cavities added up, and the frame's glazing rebate sets the ceiling.
| Unit | Build-up | Type | Typically used in |
|---|---|---|---|
| 24mm | 4-16-4 | Double | Older frames and shallow heritage rebates |
| 28mm | 4-20-4 | Double | Our standard unit; uPVC casement, sash, bay and French windows |
| 28mm | 4-8-4-8-4 | Triple | Retrofitting triple glazing into a frame built for a 28mm double unit |
| 32mm | 4-10-4-10-4 | Triple | Aluminium systems with a mid-depth rebate |
| 36mm | 4-12-4-12-4 | Triple | uPVC triple glazing; the standard triple build |
| 44mm and above | Wider cavities or thicker panes | Triple / acoustic | Aluminium casements to 44mm, tilt and turn to 40mm, sliding door systems to 56mm and beyond |
Two practical points. A 28mm triple unit is not the same product as a 36mm triple unit: squeezing three panes into 28mm means two 8mm cavities instead of two 12mm ones, which costs you thermal performance, so it is a compromise for retrofit rather than an upgrade. And a triple unit weighs roughly half as much again as the double it replaces, so hinges, hardware and sash sizes have to be specified for it rather than assumed.
Glazing capacities across the systems we fit: uPVC windows take 28mm double or 36mm triple; aluminium casements take 23 to 44mm; the aluminium tilt and turn takes up to 40mm; steel-look heritage takes 24 to 36mm; structural corner windows take 28 to 41.5mm; and sliding door systems run from 22mm up to 65mm depending on the system.
Double or triple glazing
Triple glazing adds a third pane and a second argon-filled cavity. It is a real improvement in heat loss and it is the right specification in specific circumstances, but it is not automatically the better buy.
| Factor | Double | Triple |
|---|---|---|
| Standard build | 28mm, 4-20-4 | 36mm, 4-12-4-12-4 |
| Centre-pane U-value | From 1.0 W/m²K | As low as 0.5 W/m²K |
| Cavities | One | Two |
| Weight | Baseline | Roughly 50% heavier |
| Hardware implications | None | Heavier hinges, reduced maximum sash sizes on some systems |
| Energy rating | Up to A+ | A++ |
| Best for | Most replacement work in existing London housing | New build, deep retrofit, Passivhaus targets, north-facing rooms, combined thermal and acoustic upgrades |
A note on acoustics, because triple glazing is frequently sold as a noise solution and it is a weak one. Sound reduction comes from mass and from asymmetry between panes, not from the number of panes. A laminated acoustic double-glazed unit with dissimilar pane thicknesses will usually outperform a symmetrical triple unit against traffic noise, and it is cheaper. If noise is the problem, specify for noise. The longer comparison is on our double versus triple glazing page.
Glass options by problem
The standard argon-filled Low-E unit is the right answer for the large majority of rooms. These are the exceptions, and what to specify instead.
| Problem | Specify |
|---|---|
| Traffic, railway or flight-path noise | Acoustic laminated outer pane with dissimilar pane thicknesses |
| West or south-facing room overheating | Solar-control coating on the outer pane |
| Ground-floor or accessible window security | Laminated inner pane |
| Glazing near a door, low level or in a bathroom | Toughened or laminated safety glass, as Building Regulations Part K requires |
| Bathroom or overlooked window | Obscured, patterned or sandblasted glass by privacy level |
| Period property with glazing bars | Astragal bars or through bars set out to the original rhythm |
| Listed building where replacement is refused | Secondary glazing with a low-emissivity inner pane |
| Passivhaus or net-zero target | Triple glazed Passive specification, with the frame chosen to match |
What the regulations require
Replacement windows are controlled work under the Building Regulations regardless of whether planning permission is involved.
| Requirement | Standard |
|---|---|
| Maximum whole-window U-value on a replacement | 1.4 W/m²K, or Window Energy Rating Band B minimum |
| No downgrade | The new unit must also be no worse than the one it replaces |
| Replacement doors with over 60% of the face glazed | 1.4 W/m²K, or Doorset Energy Rating Band C minimum |
| Background ventilation, habitable rooms and kitchens | Minimum 8,000mm² equivalent area |
| Background ventilation, bathrooms | Minimum 4,000mm² equivalent area |
| Heritage exception where character must be maintained | Centre-pane U-value no worse than 1.2 W/m²K, or low-E secondary glazing |
Sources: Approved Document L Volume 1, 2021 edition (Table 4.2, paragraphs 4.8 and 4.10) and Approved Document F Volume 1, 2021 edition (paragraphs 3.14 and 3.15). The 2026 edition of Approved Document L has been published but does not take effect until 24 March 2027, so the 2021 edition is the standard in force for work today. Apex Glazing is FENSA-registered (Reg 40373), so the installation is self-certified and notified to your council, and you receive a certificate for the property file.
On planning: double glazing is a specification, not a planning category, and on an unlisted house outside an Article 4 Direction it needs no permission. See planning permission for new windows.
Manufacturer, supplier or installer?
Searches for double glazing manufacturers and suppliers in London usually come from one of three people, and they need different things.
| What you need | What to ask for |
|---|---|
| Whole windows, measured, made and fitted | A supply-and-fit installer with a competent person scheme registration, so the work is self-certified |
| Replacement sealed units for existing frames | Made-to-measure units to your surveyed sizes, fitted into the frames you already have |
| Glass only, for a builder or joiner to install | Supply of units to a cutting list, with the toughening and coating specification confirmed in writing |
Apex Glazing specifies the system, has the frames and units made to your surveyed openings, and fits them with our own installation teams rather than subcontracting the fix. One survey, one quote, one guarantee, one number to call. That matters most where things get awkward: an out-of-square Victorian reveal, a bay window carrying load from above, a lintel that needs replacing, or a large unit that has to physically reach a rear elevation in a terrace.
You get a 10-year manufacturing guarantee on the product, a 15-year guarantee on the sealed units against misting and seal failure, and a 12-month workmanship guarantee on the installation.
Misted and failed units
Condensation between the panes means the seal has failed and the cavity is no longer sealed against moisture. It is the most common fault on older double glazing and it is not repairable by drilling and draining, whatever you have been offered: once the desiccant is saturated and the gas is gone, the unit is doing very little.
The good news is that the unit and the frame are separate things. If the frames are sound and the hardware works, replacing just the sealed units is usually the right repair, and it is a fraction of the cost of new windows. If the frames are draughty, the hardware is worn, or the windows predate thermal breaks and warm-edge spacers, replacing the unit alone is spending money on the smaller half of the problem. We will tell you which it is at survey.
Three symptoms worth distinguishing. Misting between the panes is seal failure. Condensation on the inside face in winter is room humidity plus a cold surface, often a sign of blocked or missing background ventilation rather than a faulty window. Condensation on the outside face on a clear morning is the opposite: it means the glass is losing almost no heat, and it is a sign the unit is working well.
Double glazing prices
Indicative fitted prices per window for London, including VAT, frame, sealed units, fitting, removal and disposal, and FENSA self-certification.
| Window type | Frame | From | To |
|---|---|---|---|
| Casement | uPVC | £550 | £1,200 |
| Casement | Aluminium | £900 | £1,800 |
| Flush casement | uPVC | £650 | £1,400 |
| Flush casement | Aluminium | £1,100 | £2,000 |
| Tilt & turn | uPVC | £700 | £1,500 |
| Tilt & turn | Aluminium | £1,100 | £2,200 |
| Sliding sash | uPVC | £900 | £1,800 |
| Bay window, 3 piece | uPVC | £1,800 | £3,500 |
| Bay window, 5 piece | uPVC | £2,500 | £4,500 |
| Whole house, 8 windows | uPVC | £5,500 | £10,000 |
| Whole house, 8 windows | Aluminium | £8,500 | £18,000 |
Triple glazing, acoustic and solar-control units and dual-colour finishes sit above these ranges. See the cost of new windows in London for a component-level breakdown.
Frequently asked
How much does double glazing cost in London?
As an indicative fitted range including VAT and FENSA certification: a uPVC casement runs £550 to £1,200 and an aluminium casement £900 to £1,800. A three-piece bay is £1,800 to £3,500. A whole house of eight windows runs £5,500 to £10,000 in uPVC or £8,500 to £18,000 in aluminium. Size, glass specification, colour and access all move the figure, so treat these as a starting point.
What is a 4-20-4 unit?
It is a 28mm double glazed unit: a 4mm outer pane, a 20mm cavity, and a 4mm inner pane. It is our standard build, filled to 90% argon, with a soft-coat low-emissivity coating on the cavity face and a warm-edge spacer bar. It reaches centre-pane U-values from 1.0 W/m²K and a sound reduction of 31 dB Rw. Twenty millimetres is close to the optimum cavity width for argon; going much wider allows convection to start working against you.
What thickness are triple glazed units?
The standard triple build is 36mm, made up as 4-12-4-12-4. Narrower versions exist to fit frames designed for double glazing: 32mm as 4-10-4-10-4, and 28mm as 4-8-4-8-4. They fit, but the shallower cavities cost you thermal performance, so a 28mm triple unit is a retrofit compromise rather than an upgrade over a good 28mm double unit. Units above 36mm are used in aluminium systems with deeper rebates.
Is argon fill worth it?
Yes, and it is standard on every sealed unit we fit rather than an upsell. Argon is denser and less conductive than air, so it slows heat transfer across the cavity and improves the U-value measurably. Krypton performs better still in narrow cavities, which is why it appears in slim heritage units, but it is considerably more expensive and rarely justified in a standard 20mm cavity.
What U-value do double glazed windows achieve?
Our standard unit reaches centre-pane U-values from 1.0 W/m²K. Whole-window U-values, which are what the regulations work to, depend on the frame as well as the glass: from 1.1 W/m²K on our double-glazed windows, down to 0.93 W/m²K on a uPVC tilt and turn and 0.84 W/m²K on the aluminium tilt and turn. Building Regulations require a maximum of 1.4 W/m²K or Window Energy Rating Band B on a replacement window.
What does A-rated and A+ rated mean?
Window Energy Rating is a whole-window scale running from A++ down to E. It combines heat loss through the window, useful solar gain, and air leakage into a single band, which makes it a convenient shorthand but a less precise measure than the U-value. Building control works to the U-value, or accepts WER Band B as an alternative. Our double glazing is available up to A+, and A++ with triple glazing.
Does double glazing reduce noise?
Some, and our standard unit is rated at 31 dB Rw. But sound reduction comes from mass and from asymmetry between the panes, not from the number of panes, so the effective upgrade is an acoustic laminated pane of a different thickness to its partner rather than more glass. Triple glazing is often sold for noise and usually underperforms a properly specified acoustic double unit against traffic.
My double glazing has misted between the panes. Can it be fixed?
Not meaningfully. Misting means the perimeter seal has failed, moisture is inside the cavity and the desiccant is saturated, and the gas fill has gone. Drilling and draining a unit does not restore any of that. The unit needs replacing. If the frames and hardware are still sound, replacing sealed units alone is a sensible repair; if the frames are draughty or predate warm-edge spacers, replacement windows are usually the better spend.
Why do I get condensation on the outside of my windows?
Because they are working. External condensation forms when the outer pane is cold, which happens precisely when very little heat is escaping through the glass to warm it. It is most common on clear, still autumn mornings on new low-emissivity units, and it clears as the sun comes up. Condensation on the inside face is a different matter and usually points to room humidity plus insufficient background ventilation.
Do I need planning permission for double glazing?
Usually not. On a house that is not listed and not covered by an Article 4 Direction, replacing single glazing with double glazing in the same openings is not development and needs no application. Listed buildings need Listed Building Consent whatever the glazing, and flats have no householder permitted development rights. Building Regulations apply in every case.
Can I get double glazing in a listed building or a period property?
Often the Building Regulations themselves provide the route. Where windows cannot meet the normal standard because the character of the building has to be maintained, Approved Document L accepts either a centre-pane U-value no worse than 1.2 W/m²K or single glazing supplemented with low-emissivity secondary glazing. In practice that means slim heritage units where they are permitted, and secondary glazing where replacement is refused.
Do you supply sealed units without fitting?
Yes, made to your surveyed sizes, with the toughening, coating and cavity specification confirmed in writing before manufacture. Most people are better served by supply and fit, because the installation, sealing and Building Regulations certification are where the performance and the paperwork actually come from, but if you have a builder or joiner doing the frames we can supply the glass to a cutting list.
Where we supply and fit across London
We survey, supply and install with our own teams across west, south-west and central London and into Surrey, including Richmond, Twickenham, Kingston, Chiswick, Ealing, Hounslow, Putney, Wimbledon, Fulham, Hammersmith and Teddington. The full list is on areas we cover.
Related: uPVC windows, aluminium windows, double versus triple glazing, window U-values explained and secondary glazing.
