Double glazing is two panes of glass bonded around a hollow spacer bar and hermetically sealed at the factory, with a gas-filled cavity trapped between them. The trade calls that assembly a sealed unit, or an insulated glazing unit (IGU), and it is the whole of what the phrase means. Everything else about a window — the frame material, the opening style, the handles — sits around it. This page covers what is inside a unit, the physics that makes the cavity work, what a double glazed window costs in 2026 and how to tell when yours has stopped doing its job. Any term here that reads as jargon has a one-line entry in the Double Glazing Glossary, and you can price your own house in the double glazing cost calculator.
What is inside a double glazed unit
A sealed unit is a manufactured component, not something assembled on site. Two panes of float glass, 4 mm each on a standard unit, are held apart by a hollow spacer bar running right around the perimeter. The spacer is packed with a desiccant, typically molecular sieve or silica beads, which absorbs the trace moisture sealed in at manufacture and is the reason the cavity stays optically clear on a cold morning. The perimeter is then bonded with an edge seal that keeps the gas in and the weather out. The cavity itself is filled with argon rather than air, and one of the two internal glass faces carries a Low-E coating. Every part of that list except the glass is invisible once the window is in the wall, which is exactly why a sealed unit is easy to under-specify in a quote.
| Component | What it is | What it does |
|---|---|---|
| Outer and inner panes | Float glass, 4 mm each on a standard unit | The outer pane takes the weather and runs cold; the inner pane runs close to room temperature |
| Sealed cavity | A gap of roughly 12 to 16 mm between the panes | The insulating layer. Argon performs best across 12 to 16 mm; krypton suits a narrow 9 to 12 mm gap instead |
| Argon fill | An inert gas at about 0.016 W/m·K, against roughly 0.0262 for air | Conducts heat around 35% less readily than the air it replaces, and is about six times denser so it circulates less |
| Spacer bar | A hollow perimeter bar holding a desiccant | Sets the cavity width and keeps it dry. A warm-edge spacer replaces the old aluminium bar, which was a thermal bridge |
| Edge seal | The bond around the perimeter of the unit | Holds the argon in and moisture out. When it degrades the unit mists and the gas fill is lost |
| Low-E coating | A metal-oxide layer on one internal glass face | Reflects long-wave heat back into the room; cuts the centre-pane U-value by around 40% |
Gas conductivity figures and cavity widths from DGCC 2026 research (Mumford Wood, KLG Glass). The 40% Low-E figure is the DGCC 2026 research value for a soft-coat product.
It is worth separating two things the phrase "double glazed window" runs together. The sealed unit is the glass component. The window is that unit plus the frame it is glazed into, in uPVC, aluminium or timber. They do not share a life expectancy: a sealed unit typically lasts 15 to 20 years and a uPVC frame 20 to 25, with aluminium at 30 to 40 and maintained timber at 30 to 60. The glass therefore reaches the end of its life first, which is the single fact behind most repair decisions and is covered in full in our guide to how long double glazing lasts.
Double glazing is sold on the frame and bought on the glass. The frame decides the price and the look; the sealed unit inside decides whether the window meets Part L. When we review a quote, the unit specification — Low-E, argon, warm-edge spacer — is the part most often left off the paperwork, and it is the part that does the work.
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How double glazing actually keeps heat in
A window loses heat three ways, and a sealed unit is engineered against all three. Conduction is heat travelling through solid material; glass conducts readily, so a single pane offers almost no resistance and two panes with a gap between them break the path. Convection is the gas inside the cavity circulating, collecting heat at the warm inner pane and dumping it at the cold outer one. That is why the cavity width is engineered rather than arbitrary: argon works best across a gap of roughly 12 to 16 mm, and opening it wider lets convection currents form and gives the gain back. Krypton insulates better in a narrow cavity of about 9 to 12 mm, which is why it appears in triple glazing and slim heritage units rather than ordinary double glazing; argon versus krypton gas works through when that upgrade is worth paying for.
Radiation is heat leaving the warm pane as long-wave infrared, and neither an air gap nor a gas fill stops it. That is the job of the Low-E coating: a metal-oxide layer thin enough to see straight through, which reflects that heat back into the room and cuts the centre-pane U-value by around 40%. The gas fill handles the other two mechanisms. Argon conducts heat at about 0.016 W/m·K against roughly 0.0262 for air, around 35% less readily, and it is about six times denser than air so it circulates more sluggishly in the cavity. The last piece is the warm-edge spacer, which replaces the aluminium bar that used to conduct heat straight across the edge of every unit.
All of that is reported as one number: the U-value, in watts per square metre per kelvin, where lower is better. It is the figure Building Control checks, and the one worth asking for in writing as the whole-window Uw rather than the flattering centre-pane Ug, as our window U-values guide explains.
| Glazing type | Typical U-value | Notes |
|---|---|---|
| Single glazing | ~4.8-5.8 W/m²K | Old, uninsulated. Loses the most heat. |
| Old double glazing (pre-2002) | ~2.8-3.2 W/m²K | No low-E coating or gas fill. |
| Modern A-rated double glazing | 1.0-1.4 W/m²K | Low-E glass, argon fill, warm-edge spacer. Meets Part L. |
| Triple glazing | 0.6-1.0 W/m²K | Best performance, around 0.8 typical. Heavier and pricier. |
Source: DGCC 2026 research, drawn from UK installer and manufacturer technical pages.
Since 15 June 2023, Part L has required a replacement window to reach a whole-window U-value of 1.4 W/m²K or lower, or Window Energy Rating band B or better as the alternative compliance route. A properly specified modern unit clears that; single glazing misses it by a factor of three to four. Our guide to Building Regulations Part L covers how the rule is enforced and who signs it off. On the bill, the Energy Saving Trust puts the saving from replacing single glazing with A-rated double glazing at around £85 a year in Great Britain, and £140 a year in Northern Ireland, on a typical semi-detached house.
What double glazing costs in 2026
Fully fitted, a uPVC casement runs £400 to £1,000 per window, averaging around £650. Aluminium costs £550 to £1,300, a premium of 30 to 40% over uPVC for slimmer sightlines and a longer frame life, and timber £900 to £2,000, a premium of 60 to 110%. Across a whole house in uPVC that comes to £1,800 to £5,000 for a four-window flat, £4,000 to £10,000 for a mid-terrace with eight to ten windows, and £5,000 to £12,000 for a semi-detached with ten to twelve. The average double glazing cost guide breaks those totals down, and cost by house size gives the low, average and high figure for each property type.
Between 40 and 55% of a fitted price is the sealed unit and frame themselves; the rest is labour, removal, disposal and the certificate. A supply-only basic uPVC casement starts from around £160, with fitting labour adding roughly £300 to £600 a window on top, which is the gap our cost per window guide works through in detail. Regional variation is wider than most people expect: the same job is priced about 20% above the UK average in London and about 13% below it in Wales.
How it differs from single, triple and secondary glazing
Single glazing is one pane with no cavity at all, at 4.8 to 5.8 W/m²K, and it cannot be fitted as a replacement window because it fails Part L outright. Old pre-2002 double glazing is still double glazing, but built without a Low-E coating, an argon fill or a warm-edge spacer, so it lands at 2.8 to 3.2 W/m²K and also falls short of the current floor. Triple glazing adds a third pane and a second cavity, reaching 0.6 to 1.0 W/m²K with 0.8 typical, for roughly 10 to 20% more cost and a further 15 to 20% off the heat loss. Our single glazed vs double glazed comparison sets the three tiers side by side, and double versus triple glazing works the third-pane decision through for a typical house.
Secondary glazing is the one that is not double glazing at all, despite involving two panes. It is a separate second window fitted to the room side of the existing one, with a wide open air gap rather than a sealed gas-filled cavity, and the original window stays in place unaltered. It costs £300 to £900 a window fitted, and Historic England puts its U-value at around 2.5 W/m²K, or below 2.0 with Low-E glass. That is well behind a modern sealed unit thermally, but its reversibility is why it is the recommended route in listed buildings — secondary versus double glazing compares the two properly.
How to tell if your double glazing has failed
There is one unmistakable symptom: a foggy or misty patch between the panes that you cannot wipe off from either side, cycling between cloudy when cold and clear when warm, and eventually leaving permanent staining on the inner glass faces. That means the edge seal has degraded, outside air and moisture have entered, the desiccant in the spacer bar is saturated and the argon fill has escaped. It cannot be reliably repaired; drilling and drying the unit restores neither the desiccant nor the airtight seal. A failed unit raises heat loss through that window by around 50 to 70% against a working one.
It is not an emergency. Act within three to six months if bills are rising or several windows are affected, and within six to twelve months for a single window in a low-use room, and check the guarantee first because sealed units often carry a ten-year warranty. The fix is normally glass only: replacing a misted sealed unit costs £55 to £145 for a standard pane with the frame left in place, against £100 to £850 to replace a whole window unit and £250 to £750 to replace a uPVC frame. Resealing a window is £50 to £100, a handle £45 to £150 and hinges £100 to £250. Price your own symptom in the double glazing repair cost calculator, work out which way to go in repair versus replace, or identify the fault first with the window fault checker.
Two symptoms that look like failure and are not. Condensation on the inside face of the glass is room humidity meeting a cold surface, a ventilation problem rather than a window fault. Condensation on the outside face is the opposite of a fault: the outer pane is staying cold because so little heat is escaping through the unit. Our guide to condensation on double glazing separates all three cases.
The commonest misunderstanding in the quotes we check is that a misted window needs a new window. It usually needs new glass. The sealed unit lasts 15 to 20 years and the uPVC frame 20 to 25, so the glass reaches the end of its life first, and swapping it costs £55 to £145 a pane against £400 to £1,000 for the whole window.
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Frequently asked questions
Double glazing is a window made from two panes of glass sealed together with a gap between them. The gap is filled with argon rather than air and is what holds the heat in. Compared with a single pane it cuts heat loss through the glass by around 50%, taking a window from a whole-window U-value of 4.8 to 5.8 W/m²K down to 1.0 to 1.4 W/m²K.
Two panes of float glass, a hollow spacer bar around the perimeter that holds them apart and carries a desiccant, an edge seal bonding the whole perimeter, an argon fill in the cavity between the panes, and a Low-E metal-oxide coating on one of the internal glass faces. That assembly is the sealed unit. The window is the sealed unit plus the frame it is glazed into, in uPVC, aluminium or timber.
Not quite. The sealed unit, also called an insulated glazing unit or IGU, is the glass component: the two panes, the spacer bar and the sealed cavity. A double glazed window is that unit plus its frame. The distinction matters for repairs, because a sealed unit typically lasts 15 to 20 years and a uPVC frame 20 to 25, so the glass usually fails first and can be replaced on its own for £55 to £145 a standard pane.
Look at the edge of the glass in good light, where the frame meets the pane. A double glazed window shows two separate sheets of glass with a spacer bar between them, and the bar is often stamped with the manufacturer and the unit specification. A single glazed window shows one sheet. If the windows went in after 2002 and the work was certified, the FENSA or Certass certificate for the installation also records the product and its rating.
A fully fitted uPVC casement costs £400 to £1,000 per window, averaging around £650. Aluminium runs £550 to £1,300, a premium of 30 to 40% over uPVC, and timber £900 to £2,000, a premium of 60 to 110%. Whole-house totals in uPVC run £1,800 to £5,000 for a four-window flat and £5,000 to £12,000 for a ten to twelve window semi-detached house.
The sealed unit lasts 15 to 20 years before the edge seal degrades and the unit mists. The frame outlives it: 20 to 25 years for uPVC, 30 to 40 for aluminium and 30 to 60 for timber if it is maintained. Because the glass reaches the end of its life first, a misted window is usually a glass replacement rather than a whole-window replacement.
It reduces condensation on the room side of the glass, because the inner pane runs warmer and is less likely to fall below the dew point of the air in the room. Condensation on the outside of a double glazed window is a sign it is working, as the outer pane stays cold when little heat is escaping. Condensation between the panes is different: it means the edge seal has failed and the desiccant in the spacer bar is saturated.
No. Secondary glazing is a separate second window fitted to the room side of the existing one, leaving the original in place, with a wide open air gap rather than a sealed gas-filled cavity. It costs £300 to £900 a window fitted against £400 to £1,000 for replacement double glazing, and Historic England puts its U-value at around 2.5 W/m²K, or below 2.0 with Low-E glass, against 1.0 to 1.4 for modern A-rated double glazing.