The glazing trade calls a double-glazed panel a sealed unit or insulating glass unit (IGU). It’s the glass part of a double-glazed window: two panes held apart by a spacer bar around the edge, with the gap filled with air or an insulating gas such as argon, and the whole edge sealed to keep moisture out. In most modern units one pane also has an invisible low-emissivity (low-E) coating that reflects heat back into the room. The unit sits inside the window frame but is a separate component, so if it fails it can usually be replaced without replacing the window.
The parts of a double-glazed unit
| Part | What it does | What to ask about |
|---|---|---|
| Two panes of glass | The inner and outer layers. The Glass and Glazing Federation (GGF) says domestic windows usually use glass 4mm to 6mm thick. | Whether either pane needs to be safety glass (toughened or laminated), and whether different thicknesses would help with noise. |
| Spacer bar | Runs around the edge and keeps the panes apart. The GGF says spacers are made of metal or polymer and can contain a drying agent (desiccant) that deals with moisture trapped between the panes. | “Warm edge” spacers. The Energy Saving Trust says spacers containing little or no metal are generally found in more efficient windows. |
| Cavity and gas fill | The sealed gap between the panes. The GGF says it holds air or a gas such as argon, krypton or xenon. The Energy Saving Trust says argon is more insulating than air. | Which gas is used, and how wide the cavity is. |
| Low-E coating | A thin metal oxide coating on one of the internal glass surfaces. It reflects heat back into the home but still lets daylight in. The Energy Saving Trust calls low-E the most energy-efficient type of glass for double and triple glazing. | Which coating it is and which surface it’s on. |
| Edge seal | Seals the unit so the gas stays in and moisture stays out. | Whether the unit is made to BS EN 1279, the standard series for insulating glass units. |
BSI says meeting BS EN 1279 indicates that a unit’s visual, energy, acoustic and safety properties do not change significantly over time.
How a sealed unit is specified
Glaziers describe a unit pane by pane, from outside to inside. The four glass surfaces are numbered too. In Saint-Gobain Glass’s description, surface 1 faces outdoors, surfaces 2 and 3 face each other across the cavity, and surface 4 faces into the room. Here is one example from Saint-Gobain’s published product data:
- 4mm clear outer pane
- 16mm cavity filled with 90% argon
- 4mm inner pane with a low-E coating on surface 3
That’s usually written as 4/16/4, which makes the unit 24mm thick overall. For this build-up with its PLANITHERM NEO coating, Saint-Gobain gives a centre-pane U-value of 1.0 W/m²K (calculated to EN 673), a solar factor (g-value) of 0.54 and light transmission of 73%. It says the figures were correct on 21 October 2026. We’ve used it only to show how a specification reads; it isn’t a recommendation. Other glass makers publish similar data for their own products.
A written quote for a sealed unit should cover:
- Size and overall thickness. Units are made to order. The GGF says a replacement needs exact measurements so the unit can be made to that size, and the thickness has to suit your frame.
- Glass for each pane. For example clear, toughened, laminated, obscure (patterned) or solar control.
- Coating. Which low-E product is used, and on which surface.
- Gas fill and spacer. Air or argon, and whether the spacer is a warm edge type.
- Performance figures. The centre-pane U-value and g-value. If noise is the problem, ask for a sound reduction figure. The GGF says any noise reduction claim should be backed by test evidence.
Centre-pane U-value or whole-window U-value?
A U-value shows how easily heat passes through; lower is better. Approved Document L defines the centre-pane U-value as the figure for the central area of the glazing unit, “making no allowance for edge spacers or the window frame”. A whole-window U-value includes the frame and edges, so it’s a different number. Don’t compare one with the other.
When a complete window is replaced in England, Approved Document L sets a limit of 1.4 W/m²K for the whole window, or a Window Energy Rating of band B or better. If that can’t be met because the building’s character has to be kept, the guidance allows a centre-pane U-value of 1.2 W/m²K instead. The British Fenestration Rating Council (BFRC) energy rating also judges the whole window, combining heat loss, solar gain and air leakage. Our guide to window U-values explains these figures in more detail.
Glass options inside the unit
Safety glass
Approved Document K (England) lists “critical locations” where glazing must break safely, be robust, be in small panes or be permanently protected. The Planning Portal summarises them as:
- any glazed panel up to 800mm above floor level
- door side panels within 300mm of the door edge, up to 1,500mm above floor level
Approved Document K’s diagram also includes glass in the doors themselves. “Break safely” is defined by impact test classes in BS EN 12600 and BS 6206. Toughened or laminated glass is the usual answer, and either can be one pane of a sealed unit.
Noise
The GGF says thicker glass usually cuts more noise. Panes of different thicknesses, such as 4mm and 6mm, reduce noise across a wider range of frequencies than two identical panes, and a larger gap between the panes also helps. Acoustic laminated glass is available too. The GGF adds that gaps around the frame, open trickle vents and noise coming through walls and roofs can all limit how much difference new glass makes.
Solar control and appearance
The GGF says some low-E glass lets in a lot of the sun’s heat, which can cause overheating in large glazed areas, and suggests considering solar control glass for areas such as south-facing conservatories. It also notes that low-E coatings can differ slightly in colour from batch to batch. That can show when a single unit is replaced next to older ones.
When a sealed unit fails
The Energy Saving Trust says that condensation between the panes “likely means that the double glazing is letting air in”, which reduces its effectiveness. It also says the glazed unit can be replaced rather than the whole window. Mist on the room side of the glass, or on the outside, is a different issue. See our guide to the signs of double glazing failure.
A unit’s insulating performance can also fade without visible misting. The BFRC offers on-site tests that measure the argon fill level, the emissivity of the low-E coating and the dew point inside the unit. It describes them as a way to check how ageing units have kept their performance.
Replacing only the glass doesn’t trigger the energy-efficiency rules. Approved Document L says that replacing glazing, or a window in its existing frame, “is not providing a controlled fitting” and so doesn’t have to meet the energy requirements. FENSA says its certificates don’t cover repairs where the frame isn’t replaced. It’s still worth asking for a low-E, gas-filled unit with a warm edge spacer so the new glass is at least as good as the old, and for safety glass in any critical location. Our guide to double glazing panel replacement covers typical costs and the fitting process.
Before you order a replacement unit
- Check your paperwork. The installer’s guarantee may cover failed units. FENSA says installations completed after 2014 may also come with a separate insurance-backed guarantee that covers the installer’s written guarantee if the firm stops trading.
- Ask for the full specification in writing: size, thickness, glass for each pane, coating, gas fill and spacer type.
- Ask whether the unit is made to BS EN 1279 and how long its own guarantee lasts.
- Check where the unit sits. If it’s in a door, beside a door or low in a window, ask for safety glass.
Frequently asked questions
Is a double-glazed panel the same as a double-glazed window?
No. The panel is the sealed glass unit. The window is the frame, opening sashes, seals and hardware with one or more units fitted. See our guide to double-glazed windows.
Can I replace just the glass panel?
Usually, yes, if the frame is sound. The Energy Saving Trust and the GGF both say a failed sealed unit can be replaced without replacing the whole window.
What does 4/16/4 mean?
It’s the build-up in millimetres, from outside to inside: a 4mm pane, a 16mm cavity and a 4mm pane, 24mm in total.
Do I need Building Regulations approval to replace a sealed unit?
Not for energy efficiency. Approved Document L says replacing glazing in an existing frame isn’t a controlled fitting. A complete replacement window is different: use a registered installer or apply to building control.
Sources
- Energy efficient windows and doors, Energy Saving Trust, last updated 20 May 2026. Accessed 12 November 2026.
- Energy efficient glazing: a useful guide, Glass and Glazing Federation (MyGlazing), undated. Accessed 12 November 2026.
- Glazing and noise reduction in homes, Glass and Glazing Federation (MyGlazing), undated. Accessed 12 November 2026.
- Low-E (Low Emissivity) Glass and Thermal Efficiency, Glass and Glazing Federation (MyGlazing), undated. Accessed 12 November 2026.
- When to repair or replace your windows, Glass and Glazing Federation (MyGlazing), undated. Accessed 12 November 2026.
- BS EN 1279-1:2018 Glass in building. Insulating glass units, BSI Knowledge, standard published 2018. Accessed 12 November 2026.
- PLANITHERM NEO product page, Saint-Gobain Glass UK, performance data stated as correct on 21 October 2026. Accessed 12 November 2026.
- Energy Ratings, British Fenestration Rating Council, page updated 22 September 2026. Accessed 12 November 2026.
- On-Site IGU Testing, British Fenestration Rating Council, page updated 9 July 2025. Accessed 12 November 2026.
- Approved Document L, Volume 1: Dwellings, 2021 edition incorporating 2023 amendments (England), paragraphs 4.10 and 10.6, Table 4.2 and glossary, MHCLG (GOV.UK), page last updated 2 February 2023. Accessed 12 November 2026.
- Approved Document K: protection from falling, collision and impact, 2013 edition (England), section 5, GOV.UK. Accessed 12 November 2026.
- Doors and windows: Building Regulations, Planning Portal, undated. Accessed 12 November 2026.
- Homeowners, FENSA, undated. Accessed 12 November 2026.
Last reviewed: 12 November 2026.
Costs and figures in our guides are indicative and change over time. Always get written quotes for your property. Spotted an error? Tell us.
