The gap between glass panes in UK double glazing is not a single standard size
The most common cavity width in UK replacement double glazing installed since 2022 is 20 mm or 28 mm, according to the Glass and Glazing Federation (GGF) 2025 technical guidance (GGF technical fact sheet “Double Glazed Sealed Units – Common Dimensions”). Standard sealed units (the glass part) typically have an overall thickness of 24 mm (4 mm glass / 16 mm gap / 4 mm glass) or 28 mm (4 mm glass / 20 mm gap / 4 mm glass). The frame depth, the part you see from the window sill, is almost never the same as the glass-unit thickness; uPVC frames commonly range from 58 mm to 70 mm. The misconception that double glazing has “one standard thickness” arises because many replacement windows use a 28 mm sealed unit, but this is a common choice, not a mandatory standard (FENSA “Replacement Windows – Technical Standards” guide, 2025).
No, double glazing has no single standard thickness. Most UK sealed units are either 24 mm overall (4 mm glass / 16 mm gap / 4 mm glass) or 28 mm overall (4 mm glass / 20 mm gap / 4 mm glass). The cavity width, typically 20 mm or 28 mm, drives energy efficiency.
- No single standard thickness exists for UK double glazing units.
- Most common sealed units are 24 mm or 28 mm overall.
- Cavity width is 20 mm or 28 mm in most replacement windows.
- A 20 mm argon gap gives centre-pane U-value around 1.2 W/m²K.
- Cavities wider than 28 mm can reduce insulation due to convection.
- The gap between glass panes in UK double glazing is not a single standard size
- Why the cavity width matters most for energy efficiency
- Quick numbers common double glazing thicknesses in UK homes
- The direct answer no, double glazing does not have a single standard thickness
- How to check if a new double-glazing unit will fit your existing frame
- Eligibility, certification, and how to verify a double-glazing installer
Why the cavity width matters most for energy efficiency
The thermal performance of a double-glazed unit depends primarily on the width of the air or gas-filled gap between the panes, not the total glass thickness. A 20 mm argon-filled gap typically achieves a centre-pane U-value of approximately 1.2 W/m²K; a 28 mm gap can reach 1.0 W/m²K or lower, per DESNZ’s “Standard Assessment Procedure (SAP) 2025” data tables (DESNZ SAP 2025 Table U2). Gaps wider than 28 mm (e.g. 32 mm) can cause convection currents inside the unit that actually reduce insulation, as noted in the Building Research Establishment (BRE) report “Thermal Performance of Glazing” (2024) (BRE report “Thermal Performance of Glazing”, 2024). The Building Regulations Part L (2025 edition) does not prescribe a specific cavity width, but requires a whole-window U-value of ≤ 1.4 W/m²K for replacement windows in England (GOV.UK “Approved Document L”, 2025).
Quick numbers common double glazing thicknesses in UK homes
| Glass-unit type | Typical overall thickness | Typical cavity width | Common frame depth | Typical centre-pane U-value |
|---|---|---|---|---|
| Standard retrofit (4/16/4) | 24 mm | 16 mm | 58–70 mm | 1.4 W/m²K (air) |
| Standard replacement (4/20/4) | 28 mm | 20 mm | 58–70 mm | 1.2 W/m²K (argon) |
| High-performance (4/28/4) | 36 mm | 28 mm | 70 mm | 1.0 W/m²K (argon) |
| Triple glazing (4/12/4/12/4) | 36 mm | 12 mm x2 | 70–90 mm | 0.7–0.8 W/m²K (argon) |
Source: GGF sealed-unit dimension guide; DESNZ SAP 2025 Table U2; FENSA “Window Energy Ratings” data (GGF sealed-unit dimension guide; DESNZ SAP 2025 Table U2; FENSA “Window Energy Ratings” data).
The direct answer no, double glazing does not have a single standard thickness
The phrase “standard double glazing” is a shorthand used by installers, but it refers to a common configuration (typically a 28 mm sealed unit with a 20 mm argon-filled gap), not a regulated or mandatory dimension. Building Regulations (Part L, 2025) set a performance target for the whole window, not a fixed thickness, a 24 mm unit can meet the target if the frame is well-insulated and the glass has a low-emissivity coating (GOV.UK “Approved Document L: Conservation of fuel and power”, 2025). The Energy Savings Trust (EST) “Home Insulation Guide” (2025) explicitly states: “There is no single standard thickness for double glazing; the right unit depends on your frame, budget, and target U-value” (Energy Saving Trust “Home Insulation Guide”, 2025). Homeowners should ask for the centre-pane U-value (or whole-window U-value) rather than specifying a thickness in millimetres.
How to check if a new double-glazing unit will fit your existing frame
Replacement sealed units must match the original frame’s rebate depth, the space inside the frame where the glass sits, which is typically 18–28 mm for uPVC windows installed after 2000. If your current frame has a rebate depth of only 18 mm, the maximum sealed-unit thickness you can fit is 24 mm (4/16/4), not a 28 mm unit (GGF “Replacement Sealed Units – Sizing Guidelines”). The Window Energy Rating (WER) label on a new unit includes the overall thickness in millimetres, check this against the frame manufacturer’s specification. For older timber or metal frames (pre-1990), the rebate may be as shallow as 12–15 mm, limiting you to a 20 mm sealed unit or requiring a frame adaptor (FENSA “Window Frame Compatibility Guide”, 2025).
How to measure your window rebate depth before ordering replacement units
Eligibility, certification, and how to verify a double-glazing installer
All replacement double glazing in England and Wales must be installed by a company registered with FENSA, CERTASS, or a similar competent-person scheme, or be signed off by a building-control body, per GOV.UK “Building Regulations: Competent Person Schemes” (2025) (GOV.UK “Building Regulations: Competent Person Schemes”, 2025). The installer must provide a FENSA certificate (or equivalent) for each window, which confirms the unit thickness, U-value, and compliance with Part L. Check the installer’s MCS registration only if you are installing solar-control glass or integrated photovoltaic glazing; for standard double glazing, MCS is not required. For work done under a government grant (e.g. Great British Insulation Scheme), the installer must be TrustMark-registered and the product must appear on the “Product Characteristics Database” managed by DESNZ (DESNZ “Great British Insulation Scheme: installer requirements”, 2025).
What to do if your double glazing installer refuses to provide a FENSA certificate
Frequently Asked Questions
There is no single standard thickness. According to the Glass and Glazing Federation (GGF) 2025 guidance, the most common sealed units are 24 mm overall (4 mm glass / 16 mm gap / 4 mm glass) or 28 mm overall (4 mm glass / 20 mm gap / 4 mm glass).
Yes, 28 mm is a very common sealed unit size, but it is not a mandatory standard. The GGF notes it is a widely used choice for replacement windows in the UK.
The required thickness depends on your frame. Building Regulations Part L (2025 edition) requires a whole-window U-value of ≤ 1.4 W/m²K, which often means a 20 mm or 28 mm argon-filled cavity.
Yes, 24 mm (4 mm glass / 16 mm gap / 4 mm glass) is a standard sealed unit size, common in older or budget-friendly installations per GGF technical guidance.
Yes, but cavity width matters more than total thickness. A 20 mm or 28 mm gap with argon fill improves thermal and acoustic performance, per BRE report 2024.
