Double Glazing Glass Types, Thickness, and What's Right for Your Home

Double glazing glass is not a single product – it is a broad family of glass types, coatings, and configurations, each engineered for a specific purpose. Choosing the right glass for every window in your home affects energy efficiency, safety, security, noise levels, privacy, and maintenance. Yet most homeowners order replacement sealed units without fully understanding the options available to them, often because the differences are never clearly explained. This guide changes that. It covers every type of double glazing glass used in UK homes today, from basic float through to fire-rated and self-cleaning variants, explaining how each is made, where it should be used, what standards apply, and how the various types combine inside a sealed unit to deliver the performance you need.

Whether you are replacing a single misted unit or specifying glazing for an entire new build, understanding the types of double glazing glass available in the United Kingdom puts you in a far stronger position to make the right choice – and avoid paying for features you do not need or, worse, missing features you do.

Float Glass – The Foundation of All Glazing

Float glass is the starting point for virtually every other type of glass used in double glazing. It is manufactured by floating molten glass at approximately 1,100 °C onto a bath of molten tin, where it spreads into a perfectly flat, uniform sheet with parallel surfaces and consistent thickness. The process – invented by Sir Alastair Pilkington in 1952 and still used worldwide – produces glass that is optically clear, free from distortion, and available in a range of standard thicknesses: 3 mm, 4 mm, 6 mm, 8 mm, 10 mm, and 12 mm.

In the context of double glazing, 4 mm float glass is the most commonly used thickness. A standard sealed unit configuration in the UK is 4-16-4 – two panes of 4 mm float separated by a 16 mm cavity – giving a total unit thickness of 24 mm. Thicker 6 mm glass is sometimes used for larger panes where additional rigidity and wind-load resistance are needed, or as part of an acoustic configuration where asymmetric pane thicknesses improve sound insulation.

On its own, standard clear float glass has relatively modest insulating properties. A 4-16-4 air-filled unit with two panes of clear float achieves a centre-pane U-value of approximately 2.8 W/m²K – acceptable 20 years ago, but well below the performance expected by modern UK Building Regulations. Float glass is, however, the base material from which toughened, laminated, Low-E, acoustic, and every other specialist glass type is produced. Think of it as the raw ingredient – essential, but rarely used alone in a modern sealed unit.

Toughened Safety Glass

Toughened glass, also called tempered glass, is manufactured by heating standard float glass to approximately 620 °C and then rapidly cooling it with jets of air in a process known as thermal tempering. This creates a state of balanced internal stress: the outer surfaces are in compression and the core is in tension. The result is glass that is four to five times stronger than float glass of the same thickness, with significantly greater resistance to thermal stress and mechanical impact.

When toughened glass does break – whether from impact, extreme thermal shock, or deliberate force – it fractures into small, roughly cube-shaped granules rather than the large, sharp shards produced by standard annealed glass. These granules are far less likely to cause serious injury, which is why toughened glass is classified as a safety glass under BS EN 12150.

When is toughened glass required by UK Building Regulations? Approved Document N (England and Wales) mandates safety glass in what are termed "critical locations":

• Glazing in doors and door side panels within 300 mm of the door edge
• Any glazing with its lowest point below 800 mm from finished floor level
• Glazing in bathrooms, shower rooms, and other wet areas below 800 mm
• Internal glazed screens or partitions in domestic and commercial properties

All toughened glass used in sealed units must carry a permanent etch mark identifying the manufacturer, the standard (BS EN 12150), and the glass type. Because toughened glass cannot be cut or drilled after tempering – any attempt to do so will cause it to shatter immediately – it must be manufactured to the exact finished size, with all holes, notches, and edge treatments completed before the tempering process. Browse our full range of glass types to see toughened options available for your sealed units.

Laminated Glass

Laminated glass is made by bonding two or more panes of glass together with one or more interlayers of polyvinyl butyral (PVB) or ethylene-vinyl acetate (EVA) under heat and pressure. The interlayer is a tough, flexible plastic film – typically 0.38 mm thick per layer – that holds the glass together if it is broken. Instead of collapsing into fragments, a broken laminated pane remains intact in the frame with a characteristic "spider web" crack pattern, and the interlayer prevents pieces from falling free.

Laminated glass conforms to BS EN 14449 and offers several distinct advantages over toughened glass:

• Security. Because laminated glass stays in the frame when broken, it is far more resistant to forced entry than toughened glass, which shatters completely and leaves an open hole. A burglar can break through a toughened pane with a single blow, but penetrating a laminated pane – particularly one with multiple PVB interlayers – requires sustained effort, time, and noise, all of which deter opportunistic break-ins.
• Noise reduction. The PVB interlayer acts as a dampening layer that absorbs sound energy, particularly in the mid-frequency range where traffic and human speech are most prominent. Laminated glass typically provides 3 to 5 dB more sound reduction than monolithic glass of the same total thickness.
• UV protection. Standard PVB interlayers block over 99 per cent of UV radiation, significantly reducing fading and degradation of furniture, carpets, curtains, and artwork near windows.
• Overhead and balustrade use. UK Building Regulations require laminated glass – not toughened – for overhead glazing (roof lights, canopies, atriums) and certain balustrade applications, because it remains in place when broken rather than falling as fragments.

Toughened versus laminated – which should you choose? For standard doors and low-level glazing where the primary concern is impact safety, toughened glass is more common and generally more cost-effective. For security-sensitive locations, noise reduction, UV protection, and overhead applications, laminated glass is the superior choice. In many modern sealed units, the two are combined: a toughened outer pane for impact resistance and a laminated inner pane for security and acoustic performance.

Low-E (Low Emissivity) Glass

Low-E glass is the single most important upgrade you can make to a double glazed unit, and it is far and away the most popular glass type specified in UK replacement and new-build glazing. The "E" stands for emissivity – a measure of how readily a surface radiates heat. Standard float glass has a high emissivity of around 0.89, meaning it radiates the vast majority of long-wave infrared heat that strikes it. Low-E glass has a microscopically thin metallic oxide coating – usually tin oxide, silver, or a multi-layer stack of metallic oxides – that reduces the emissivity to as low as 0.03, reflecting up to 97 per cent of radiant heat back towards its source.

In a sealed unit, the Low-E coating is applied to what is technically called "surface 3" – the room-facing surface of the inner pane – or "surface 2" – the cavity-facing surface of the outer pane. When positioned on surface 3, the coating reflects heat radiated by furniture, people, and radiators back into the room, dramatically reducing the amount of heat lost through the window. The result: a standard 4-16-4 unit with a soft-coat Low-E pane and argon gas fill achieves a centre-pane U-value of approximately 1.1 W/m²K, compared to 2.8 W/m²K for plain float with air – a reduction in heat loss of over 60 per cent.

Hard-coat versus soft-coat Low-E. There are two manufacturing processes for Low-E coatings. Hard-coat (pyrolytic) Low-E is applied while the glass is still hot on the float line, fusing the coating permanently into the glass surface. It is extremely durable and can be handled, cut, and toughened after coating, making it versatile for manufacturers. However, hard-coat Low-E has a higher emissivity (typically 0.15 to 0.20) and is therefore less thermally efficient than soft-coat. Soft-coat (sputtered) Low-E is applied in a vacuum chamber after the float glass has been manufactured, depositing ultra-thin layers of silver and dielectric materials onto the surface. Soft-coat achieves much lower emissivity values (0.03 to 0.05) and correspondingly better U-values, but the coating is delicate and must be protected within the sealed unit cavity – it cannot be exposed to the elements or to cleaning.

In the UK market, soft-coat Low-E is overwhelmingly dominant in residential double glazing, and it is the type used in units rated A+ and A++ under the BFRC (British Fenestration Rating Council) scheme. When combined with argon gas and a warm-edge spacer bar, a soft-coat Low-E unit comfortably meets and exceeds the minimum thermal requirements of Building Regulations Part L. Visit our products page for Low-E sealed unit specifications and pricing.

Acoustic Glass

Acoustic glass is engineered specifically to reduce the transmission of sound through a window. Standard double glazed units provide a reasonable level of sound insulation – typically around 29 to 32 dB Rw (weighted sound reduction index) – but for properties on busy main roads, near railway lines, under flight paths, or in noisy city centres, dedicated acoustic glazing can make a transformative difference to comfort and quality of life.

The key principle behind acoustic performance in sealed units is asymmetry. When the two panes in a unit are the same thickness, they resonate at the same frequency, creating a "coincidence dip" where sound passes through more easily. By using panes of different thicknesses – and ideally different types – the coincidence frequencies are separated, and overall sound transmission is reduced across a wider range of frequencies.

A typical high-performance acoustic sealed unit might comprise a 6.8 mm acoustic laminate outer pane (two layers of 3 mm glass with a 0.8 mm acoustic PVB interlayer), a 16 mm argon-filled cavity, and a 4 mm float inner pane. This configuration achieves an Rw value of 39 to 42 dB – a substantial improvement over the 29 to 32 dB of a standard symmetric unit. In practice, that additional 10 dB represents a perceived halving in loudness to the human ear.

Products such as Pilkington Optiphon and similar acoustic laminates from UK manufacturers use specially formulated PVB interlayers with enhanced dampening properties tuned to the frequencies most associated with traffic, aircraft, and urban noise. For the best results, acoustic glazing should be combined with gas-filled units (argon or krypton), properly sealed frames, and acoustic trickle vents that allow ventilation without compromising sound insulation. Our supply and installation service can help you specify the right acoustic configuration for your property.

Obscure and Patterned Glass

Obscure glass – also called patterned or textured glass – is used wherever privacy is needed without sacrificing natural light. In UK double glazing, there is a well-established range of standard patterns, each offering a different level of obscurity. Suppliers typically rate obscurity on a scale of 1 to 5, where 1 is lightly textured and 5 is fully obscured.

The most commonly specified UK patterns include:

• Stippolyte – a fine, even texture with high obscurity (level 5). The most popular choice for bathrooms and WCs.
• Cotswold – an irregular, rippled surface (level 3–4). Widely used for front door side panels and bathroom windows in period-style properties.
• Arctic – a heavily frosted appearance (level 5). Similar obscurity to Stippolyte with a slightly different aesthetic.
• Contora – narrow vertical reeded lines (level 4). Popular for bathroom windows and office partitions.
• Flemish – a traditional leaded-light appearance (level 2–3). Decorative rather than heavily obscuring, often used in front doors.
• Autumn – a leaf-like organic texture (level 3). Used in decorative applications where moderate privacy is required.
• Digital – a geometric, contemporary pattern (level 3). A modern alternative to traditional textures.
• Minster – a diamond-shaped repeating pattern (level 2). Commonly used in heritage and conservation-area properties.

Patterned glass can be incorporated into sealed units as one pane (typically the outer pane for external privacy) while the inner pane remains clear float, Low-E coated, or toughened as required. In bathrooms and WCs – where both obscurity and safety glass are often needed – toughened patterned glass satisfies both requirements in a single pane.

Self-Cleaning Glass

Self-cleaning glass uses a dual-action coating to keep the external surface of the glass cleaner for longer, reducing the frequency of manual cleaning. The technology relies on two distinct properties: a photocatalytic coating that uses UV light from the sun to break down organic dirt on the glass surface, and a hydrophilic surface that causes rainwater to sheet evenly across the glass rather than forming droplets – carrying away the loosened dirt as it flows.

The most widely available self-cleaning glass in the UK is Pilkington Activ, which has been on the market since 2001. The coating is applied during manufacture and is permanent – it does not wear off or need reapplying. Self-cleaning glass is particularly well suited to hard-to-reach windows (upper-storey fixed lights, skylights, dormer windows), conservatory roofs, and any glazing where regular manual cleaning is impractical or expensive. While it does not eliminate the need for cleaning entirely – inorganic dirt such as paint splashes or cement dust must still be removed manually – it significantly reduces the build-up of everyday grime, algae, and water staining.

Self-cleaning glass can be combined with Low-E coatings and toughened or laminated glass in a single sealed unit, so specifying it does not mean compromising on energy efficiency or safety.

Solar Control Glass

Solar control glass is designed to reduce the amount of solar heat that enters a room through the glazing, while still allowing natural light to pass through. It is particularly beneficial for south-facing rooms, large glazed extensions, conservatories, and any space that suffers from overheating during the summer months.

Solar control is achieved either through body-tinted glass – where metal oxides are added during the float process to absorb a proportion of the sun's energy – or through a sputter-coated metallic film similar to a Low-E coating but optimised to reflect solar infrared radiation rather than long-wave heat from inside the room. High-performance solar control glass can reject 60 to 80 per cent of solar heat gain while still transmitting 40 to 70 per cent of visible light, maintaining a bright, naturally lit interior without the greenhouse effect that plagues many UK conservatories.

Modern solar control coatings can be combined with Low-E coatings on the same unit, giving you excellent summer comfort and excellent winter insulation in a single sealed unit. This dual-function approach is often marketed as "four-season glass" and is increasingly popular in new-build and high-specification refurbishment projects across the UK.

Fire-Rated Glass

Fire-rated glass is required by UK Building Regulations Part B (Fire Safety) in specific locations where glazing must provide a defined period of fire resistance – typically in fire doors, fire-rated partitions, escape route walls, and glazed screens separating different fire compartments within a building.

Fire-rated glass is classified by two key properties:

• Integrity (E). The ability of the glass to prevent the passage of flames and hot gases for a specified period. An E30 rating means the glass maintains integrity for a minimum of 30 minutes.
• Insulation (I). The ability of the glass to limit the temperature rise on the unexposed face, preventing radiant heat from igniting materials on the other side. An EI30 rating means the glass provides both integrity and insulation for 30 minutes; EI60 provides both for 60 minutes.

Integrity-only glass (E-rated) is typically a toughened or wired glass product that stays in the frame during a fire but does not prevent heat radiating through. Integrity-and-insulation glass (EI-rated) uses intumescent interlayers that expand when exposed to heat, forming an opaque, insulating barrier. EI-rated glass is more expensive and thicker than E-rated alternatives, but it is essential wherever people or combustible materials may be close to the glass on the unexposed side.

Fire-rated glass must be installed in compatible fire-rated frames and tested to BS EN 1364 as a complete system – the glass, frame, beads, and seals must all be certified together. It is not permissible to fit fire-rated glass into a standard window frame and claim fire resistance.

Understanding UK Energy Ratings

The BFRC (British Fenestration Rating Council) energy rating system is the standard method used in the UK to compare the overall energy performance of windows and sealed units. It uses an A++ to E scale – similar to the energy labels on household appliances – where A++ represents the best performance and E the worst.

A window's BFRC rating is determined by three factors working together:

• The glass type. Low-E coated glass dramatically reduces heat loss (lower U-value), which is the biggest single factor in achieving a high energy rating.
• The gas fill. Argon reduces convection within the cavity compared to air, improving thermal performance. Krypton offers even greater improvement but at higher cost – see our gas fills guide for a detailed comparison.
• The spacer bar. Warm-edge spacer bars reduce heat loss at the glass edge, eliminating the thermal bridge created by traditional aluminium spacers and reducing the risk of internal edge condensation.

Building Regulations Part L (England and Wales) sets the minimum energy performance standards for replacement windows. The current requirement is a maximum whole-window U-value of 1.4 W/m²K, which corresponds approximately to a BFRC rating of C or above. In practice, most reputable suppliers now specify Low-E glass with argon gas as standard, which typically achieves a BFRC rating of B or A – comfortably exceeding the minimum. For new-build properties, the requirements are tighter, and specifying A+ or A++ rated glazing is increasingly common to meet the demands of Part L 2021 and the Future Homes Standard.

Scotland has its own requirements under Section 6 of the Building Standards, with slightly different minimum U-value thresholds. Northern Ireland follows broadly similar requirements. Regardless of where in the UK you are located, choosing the highest-rated glass and gas fill combination that fits your budget delivers long-term savings on energy bills and adds value to your property. See our cost guide for transparent pricing on all glass types and configurations.

We supply double glazing glass across London, Manchester, Birmingham, Leeds, and areas throughout the UK. All units are manufactured to BS EN 1279 and delivered to your door.

Frequently Asked Questions