"Glass" is the loosest word in glazing. What you actually need depends on where the window is, what room it's in, who lives in the house and what the original glass was. This is the complete guide to every glass type used in UK homes.
Annealed (float) glass
The default. Ordinary, clear glass produced by floating molten glass on a bed of liquid tin. It's what you'd find in most non-safety locations - upper windows, fixed lights above shoulder height, and the outer pane of most sealed units. When it breaks it shards into long jagged pieces, which is why it's restricted in safety-critical areas (see building regs section below).
Cost: the baseline - every other glass type is priced relative to annealed.
Toughened (tempered) glass
Annealed glass that's been reheated to around 620°C then rapidly cooled with jets of air. This puts the outer surface under compression and the core under tension, making the finished glass 4-5 times stronger than annealed and dramatically more resistant to thermal stress. When toughened glass breaks, it shatters into thousands of small blunt cubes rather than long shards - which is why it's a "safety glass" under BS EN 12150.
Where you need it: doors, side-panels next to doors, any glass within 800mm of floor level in a wall, any glass within 300mm of a door and below 1,500mm. Also Velux roof windows and low-level conservatory glazing.
Cost: roughly 40-60% more than annealed for the same size.
One quirk: toughened glass cannot be cut or drilled after manufacture. The exact size has to be specified before the toughening process. This is why a toughened unit lead time is usually a few days longer.
Laminated glass
Two panes of glass bonded together with a clear plastic interlayer (PVB - polyvinyl butyral). When laminated glass breaks, the shards stick to the plastic film rather than falling out - exactly like a car windscreen. It's the most secure standard glass type because an intruder cannot simply punch through it; even repeated hammer blows leave the pane in one piece.
Where you'd choose it:
- Ground floor windows in security-sensitive areas
- Overhead glazing (skylights, conservatory roofs) - shards can't fall
- Acoustic glazing - the PVB layer damps sound waves; a 6.4mm laminated pane is significantly quieter than a 6mm toughened pane
- Anywhere on a busy road or flight path
Cost: 50-80% more than annealed. A laminated outer pane in a sealed unit is one of the cheapest ways to reduce traffic noise without changing the window.
Low-E (low emissivity) glass
Now the standard inner pane in virtually every new sealed unit sold in the UK. Low-E glass has a microscopically thin metallic oxide coating - usually based on silver - on one face. The coating is invisible to the eye but reflects long-wave infrared radiation. In practical terms: in winter it bounces escaped room heat back into your home; in summer it reflects solar heat back outside.
A standard 4mm-16-4mm sealed unit with Low-E and argon achieves a U-value of around 1.2 W/m²K, compared to 2.8 W/m²K for the same unit with two clear panes. That's roughly half the heat loss. Building regulations have required this performance level on replacement windows since 2010.
Cost: a Low-E unit is barely more expensive than a clear unit - manufacturers buy Low-E glass by the thousand-tonne. There's no reason to specify anything else for the inner pane.
Argon and krypton gas fills
The space between the two panes of a sealed unit isn't air - or shouldn't be, if it's modern. It's filled with an inert gas, almost always argon. Argon is denser and less conductive than air, so it slows heat transfer between the panes. Krypton performs even better but costs roughly five times as much and is reserved for high-performance triple glazing.
You cannot see whether a unit is gas-filled. The small spacer bar (the metal or warm-edge plastic strip around the perimeter) holds the gas in. Once the seal fails, the gas escapes - which is why "blown" units perform so much worse than healthy ones, even before the misting becomes visible.
Obscure and patterned glass
Used wherever you want light through the window but not visibility - bathrooms, downstairs WCs, front doors, sometimes hallways. The glass is rolled past a textured roller during manufacture, leaving a pattern that distorts the view. Common UK patterns include:
- Pattern 7 / Sandblast (Privacy level 4) - smooth fine texture, high obscurity
- Pelerine - parallel narrow lines, modern look
- Contora - the classic "rippled" bathroom glass
- Cotswold - chunky organic pattern, common in 80s/90s builds
- Stippolyte - fine even stipple, allows lots of light through
When replacing a blown unit in a bathroom, you must match the existing pattern or it'll look like an obvious patch from outside. Ask your glazier to photograph the existing pane and confirm the pattern before ordering.
Georgian bar and leaded units
Decorative units with either flat metal bars (Georgian) or hand-cut diamond/rectangular lead patterns (leaded lights) sandwiched between the two panes. They're factory-applied and cannot be retrofitted - the bars or leadwork sit inside the sealed unit during manufacture.
If you're replacing a blown leaded unit, the new one needs to match the surrounding windows' bar spacing, lead width and corner pattern. We template these in person rather than working from measurements alone.
Where building regulations force your hand
UK Building Regulations Approved Document K and Document Q dictate where safety glass is mandatory. The short version:
- All glass below 800mm from finished floor level must be toughened or laminated
- All glass within 300mm of a door and below 1,500mm from the floor must be toughened or laminated
- Doors themselves and full-height side panels must be safety glass throughout
- New windows in habitable rooms must meet a U-value of at least 1.4 W/m²K (Low-E argon as a minimum)
A FENSA-registered installer is legally required to certify this and notify Building Control on your behalf. A "bloke down the pub" is not.
How to specify a replacement unit
Whether you're ordering yourself or briefing a glazier, you need six pieces of information:
- Finished size (width x height in mm, after the 10mm deduction)
- Overall thickness (typically 20mm, 24mm or 28mm)
- Construction (e.g. 4mm clear / 16mm argon spacer / 4mm Low-E)
- Safety glass status (toughened, laminated or annealed - and which pane)
- Pattern if obscure (Contora, Pelerine, etc.)
- Decorative work if applicable (Georgian bar colour and spacing, lead pattern)
If any of that feels like a foreign language, that's our job. Get a free quote and we'll specify the right unit for every window.