Most people choose a window by its frame — the colour, the profile, the handle — and leave the glass to whoever is writing the quote. That is a bit like buying a car on paintwork alone. The glazing decides how much heat escapes, how much noise gets in, how quickly the sofa fades, and what happens when a football, a hailstorm or an elbow arrives at speed.
The vocabulary is smaller than it looks. Four terms cover almost everything sold for houses: float, tempered, laminated and low-E. Here is what each one actually does, where it tends to be required, and how to work out which of them you already have.
Float Glass: The Base Material Everything Starts From
Nearly all flat glass begins life as float glass. A continuous ribbon of molten glass is floated across a bath of molten tin, and gravity plus surface tension do the rest, producing a flat, uniform sheet with none of the ripples you see in old hand-made panes. When a glazier says "float", they normally mean annealed float — the ordinary, untreated version, sold in 4mm, 6mm and other standard thicknesses.
It is perfectly good glass: clear, inexpensive, easy to cut and easy to drill, which is why it still ends up in picture frames, cabinet doors, interior partitions and plenty of older windows. It is also the starting point for most of the specialist glass sold today: tinted, reflective, patterned and coated products usually begin as a float sheet before something else is done to them. What float glass is not is safety glass. Break it and you get large, sharp, dagger-shaped pieces — the reason a single-glazed French door makes a careful parent wince. A single 4mm annealed pane in a full-height door is a genuine hazard; the same opening fitted with tempered or laminated glass is not. Almost everything else in this list is float glass that has been put through something extra.
Tempered Glass: Very Strong, Then Very Sudden
Tempered glass — toughened, in British usage — is float glass heated until it softens, then chilled hard and fast. The surface cools first and pulls tight around a still-hot core, locking a layer of compression into the outside of the sheet. That compression is what makes the pane considerably more resistant to impact than annealed glass of the same thickness, and it changes how the glass fails: instead of shards, you get small, comparatively blunt granules.
The catch is that tempering is permanent and happens last. Once a pane is tempered it cannot be cut, drilled or re-edged, so every hole for a handle and every cut-out for a hinge has to be in the drawing before the glass goes into the oven. This is where renovation projects lose a week: someone measures optimistically, the pane arrives 8mm too wide, and it is scrap.
You will find tempered glass in patio and balcony doors, in glazed panels beside a door, in any pane that comes close to floor level, in bathroom windows and shower screens, stairwells and landings, balustrades, shopfronts and glass furniture. Building codes in most countries require safety glass in these positions, though the exact heights and distances vary by country and by building type. Check the local rules, or ask your glazier to confirm in writing, before you order.
Two practical footnotes. First, tempering increases strength, but it does not make glass unbreakable: a sharp knock to the edge, where the compression layer is weakest, can still start a failure. Second, a very small number of tempered panes break spontaneously because of nickel-sulphide inclusions in the raw glass; heat-soak testing reduces that risk, and is worth asking about for large or high-level panes where replacement would be awkward and expensive.
Laminated Glass: The One That Stays in the Frame
Laminated glass is two or more sheets bonded together with a transparent plastic interlayer — usually PVB, sometimes EVA or a stiffer ionoplast. The glass will crack, but the interlayer holds the pieces in place. Push hard and the pane flexes; it does not open a hole for a hand, a brick or a gust of wind.
That is a different kind of safety from tempering. Tempered glass resists the impact; laminated glass survives it. Which is why laminated is the default for anything overhead — rooflights, canopies, glass roofs — and for balustrades, shopfronts, secure glazing, and windows where a broken pane must not drop out of the opening. It is also the usual choice where flying debris is a genuine risk.
Two more things it does quietly well: the interlayer damps sound, especially when the two panes are of different thicknesses, so it is a common fix for a house on a busy road; and it blocks most of the ultraviolet light that fades timber, textiles and artwork. The categories combine, by the way. A tempered laminated build — two toughened plies around an interlayer — is used where both strength and post-breakage integrity matter, such as large structural panes high up a facade.
The thickness of the interlayer matters too. A standard 0.38mm PVB layer will hold broken glass together, but a 0.76mm or 1.52mm interlayer — or a stiff ionoplast such as SentryGlas — is specified where the pane must resist a sustained push, a repeated attack, or a person falling against it. For a bedroom window above a busy road, a laminated unit with unequal glass thicknesses, say 6mm and 4mm, is a common acoustic upgrade; for a ground-floor window facing a pavement, the same construction doubles as a security measure, because the interlayer makes quiet entry much harder.
Low-E Glass: The Coating You Cannot See
Low-emissivity glass carries an invisible metallic coating, thinner than a soap bubble, that changes how the surface handles radiant heat. Put simply, it lets daylight through but reflects long-wave heat back where it came from. In a cold climate that means less of your heating drifting out through the pane; in a hot one, with the right coating, it means less of the afternoon sun cooking the living room.
Low-E is not a glass type in its own right. It is a coating applied to float glass, and that glass can then be tempered or laminated as needed. Almost every sealed double- or triple-glazed unit sold for housing today has at least one low-E surface, usually facing into the gas-filled cavity where it is protected from handling and dirt.
Soft coat and hard coat
There are two families, and the difference matters when you are repairing rather than building new. Hard coat, or pyrolytic, is applied while the glass is still being made, fuses into the surface, and is tough enough to be handled, cleaned and used as a single pane. Soft coat, or sputtered, is applied afterwards in a vacuum chamber; it performs better but is delicate, so it must sit sealed inside an insulating unit. Ask which one a quote specifies. If a single-glazed window is being sold as low-E, it will be hard coat.
Orientation and climate change the specification. A west-facing living room that overheats in summer needs a solar-control low-E coating that limits heat gain without making the room gloomy; a north-facing bedroom in a cold climate may be better served by a coating tuned to retain heat. Two panes of the same thickness, both called "low-E", can perform quite differently. Ask for the U-value and the solar heat gain coefficient, not just the label. Also, low-E is not the same as tinted or reflective glass: tinting reduces light and heat by absorbing or reflecting it, while a low-E coating is selective, and the better products keep more daylight while blocking more unwanted heat.
How the Four Types Combine
In practice, the four terms describe different jobs, and a modern window usually stacks several of them into one sealed unit. A typical double-glazed window might be 4mm float, a 16mm argon-filled cavity, and 4mm low-E float; a safety-critical window might replace one of those plies with tempered glass; an acoustic or security window might use laminated glass instead. A common specification reads something like "6mm low-E tempered + 16mm argon + 4mm float", which tells you the outer pane is coated and toughened, the cavity is gas-filled, and the inner pane is ordinary float. The order matters, and so does which face carries the coating. That is why a quote that says only "double glazing" is not a specification.
How to Tell Which Glass You Already Have
You can usually narrow it down in ten minutes with a torch, a step ladder and some patience. Work through these in order:
- Look for a corner stamp. Tempered panes are normally marked near one corner with the manufacturer's name, the relevant standard, and the word "tempered" or "toughened". The mark is fired into the glass and will not rub off. If you cannot find one, do not jump to conclusions: the stamp may be hidden behind the frame or on a face you cannot see.
- Inspect the edge of the pane. Laminated glass usually shows a thin plastic interlayer line where the glass meets the frame. Float glass has a clean, uniform edge; tempered glass may show small roller marks or a slightly different edge finish, though this is not a reliable test on its own.
- Try a polarising filter. Viewed through polarised sunglasses or a camera filter, tempered glass often shows dark, patchy stress patterns across the pane. Annealed float glass normally looks clear. It is a useful clue, not a certificate.
- Check for a sealed unit. If the glass has a metal or plastic spacer between two panes, it is an insulating unit, and the chances are high that at least one surface is low-E. A low-E detector or a thermal-imaging camera can confirm the coating, especially on a cold day.
- Read the paperwork. Glazing certificates, window stickers and the original order form are more reliable than any field test. If none exist, ask a glazier to inspect the unit before you drill, replace or build around it.
What to Ask Before You Sign
Glass is one of the few building products where the cheapest line item can quietly undo the rest of the specification. Before you accept a window quote, ask these questions:
- What is the make-up of each pane? Ask for thicknesses, glass types and cavity width, not just "double glazed".
- Is safety glass required here? Mention doors, floor-level panes, stairwells and bathrooms, and ask the supplier to confirm the local rule in writing.
- Which low-E coating, and on which surface? Soft coat and hard coat are not interchangeable, and the coating face affects performance.
- What are the U-value and solar heat gain figures? These let you compare products on performance rather than marketing language.
- Can the glass be tempered or laminated after order? If holes, cut-outs or exact sizes are involved, confirm that all fabrication happens before tempering.
None of this requires you to become a glazing specialist. It requires only that you treat the glass as part of the specification rather than an afterthought — and that you ask for the numbers before the frame is chosen, not after.
Photo: ELG21 / Pixabay
