Types of window glass
Most people don’t pick their glass. They choose a frame, a colour, and a style; the glass is whatever the installer included in the quote. We get it. The frame is what you see, while the glass is less obvious, and all the technical terms can make it hard for homeowners to feel confident about their choice.
It’s a pity, because when it comes to how a window works, the glass does most of the job. The frame just holds it and keeps out the weather. The glass affects how much heat escapes, how much sunlight gets in, how much noise you hear, how safe the window is if someone bumps into it, and how private your bathroom is. Two windows with the same frame can act very differently depending on the glass inside.
We’ve had this conversation many times. Someone gets three quotes for windows that look alike, and one is much cheaper. Most of the time, the difference is in the sealed units, and you only notice it after your first winter. People who have experienced this usually ask about the glass first the next time.
So, we want to make this topic practical, not just technical. We’ll start with ordinary float glass and when it’s still the best choice, then cover the safety glass options—toughened and laminated—what low-emissivity coatings do and how to understand U-values, when solar control glass is useful, what acoustic glass can and can’t do, and finally, privacy options and the sealed unit that brings it all together. If there’s a real trade-off, we’ll be honest about it instead of pretending every option is an upgrade.
Float glass and where it still works
Most of the glass in your home starts as float glass, also known as plate or annealed glass. It’s made mainly from silica, soda ash, and limestone, with a bit of magnesia and alumina added.
The float process was invented in Britain by Alastair Pilkington at Pilkington Brothers in St Helens, not far from our workshop. In this method, molten glass is poured onto molten tin, spreading out to form a sheet with two flat surfaces. Before this, making flat glass meant grinding and polishing, which was slow and costly. The float process made large, clear glass sheets affordable, shaping the look of modern buildings.
Ordinary float glass is still the right choice for most window openings. It’s clear, colourless, stable, and affordable. The main drawback is that it breaks into sharp pieces, which is why the next section covers other options.
A quick tip: thicker glass sheets can look slightly green due to iron in the glass. If you need large panes where true colour is important, you can get low-iron glass, though it costs more.
Toughened or laminated: the safety choice
People often mix these two up, so let’s explain them clearly. They both address the same issue, but in different ways.
Toughened glass, also called tempered glass, is heated to around 620 degrees and then rapidly cooled. This puts the surface into compression and the core into tension, making the finished pane roughly four to five times stronger than annealed glass of the same thickness. When it breaks, it shatters into small, blunt granules rather than shards.
Laminated glass consists of two or more panes bonded around a polyvinyl butyral interlayer. It is not stronger in the sense that toughened glass is, but when it breaks, the fragments stay adhered to the interlayer and the pane remains in place. That single property is why it is used for shopfronts, overhead glazing and anywhere security matters.
What really matters is what happens if the glass breaks. Toughened glass breaks all at once, leaving a hole. Laminated glass breaks but stays in place. For ground-floor windows on quiet streets, toughened glass is usually fine and costs less. For door side panels, vulnerable back windows, or anywhere a hole would be a big problem, we recommend laminated glass.
This is not entirely a matter of preference. Approved Document K requires glazing in critical locations to break safely, resist impact, or be protected. Critical locations include glazing in doors and door side panels below 1,500mm from floor level, and glazing in walls below 800mm. Any competent installer builds this into the specification automatically, but it is worth checking that your quotation identifies which panes are safety glass. If it does not mention it at all, ask.
Low E coatings and what U-values mean
This is the part where you can really save money, but it’s also where people get most confused.
Low-emissivity glass has a microscopically thin metallic oxide coating that is transparent to short-wavelength visible light but reflects long-wavelength infrared. In winter, that means daylight comes in, and the heat radiating from your radiators gets bounced back into the room instead of escaping through the glass.
There are two types, and the difference matters. Hard coat, or pyrolytic, is baked onto the glass during manufacturing. It’s tough and can be cut later, but its emissivity is only about 0.15-0.20. Soft coat, or sputtered, is applied in a vacuum after the glass is made and can achieve an emissivity as low as 0.02-0.04, which is much better.
People often say low-E coatings are fragile and can be scratched during installation. Soft coat is delicate, which is why it’s always sealed inside the unit’s cavity where nothing can reach it. Once the unit is made, the coating is protected for the unit's entire life. So, while this is a real concern in the factory, it doesn’t matter on-site.
The numbers make the case better than any description. A plain 4-16-4 clear double-glazed unit with air achieves around 2.7 W/m²K. The same unit with argon and a soft-coat low-E drops to roughly 1.2 W/m²K. Add a polymer warm-edge spacer instead of aluminium, and you gain another tenth or two on the overall window figure.
Our clear recommendation for almost every replacement window is a 4-16-4 argon-filled unit with soft-coat low-E and a polymer warm-edge spacer. It reliably achieves 1.0 to 1.2 W/m²K whole window, comfortably beating the 1.4 W/m²K maximum that Building Regulations now set for replacement windows in England, and it costs very little more than a specification that only just complies.
Always ask for the U-value of the whole window, not just the centre pane. These are different numbers, and the difference is often where marketing can be misleading.
Solar control glass and summer overheating
Low E glass helps keep heat inside. Solar control glass helps keep heat out. They’re related but not the same, though modern coatings often combine both features.
Solar control glass reflects some of the sun’s energy while letting most visible light through. The key measure is the g-value, or solar factor, which shows how much solar energy gets through. A lower g-value means less heat comes in and your room stays cooler.
This used to be a commercial concern only. It is now a domestic one, because Approved Document O introduced overheating requirements for new residential buildings in England from June 2022, and one of the compliance routes is glazing with a maximum g-value of 0.4 combined with a light transmittance of at least 0.7. Notably, parts of central Manchester are treated as high-risk locations alongside urban London, bringing this closer to home than many people expect.
Part O only applies to new homes, not replacement windows, so most of our work isn’t affected. It matters most for extensions with big south or west-facing windows, garden rooms, and conservatory replacements.
Our honest advice is to use solar-control glass for large south- and west-facing windows, but it’s usually not needed for north-facing ones. Keep in mind that cutting down on heat in summer also means less free warmth in winter, so using it everywhere isn’t always ideal. We prefer to recommend it only where it’s really needed.
Tinted glass is a similar but simpler option. Adding colour during manufacturing reduces glare and light, but it also causes the glass to absorb more heat, which can cause stress. That’s why tinted panes often need to be toughened.
Acoustic glass and realistic expectations
Noise is the issue we can help with the most, but it’s also where people’s expectations often need to be managed.
Sound reduction is measured as Rw in decibels, using a logarithmic scale. A 3 dB improvement halves the sound energy. A 10 dB improvement sounds about half as loud to our ears.
The realistic figures are these. Here are the real numbers: a single-glazed Victorian sash provides about 24-28 dB Rw. Standard 4-16-4 double glazing provides 28-32 dB. An acoustic laminated unit with a 6.8mm outer pane gives 36 to 39 dB, and a thicker 8.8mm version reaches 40 to 43 dB. The panes are deliberately different thicknesses so they do not resonate at the same frequency. Second, an acoustic PVB interlayer, which damps vibration across the pane and is particularly effective in the speech frequency range. A laminated acoustic pane will typically outperform a thicker plain pane by 3 to 5 dB.
Here’s an important note: lab results aren’t the same as real-life results. Once you factor in frames, seals, and gaps in walls or vents, expect three to eight decibels less. Also, glass alone can’t solve noise problems if the frame isn’t sealed well or the wall isn’t insulated.
For most windows installed in Cheshire homes on a moderately busy road, aiming for an asymmetric acoustic unit at 36 to 39 dB is a good goal and adds about 15 to 25 per cent to the cost. Beyond that, improvements get expensive fast. If you live near a main road or railway, secondary glazing with a wide gap works better than any sealed unit, and we’ll tell you that even though we don’t sell it.
Privacy glass and the sealed unit
Choosing obscure glass is the simplest decision, but it’s also easy to get a little wrong.
Patterned and frosted glass is made by adding texture during manufacturing, or by acid etching or sandblasting later. The key thing is the privacy level, rated from one to five. One is barely obscured, five is almost opaque. For a bathroom facing a neighbour, go for level four or five. For a hallway or front door where you want light but some privacy, level three is usually best. We’ve replaced plenty of bathroom windows where the original glass was too clear, so it’s an easy mistake to avoid.
Finally, the sealed unit isn’t a type of glass, but the assembly that holds your chosen glass. It has two or three panes separated by a spacer bar, with the gap filled with argon and sealed around the edge with butyl. The spacer also has a desiccant to absorb any residual moisture and keep the interior clear.
There are two important things to look for. First, a 16mm gap is best for argon-filled double glazing—narrower gaps reduce performance, and wider ones allow heat to move around. Second, choose a polymer or steel warm-edge spacer instead of aluminium, which conducts heat along the edge, often causing cold spots and condensation.
If you get those details right, along with a soft-coat low-E, you’ll have a really good window no matter what the frame costs.
Specifying glass that suits your home
Glass is what really affects how a window works, but it’s the part homeowners rarely choose. Talking about it early on is the easiest way to improve your replacement project.
If you remember three things, let them be these: use a 4-16-4 argon-filled unit with soft-coat low-E and a warm-edge spacer as your standard—it costs only a bit more than the minimum but works much better. Pick laminated glass instead of toughened wherever a hole would be a real issue, and make sure your quote says which panes are safety glass. Finally, treat solar-control and acoustic upgrades as choices for certain windows, not for the whole house.
Most importantly, ask for the glass specification. If a quote just says "double glazed," it doesn’t tell you what you’re actually getting—and it’s usually the cheapest option for that reason. If you’d like us to talk through your window options, including where we think an upgrade isn’t worth the cost, just call us at 01606 863553 or email hello@swsonline.co.uk for a free, no-obligation quote.
Select Window Systems
Phone: 01606 863553
Email: hello@swsonline.co.uk
Visit: Unit 2, Woodford Court, Winsford, Cheshire, CW7 2RB
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