What's Actually Inside the Sealed Unit: Glass Choices That Decide How Your Windows Feel

Ask most homeowners what glass they've bought and the answer is 'double' or 'triple'. Ask what's inside it and the conversation tends to stop. That's a shame, because the sealed unit — the glass, the coating, the gas and the spacer — does most of the work. The frame gets the attention and the colour swatches, but the glass is where the performance actually lives.

None of it is complicated once the jargon is unpacked. Here's what each choice really does, and where the trade-offs sit.

Low-e coatings: the invisible layer doing the heavy lifting

Low-emissivity coatings are microscopic metallic films that reflect heat back into the room. Nearly every modern unit has one, and it's the main reason a modern double-glazed window outperforms an old one by such a margin.

Soft-coat (sputtered) coatings are the most thermally efficient and are standard in domestic units. Hard-coat versions are more durable and are sometimes used where a unit is more exposed. The difference in feel is nil; the difference in the paperwork matters, so ask which one is being quoted.

One practical point: a low-e coating can give glass a very faint tint. In most rooms you won't notice. Against a dark render with a low sun angle, you might. Ask to see a sample pane outdoors, not under showroom lighting.

Solar control: for the room that cooks in July

A window facing east or west, a large sliding door, a conservatory roof — these can turn a living room into a greenhouse by mid-afternoon. Solar-control glass uses a coating or a body tint to limit how much of the sun's energy gets through, measured as the g-value. A standard unit sits somewhere around 0.6–0.7; a solar-control unit might be 0.3–0.4, meaning noticeably less heat enters.

The trade-off is genuine. Cut too much solar gain and you also lose the free winter warmth that makes a south-facing room pleasant in February. Tinted glass also changes the look from the street. If overheating is the issue, exterior shading or internal blinds are sometimes the cheaper first move — worth considering before you pay a premium for the glass.

Acoustic laminated glass: the answer to a noisy road

If you live near a dual carriageway, a railway or under a flight path, this is the specification that changes your life more than any other.

Acoustic performance comes from two things: an acoustic-grade interlayer bonding two panes together, and asymmetry — using different glass thicknesses on each side of the cavity. A 6mm pane paired with a 4mm pane breaks up sound waves far better than 4mm on both sides. The cavity width matters too. Ask for the Rw figure in decibels and compare like for like.

Be realistic, though. A window is only as good as its weakest path, and a permanently open trickle vent will undo much of the benefit. Good acoustic glass plus controlled ventilation is the combination that works.

The downsides are cost and weight. Thicker, laminated units are heavier, so hinges and opening mechanisms need to be up to it, and some older frames simply don't have the rebate depth.

Toughened or laminated, and where the rules insist

Safety glass isn't optional in certain places. Under the Building Regulations, glass in what's known as a critical location — roughly, within 800mm of the floor, and in or beside doors within 300mm — has to be a safety glass.

Toughened glass breaks into small blunt fragments. Laminated glass holds together in a cracked sheet on its interlayer. For a low-level window that someone might fall against, either works. For overhead glazing, roof lights and anything where falling glass would be dangerous, laminated is the better answer. For entrance doors and anything that might be attacked, laminated or a laminated-toughened combination is worth the extra.

Ask which panes in your quote are being upgraded and why. If the answer is 'they all are', check the price reflects it.

Obscure glass: privacy without losing the light

Bathrooms, side panels by the front door, a window overlooking a neighbour's drive. Obscure glass comes in patterns and finishes, and in standardised obscuration levels from 1 to 5, with 5 being the most private. Satin,everglade and autumn-style textures are the familiar ones.

A practical caution: heavier textures obscure more but diffuse less light, so a small bathroom can feel dimmer. A lightly textured glass paired with a good bathroom blind often works better than the most opaque option available. And note that obscure glass doesn't automatically make a pane safe — critical locations still need safety glass behind or as part of it.

Warm-edge spacers: the detail that stops black mould at the corners

The spacer bar is the frame between the panes. Traditional aluminium spacers conduct heat and cold straight through, which is why older units so often show condensation and mould right at the bottom corners. Warm-edge spacers use composite, stainless steel or thermoplastic materials that slow that transfer down.

It's a small component with a big effect on the coldest part of the window. If you're comparing two quotes and one is substantially cheaper, this is one of the places the saving has probably been found.

Gas fill and cavity width

Argon is the standard fill and is inexpensive. Krypton performs better in narrow cavities but costs more and is usually reserved for slim frames. Gas can escape over the life of a unit, which is part of why guarantees matter.

Cavity width has an optimum. Wider isn't automatically better — beyond roughly 20mm, convection currents inside the cavity start to reduce insulation. A well-built 28mm unit with a 16mm cavity and argon will comfortably outperform a badly made thicker one.

Triple glazing has real advantages for noise and for very exposed locations, but it's heavier, costs more, narrows the visible glass area and demands better hinges. It isn't automatically the right answer for a Victorian terrace.

Questions to put in front of your supplier

• Which low-e coating is specified, and on which surface?
• What is the g-value of the unit, not just the whole-window rating?
• What is the Rw figure for the acoustic glass, and are the pane thicknesses asymmetric?
• Which panes are toughened or laminated, and are they in critical locations?
• What obscuration level is being used in the bathroom?
• Is the spacer warm-edge, and what is it made of?
• What gas fill, and what cavity width?
• What is the guarantee on the sealed unit itself, separate from the frame?

Get those eight answers in writing and you'll know more about your windows than most people who signed the same week.