Low-E Glass Explained

Low-emissivity (Low-E) glass is the single most important component in a modern double-glazed unit. Its invisible coating reflects long-wave infrared heat back into the room instead of letting it escape through the glass, and in its standard soft-coat form it cuts the centre-pane U-value by around 40%. That one figure explains why Low-E glass is now fitted as standard in virtually every new unit, rather than treated as an optional upgrade. If you want to see what that improvement means for your heating bills, the energy savings calculator will put a number on it for your home.

What Low-E glass is

Glass on its own is a reasonable conductor of heat. When a room is warm and the outside is cold, heat radiates outward through the pane in the form of long-wave infrared radiation. Low-E glass addresses this with a coating so thin it is measured in nanometres, typically a metal oxide such as silver, that is applied to one of the inner surfaces of the sealed unit. The coating allows visible light to pass through largely unaffected while reflecting the infrared radiation back into the room, much as a thermos flask uses a reflective inner wall to keep a drink hot.

The result is a dramatic reduction in radiative heat loss through the glass. Because the coating is transparent and colourless, the glass looks essentially the same as ordinary float glass in everyday use. Most homeowners would never know it was there unless they were told. Understanding how this interacts with the frame and the gas fill is important; for a full explanation of how the finished unit's energy performance is measured, see our guide to U-values and WER ratings.

How the coating works

Heat loss through a window happens by three routes: conduction (heat passing through the solid material), convection (warm air circulating inside the sealed cavity and transferring heat to the cold outer pane) and radiation (the direct emission of infrared energy from the warm inner pane outward). In a sealed double-glazed unit without any special coating, radiation accounts for a significant share of the total heat loss. Low-E addresses this specific route.

The coating works by having a very low emissivity, meaning the treated surface is poor at radiating heat outward. Instead it reflects the infrared back toward its source, which is the warm interior of the building. The technical term for the coating's performance measure is emissivity (epsilon), and good soft-coat Low-E achieves an emissivity well below 0.1, compared with around 0.9 for ordinary uncoated float glass. That difference in emissivity is what produces the roughly 40% reduction in centre-pane U-value.

The coating is placed on the inner face of the inner pane (surface 3 of the unit, counting from outside in) in a standard double-glazed unit. This puts it inside the sealed cavity where it is protected from cleaning products and abrasion, which matters especially for the fragile soft-coat type.

Customers often ask if they can specify a unit without Low-E to save money. I always explain that the saving is minimal because Low-E is standard kit now, and a unit without it would struggle to hit the Part L requirement of 1.4 W/m²K. It is not an upgrade you pay extra for; it is simply part of what a compliant window is in 2026.

Tom Bradley, FENSA-registered installer

Soft-coat versus hard-coat

There are two main manufacturing methods for applying Low-E coatings, and they produce meaningfully different results.

Soft-coat (sputtered) Low-E is applied to the glass after it has been cut, in a vacuum-deposition process that lays down extremely thin, precisely controlled layers of silver and metal oxides. The result is the most thermally effective coating available, cutting the centre-pane U-value by around 40%. Because the coating is delicate, it must be sealed inside the unit during manufacture; it cannot be used on a surface exposed to the air. This is the type used in virtually all modern double-glazed units sold in the UK today.

Hard-coat (pyrolytic) Low-E is applied during the float-glass manufacturing process itself. A tin-oxide compound is deposited onto the hot glass as it comes off the production line, fusing into the surface. The result is much tougher and can be used on exposed surfaces without deteriorating, which makes it suitable for applications where a sealed unit is not practical. The trade-off is reduced performance: hard-coat Low-E cuts the centre-pane U-value by around 20%, roughly half the improvement of the soft-coat type. It is also sometimes used in secondary glazing applications.

A third category, high-solar-control Low-E, is a variant that cuts the centre-pane U-value by around 30% while also limiting the amount of solar radiation that passes through the glass. This is useful in south-facing rooms or conservatories that can overheat in summer. In a north-facing room it would be counterproductive, because it would also block the free solar warmth that helps offset heat loss.

Source: web/lib/research/u-values-wer.md

The soft-coat versus hard-coat question comes up when someone is looking at secondary glazing or a roof light where a sealed unit will not fit. For standard replacement windows the answer is always soft-coat, full stop. It is more efficient and it is what every reputable manufacturer supplies as default.

Tom Bradley, FENSA-registered installer

Low-E as part of the package

Low-E glass does not work in isolation. In a modern Part L-compliant double-glazed unit it is combined with two other components that address the remaining routes of heat loss.

Argon gas fill. The sealed cavity between the two panes is filled with argon rather than air. Argon conducts heat less readily than air, which reduces convective and conductive loss through the cavity. Krypton gas performs better still but costs significantly more; argon is the standard fill for double glazing. For a direct comparison of gas fills, see the argon versus krypton gas guide.

Warm-edge spacer. The spacer bar that runs around the perimeter of the unit, holding the two panes apart, is traditionally made from aluminium. Aluminium is a very good conductor of heat, creating a thermal bridge at the edge of the glass. A warm-edge spacer replaces the aluminium bar with a material that has much lower thermal conductivity, reducing edge-of-glass heat loss and the risk of condensation forming around the border of the pane. The full explanation of how warm-edge spacers work is in our warm-edge spacer guide.

When all three components are combined, a double-glazed unit reaches a centre-pane U-value (Ug) of about 1.0 to 1.1 W/m²K. Part L of the Building Regulations requires replacement windows to achieve a whole-window U-value of 1.4 W/m²K or lower, or a Window Energy Rating of band B or better. A well-made double-glazed unit with Low-E glass, argon and a warm-edge spacer clears both routes comfortably, with no need to go to triple glazing.

Should you pay extra for Low-E?

In most circumstances, no, because you will already be getting it. Soft-coat Low-E glass, argon gas and a warm-edge spacer are the standard specification for new double-glazed units in the UK market in 2026. Any compliant unit should include all three. If a quote specifies Low-E as an optional extra at additional cost, that is a signal to scrutinise the base specification more carefully, because the base unit without it would be unlikely to meet Part L.

The situation where you might genuinely face a cost decision is if a supplier offers high-solar-control Low-E as an upgrade for south-facing or conservatory glazing. This is a reasonable choice if summer overheating is a known problem in that room, but it is worth modelling the trade-off carefully. High-solar-control glass cuts solar gain, which reduces summer temperatures but also reduces the free warmth that solar gain provides in winter.

For most homes, the practical advice is to confirm that the specification includes soft-coat Low-E, argon and a warm-edge spacer, and then focus the buying decision on the frame material, installer quality and warranty rather than on glass upgrades. The glass itself is largely commoditised at the standard specification level. If you want to model how a better-performing unit affects your running costs, the energy savings calculator will give you figures based on your house size and heating costs.

Where the specification does matter most is in the context of new build or deep retrofit, where whole-window U-values below 1.0 W/m²K may be required. In those cases, triple glazing with two Low-E coatings and a krypton or argon fill is the standard approach, and the glass specification becomes a central part of the design brief rather than a commodity decision.