Low E glass and coatings

Low-E (low emissivity) glass carries a microscopically thin metallic coating that reduces radiative heat transfer through the glazing. For insulated roof lights it is the standard way to achieve the U-values required under Approved Document L without excessive glass thickness or wide cavities.

How low-E coatings work

Glass naturally emits long-wave thermal radiation. In winter, warm interior surfaces radiate heat towards the cold outer pane. A low-E coating reflects much of that radiation back into the room, reducing heat loss.

The coating does not work like a mirror for visible light — daylight still passes through. It selectively affects thermal infrared radiation. The result is a lower U-value (W/m²K) for the sealed insulating glass unit.

Hard coat vs soft coat

Two manufacturing routes dominate:

Pyrolytic (hard coat) — applied on the float line while the glass ribbon is still hot. The coating fuses into the surface. Hard coat is durable enough to be used on single glazing or the exposed face in some applications, though in IGUs it still sits on a cavity face.

Magnetron sputtered (soft coat) — applied offline in a vacuum chamber. Typically offers lower emissivity and therefore better U-values, but the coating is delicate until sealed in the unit.

Most high-performance residential roof lights use soft-coat low-E on the cavity-facing surface of the inner pane (face 3 in a conventional numbering system), combined with argon gas fill and a warm-edge spacer.

U-values and Part L

Building Regulations set maximum U-values for glazing elements. Roof lights often have a separate limiting value from vertical windows because the thermal bridge at the kerb and the angled or horizontal orientation affect performance.

A modern insulated roof light unit might achieve U-values in the region of 1.0–1.4 W/m²K depending on build, coating and cavity. Confirm the declared whole-unit U-value from the manufacturer — not the centre-pane figure alone, which ignores edge effects.

Low-E is one part of that performance. Cavity width (typically 16–20 mm), argon fill, spacer type and frame thermal break all contribute.

g-value and solar gain

g-value (or solar factor) describes how much solar energy passes through the glazing. Low-E coatings optimised for heat retention can have a moderate to high g-value, which is welcome in winter but may contribute to overheating in summer.

Part O (overheating) applies to new dwellings in England. Roof lights in sun-exposed rooms may need solar-control coatings or shading in addition to low-E. Thermal performance and solar control are related but not identical — specify both U-value and g-value when comparing options.

Where the coating goes

In a double-glazed unit, surfaces are numbered from the outside in:

1. Outer pane — external weather face
2. Outer pane — cavity face (low-E or solar control possible)
3. Inner pane — cavity face (most common position for soft-coat low-E)
4. Inner pane — room face

The coating must remain sealed inside the cavity. Scratching or exposing a soft coat destroys its performance.

Compatibility with safety glass

Low-E coatings are applied to annealed float before toughening or lamination, or to finished panes in some hard-coat cases. Toughened low-E glass must be processed so the coating survives heat treatment — your glass processor handles this. The finished product still complies with BS EN 12150 or BS EN 14449 as appropriate.

Specifying low-E on roof lights

• Request the whole-unit U-value for the exact size and frame system.
• State whether argon fill and warm-edge spacer are included.
• For bedrooms and south-facing spaces, discuss g-value and Part O implications.
• Low-E does not replace a laminated inner pane where safety glass is required overhead.

Vant Glass manufactures premium roof lights and glazing in Aintree, Liverpool — made in Britain, 20-year guarantee, free UK mainland delivery. Explore all products or call 03330 902 592.