What is low-e glass in a rooflight?

The “e” stands for emissivity, which is a measure of how readily a surface gives off heat as radiation. Ordinary glass has high emissivity: it soaks up warmth and re-radiates it happily out the other side. A low-e coating drops that figure close to zero on the coated face, so the glass behaves less like an open window to the cold and more like a one-way mirror for heat. Nearly every sealed unit sold in the UK today has it, because you cannot meet the thermal side of Building Regulations without it.

Heat crosses a window in three ways: conduction through the solid glass and frame, convection as air moves against it, and radiation, which is heat travelling as infrared with no need for anything to carry it. The low-e coating deals with the third one. It sits inside the sealed unit, on one of the two faces that look into the cavity between the panes, where it is protected from fingers, weather and cleaning.

On a winter evening your radiators warm the room, and every warm surface in it, including the floor, the walls and you, radiates infrared upwards at a cold rooflight. Uncoated glass would absorb most of that and pass it outside. The coating reflects the bulk of it straight back down into the room. The pane still feels cold to the touch because its outer face is still cold, but far less of your heating is being lost through it. That single change is what takes a modern sealed unit from a U-value of around 2.7 down to roughly 1.2 or better, and it is why the glass in a rooflight matters as much as the frame it sits in.

There are two ways the coating gets onto the glass, and they are not interchangeable.

For a rooflight you are almost always getting soft-coat glass inside a sealed unit, and that is the right answer. It gives the lower U-value, and because it lives protected in the cavity there is nothing exposed to wear. The reason to know the difference is that the two look identical once fitted, so if a quote simply says “low-e” it is worth asking which one, and which surface of the unit it sits on, since that changes how the glass behaves in summer as well as winter.

This is the confusion worth clearing up, because the two coatings get sold under overlapping names. Low-e is fundamentally about keeping heat you have paid for on the inside. Solar control is about keeping the sun’s heat on the outside. A basic low-e unit lets most of the sun’s warmth in during the day, which is welcome in a north-facing bedroom in February and a real problem in a south-facing kitchen in July.

That distinction lands hard in Southend. The estuary sits to the south, so a great many rear extensions and loft conversions here point their new glass straight at the summer sun with an open horizon and reflected light off the water on top. A rooflight that is merely low-e will keep that room warm in winter and then cook it in summer, because it was never designed to turn solar heat away. The better specification pairs the low-e layer with a solar-control layer, so the glass holds your heating in on a cold night and still refuses the worst of the summer gain. The number that describes the second job is the G-value, and getting the balance right between the two is the heart of our energy-efficient skylight installation work. If your roof faces anywhere near south, ask about the G-value and the light transmittance, not just whether the glass is low-e, and read our fuller note on choosing rooflight glass before you commit.

Two practical effects show up once the glass is in.

• Condensation moves. Because the inner pane stays warmer, low-e glass is much less prone to misting up on the room side on a cold morning. You are more likely to see condensation form on the outside face instead, on a clear night when the sky has drawn the outer pane below the dew point. That is a sign the glass is working, not failing, and it clears as the sun comes up.
• The view keeps its colour. A plain low-e coating is close to neutral, so the sky through it looks much as it should. Add a solar-control layer for a south-facing roof and you may notice a faint tint from below, usually a soft grey or blue, most visible on a lantern where several panes are in view at once.

None of this is about the frame, the flashing or the fit. A low-e sealed unit is only as good as the installation around it, and a coating cannot rescue a rooflight set into a poorly formed upstand. If your existing glass has misted between the panes, the seal has gone and the unit is spent: that is a replacement, and the moment to reconsider the glass specification while the opening is open anyway. If the leak is at the roof rather than the glass, a roofer is the right first call.

Ask for the glass specification sheet, not just the word “low-e”. It should give you the U-value for the whole unit, and if the roof faces the sun, the G-value and the light transmittance alongside it. Confirm it is a soft-coat unit, and for a south-facing or flat roof, confirm there is a solar-control layer as well as the low-e one, because on a horizontal opening the summer sun arrives almost square on and orientation gives you little cover.

Work out roughly which way your roof faces and whether it is pitched or flat before anyone quotes, so you can tell whether the glass they are offering suits the room or just meets the minimum on paper. If you would rather hand that over, tell us the orientation and the rough opening size and we will come back with a glass specification and a price. Ring 01702 898232 or request a quote, and we will specify the light for your roof rather than sell you a standard pane and hope.