What Is Low-E Glass on a Rooflight?

What the letters mean

Emissivity describes how readily a surface gives off heat as radiation. Ordinary glass gives it off easily. A low-emissivity surface does not, and instead reflects that heat back. Low-E glass is simply ordinary glass with a coating that behaves this way.

How the coating works

Heat leaves a warm room in three ways: conduction through materials, convection in moving air and radiation, which is energy travelling as invisible infrared. The gas fill and the spacer deal with the first two. The coating deals with the third. It is so thin that it cannot be seen or felt, yet it reflects much of the infrared back into the room while visible light passes through.

The effect is easy to picture on a cold night. Without the coating, warmth radiates out through the pane. With it, a good part of that warmth is turned back.

Where it sits in the unit

The coating is applied to a surface that faces the sealed gap, where it is protected from cleaning and weather. In a double glazed unit, the usual position for heat retention is on the face of the inner pane that looks into the gap. A triple glazed unit can carry two coated surfaces. Because it is inside the sealed unit, you cannot scratch it off, and it should last as long as the unit does.

What it does for a rooflight

A rooflight looks at the open sky, which is colder than the air around it at night, so radiation is a large part of its heat loss. That makes the coating particularly useful here. It is one reason a modern unit can reach a far lower U-value than old glass, and you can read how the figure is built up in rooflight U-values explained. It helps with the question many people ask, whether rooflights are cold in winter.

Is it worth having?

For nearly every house, yes. A coating adds little to the cost of a unit and it lowers heat loss every winter night for the life of the glass. Few homeowners would choose an uncoated rooflight once they understand the difference, and current limits for new and replacement units are met by coated glazing in practice. Fitting one is more of a default than an upgrade.

Cleaning and care

Because the coating sits inside the sealed unit, ordinary cleaning cannot harm it. Wash the outer and inner faces with water and a soft cloth or a squeegee, as you would any window. Avoid scraping at stubborn marks with a blade, which can scratch the exposed glass surface. The coating itself needs no special treatment, and you should never need to think about it again once the unit is in.

Does it make the room darker?

A little, and only slightly. Low-E coatings take a small amount of light out of the glass and can give a faint tint to the view, which you tend to notice only when comparing two panes side by side. In a room that is otherwise dark, the daylight you gain from a rooflight far outweighs that small loss.

How to see it for yourself

Hold a lit match or a phone torch beside the glass in a dim room and look at the reflections. A plain pane gives two images, one from each surface. A coated pane gives an extra, slightly different coloured image, and the tint of that reflection is the coating. Installers use the same trick to check which way round a unit has been fitted. It is a harmless test, though you only need it if you are curious or comparing two samples.

Low-E and the gas between the panes

The coating and the gas fill are partners. Argon is heavier than air and moves heat less readily, so the gap between the panes resists convection better. With the coating turning radiant heat back, the pair does a lot more than either alone. That is why a coated, gas-filled unit beats an uncoated, air-filled one by a clear margin, even when the glass thickness and the gap width are the same.

If a unit is old and the gas has leaked out over time, some of that advantage is lost, and it may be time for a new one.

Low-E and solar control are different

People often assume they are the same. They are not. A low-E coating is tuned to keep indoor heat in. Solar control coatings are tuned to reflect the sun's energy before it enters, for rooms that overheat. Some units combine them. If summer heat is your worry, read solar control glass for rooflights and our note on whether rooflights make rooms hot in summer.

Reading the specification

When a unit is described as 4-16-4 low-E argon, that means two glass panes with a 16mm gap between them, a low-E coating and argon gas in the gap. The warm-edge spacer should be listed too, and it is explained in what a warm-edge spacer is. When you read our quote, the glazing appears as its own entry, which lets you set one offer beside another fairly. See also energy efficient rooflights.