What is low-e glass?

If you have looked into replacing windows or glass doors, you have probably seen low-e glass mentioned. It sounds technical, but the idea behind it is simple, and in a Highveld home it can make a noticeable difference to how comfortable a room feels in both winter and summer.

What "low-e" means

The "e" stands for emissivity: how readily a surface gives off heat as radiation. Everything warm radiates heat, including your walls, your furniture and the people in a room. Ordinary clear glass has a high emissivity, so it absorbs that heat and passes a good share of it straight to the outside.

Low-e glass has a microscopically thin, transparent coating of metal or metal oxide on one of its surfaces. The coating reflects much of that heat back towards where it came from, while still letting visible light through. To the eye, low-e glass looks almost exactly like clear glass.

Two kinds of coating

Low-e coatings come in two broad families.

• Passive low-e (hard coat). The coating is applied while the glass is still being made, so it is fused to the surface and very durable. It lets in a good amount of the sun's warmth and is designed mainly to keep heat inside. It suits rooms you want to hold their warmth on cold nights.
• Solar control low-e (soft coat). The coating is applied after the glass is made, in a vacuum chamber, in several very thin layers. It reflects more of the sun's heat before it gets in, so it suits rooms that overheat in summer. Soft coats are more delicate and are usually placed on a protected face, for example inside a laminated pane or a double-glazed unit.

Neither is simply better than the other. The right choice depends on the room, how it is used and which way it faces.

The lit-match test

You can often tell whether a pane already has a low-e coating using a match or a lighter. Hold the flame close to the glass and look at the reflections. Clear glass shows two reflections of the flame, one from each surface, and both are the same colour. On low-e glass, one of the reflections is a slightly different colour, often with a pinkish or bluish tint, because it is bouncing off the coating. It is a quick check rather than a laboratory test, but it is useful when you are not sure what is already in a frame.

Where it helps in Highveld homes

The Highveld climate makes a good case for low-e glass. Winter nights are cold and clear, and heat escapes quickly through large areas of glass. Summer afternoons are hot, with strong sun on north- and west-facing windows.

• Living rooms with big sliding or folding doors lose a lot of heat through the glass at night. A passive coating helps keep that warmth in the room.
• West-facing rooms that bake in the afternoon benefit from solar control glass that reflects part of the heat before it gets inside.
• Bedrooms feel less chilly close to the window on winter mornings.
• Furnishings near large windows can fade more slowly, because some coatings also cut part of the sun's ultraviolet light.

How much difference it makes depends on the size of the glass, the frame, how well the window seals and which way it faces. It is best thought of as one part of the picture rather than a fix on its own.

Things to know before you choose it

• Low-e glass is a premium product compared with clear float glass, so it makes the most sense on the largest panes and the rooms where comfort matters most.
• Soft coats have to be glazed the right way round, with the coating on the protected face.
• Some coatings have a very slight tint or sheen, so if you are replacing one pane in a row of windows, it is worth matching the others.
• The frame matters as much as the glass. Draughty seals or a badly fitted frame let heat through however good the glass is.

Ask about it when you quote

If you are thinking about low-e glass for new windows or doors, mention it when you request a quote and tell us which way the rooms face. Our aluminium windows page covers the frames and window types, and you can request a free quote here.