Low-E Glass: Working Principle, Properties, and Advantages

The metal oxide coating on low-e glass is as thin as a strand of human hair.

Low-e glass is a type of modern glass developed to control heat flow across it, which is an issue with ordinary glass. Ordinary glass also poses the problem of allowing a lot of heat inside during the daytime, causing overheating, while at night it transmits all of this heat outside, leading to undesirable cooling.

Researchers began working on this problem in the 1970s, first developing a double-pane glass, with an insulated air space between the panes that provides a barrier to the transfer of heat. Such developments culminated in the invention of low-e glass in 1979.

The term 'low-e' is a short form for 'low emissivity'. Emissivity measures the ability of a substance to radiate heat energy. So, low-e glass has a tendency to emit lesser heat than conventional glass. This is possible because of a thin coating of metal oxide present on the glass.

While it does not completely block all radiation, certain wavelengths responsible for the heating effect are hindered. Let us see more about the working, properties, and benefits of low-e glass.

It works on the principle of allowing selective wavelengths of light to pass through it. Sunlight consists of visible light which helps us see things, UV light and infrared light, which are responsible for its heating effect.

Hot objects radiate longwave infrared light, while colder objects emit shortwave infrared light. Low-e glass works by blocking or reflecting longwave radiation, while allowing shortwave light to pass through.

In the summer, or in warmer regions, things outside the window, like bushes and sidewalks absorb heat from sunlight and get warmer than indoor objects. Being warmer, they emit longwave radiation which is blocked by the glass, hence keeping the room cool.

In winter, or in cooler regions, indoor objects like fireplaces, furnaces, heaters, and even our bodies are warmer than the outdoor temperature. They emit longwave light which is blocked by the low-e glass, thus effectively keeping the heat in the room itself.

✤ Low-e glass contains a thin, microscopic layer, or layers of metallic oxide particles applied on the surface of the glass pane, which, if properly fabricated, appears completely transparent. It is generally applied on multi-pane glass windows, with an inert gas like argon between the panes.

✤ Low-e glass does not work by literally 'reflecting' light. When light falls on any object, it gets absorbed, and the object then emits its own radiation. Low-e glass is capable of absorbing longwave infrared radiation, without emitting it, thus blocking the heat.

✤ The surface on which the metallic oxide coating is applied depends on the climate of the region and the reason for using the glass. In warmer areas, where the glass is intended to keep out heat, the coating is applied on the inner surface of the outer pane.

In cooler areas, where the room is to be kept warmer, the coating is applied to the outer surface of the inner pane.

✤ This type of glass also prevents heat loss by convection, in which heat flows from a warmer to a cooler area. This works by keeping the outer surface of glass warmer during summer, preventing heat from being transferred indoors. In winter, the inner surface of glass is warmer, which restricts heat from escaping outside.

✤ There are two types of low-e glass - hard-coat and soft-coat. Hard-coat low-e glass is manufactured by applying a layer of tin oxide while the glass is still hot, effectively welding it to the glass.

Soft-coat low-e glass is produced by exposing the glass to electricity in a vacuum, which causes 'sputtering' of silver enclosed in layers of metallic oxide on the glass surface. Hard-coat is common, more durable, and cheaper than soft-coat, and can only be used in multi-pane windows, because it gets oxidized when exposed to air, though it is more efficient.

✤ The rate at which heat is transferred by glass is indicated by its U-value. Low-e glass has a low U-value, which means it allows a less amount of heat to be transferred. Hard-coat glass has a higher U-value than soft-coat glass, which makes the latter more preferable for energy conservation.

✤ Despite blocking longwave heat from outside, low-e glass allows valuable solar light and heat to pass into the room. The Solar Heat Gain Coefficient (SHGC) measures this ability of glass to allow solar heat gain.

Low-e glass has a higher SHGC value, which means, it is more energy efficient by allowing solar heat to pass inside during winters, which is then trapped indoors by the glass, keeping the room warm.

✤ Low-e glass allows sunlight to enter the room, while keeping out heat in summer, and restraining the heat indoors in winter.

✤ It has a high level of energy efficiency, being able to filter out 40 to 70% of heat, that would normally be allowed to pass through by ordinary glass.

✤ This glass helps keep out ultraviolet and infrared waves that can damage human skin, carpets, drapes, and artwork, besides causing fading of color.

✤ It restrains around 40 - 60% of heat energy, which would have been lost by ordinary glass, and helps lower heating and cooling costs, which account for 4% of the total annual US energy consumption.

✤ The conservation of energy makes this glass environment-friendly, by reducing dependence on polluting technologies.

✤ It offers a high comfort level to house occupants. Soft-coat maintains a warmer temperature of 62ºF, when it is 0ºF outside, while ordinary single-pane and double-pane glass would be at 26ºF and 35ºF, respectively.

✤ The problem of condensation on ordinary glass panes during winters is not experienced with low-e glass, as its inner surface is warmer, preventing condensation of water vapor.

✤ Installing low-e glass in windows makes one eligible for a tax rebate of about $1,500, depending on local laws, which is applicable for doors and windows that meet Energy Star requirements.

✤ Hard-coat low-e glass is durable, and can last for 10 - 15 years without any damage.

While low-e glass is an important tool in energy conservation, it does have its downsides, though somewhat negligible. Due to the high amount of heat radiation reflected by this glass, it may damage vinyl sidings of buildings, and vehicle plastic parts by causing them to melt.

Its cost is also 15% higher than conventional glass, though its energy savings in the long run makes this difference negligible.