Low-E (Low-emissivity) Glass Explained

What Is Low-E Glass?

Low-emissivity, or low-e glass, as it's better known, is a glass that has a near-invisible metal-based coating.

A low-e coating is a layer of silver and metals so thin it can't be seen and can be compared to the effect of placing a penny coin on the top of the Shard building in London.

The lower emissivity means the window can reflect back more thermal energy and reduces heat loss from a room in comparison to standard glass.

How Does Low-E Glass Work?

All materials absorb energy and then reradiate that energy as radiant heat. Think about a brick wall that has been exposed to sunlight all day and how you can feel the heat radiate from the bricks.

Emissivity is the measure of the ratio of heat emitted from a material in comparison to a perfect black material. The scale is rated between zero and one (1 represents black): an emissivity value of '1' absorbs and retains heat and a value of '0' reflects energy.

Source

The measured value is a ratio, which means that for standard glass that has a rating of 0.91, the glass absorbs 91% of thermal energy and reflects only 9% of radiant energy.

The low emissivity rating of glass means that it's susceptible to absorbing a lot of thermal energy. That thermal energy can then be transferred and reradiated on the other side of the glass and heat is lost from the room.

Silver has a thermal emissivity rating of 0.02 which means that it only absorbs 2% and reflects 98% of radiant energy.

This is why aluminium blankets are used for survival because they make full benefit of all heat being generated by reradiating it back to the person that generated it in the first place. The aluminium layer stops any thermal energy from being lost and creates an insulted environment that maximises all thermal energy available. In a similar principle, aluminium foil is often used behind radiators to avoid the energy being absorbed into the wall and is instead reflected back into a room to maximise efficiency.

So, imagine if we coated our windows with a layer of aluminium foil or silver, it would reduce the transfer of thermal energy from 90% to 2% and keep our room super warm. But, we wouldn't be able to see through the glass.

Low-e glass coating was developed to take advantage of these properties. By applying a microscopic coating made of silver and metals to glass we can change the emissivity rating of the glass to a much more efficient rate similar to the metals. Energy is reflected away from the glass and the transfer of energy is reduced.

There are different types of low-e composed of different coatings and applications to the glass.

Types of Low-E Glass

There are two types of low-e coating:

• Passive low-e coating
  Designed to optimize energy efficiency by reducing the amount of heat that is lost through a window. Passive low-e glass has a higher emissivity rating than solar low-e glass, which means it is better at reflecting heat back into a room. It is typically used in colder climates where reducing heat loss is a priority.
  
  
• Solar control low-e coating
  Designed to reduce the amount of heat that enters a building from outside, making it ideal for use in warmer climates. It has a lower emissivity rating than passive low-e glass, which means it is better at allowing solar radiation to pass through the glass while reflecting back a smaller amount of the radiant heat from the sun. By reducing the amount of heat that enters a building, solar low-e glass can help reduce cooling costs and improve comfort levels inside the building.

Low-E Glass and UV Light

When light passes through a window it's comprised of:

• Ultraviolet light - This causes sun scorching and bleaching of fabrics and furnishings.
  Read more about UV light and glass.
  
  
• Infrared light - This is the thermal energy from the sun measured in short wavelengths.
  Read more about solar gain.
  
  
• Visible light - This is how much light we can actually see.

Low-e glass is designed predominantly to reduce the transfer of infrared light (thermal energy). However, some low-e coatings can reduce UV light alongside infrared light whilst allowing the maximum amount of visible light.

The most efficient glass for UV reduction is laminated glass and Everest's laminated double-glazed unit comprises low-e glass and a laminated layer for the maximum benefit of UV reduction and thermal retention.

How Low-E Glass Is Made

There are two main methods of manufacture for low-e glass:

Hard coat

Also known as pyrolytic coating, the hard coating was introduced in the early 1970s and is most often used in commercial buildings.

The coating is applied directly to the surface of the glass ribbon while it is still in a molten state during the manufacturing process. The 'hard coating' is fused to the glass surface, which makes it more durable for glass processing and the sheets are cut into size after the coating has been applied.

Hard coating is more durable and resistant to scratching, abrasion, and other types of wear and tear. It's generally used where durability is needed in high levels of exposure to wind, rain, and other weather conditions.

Soft coat

Also known as Magnetron Sputter Vacuum Deposition (MSVD) was introduced in the 1980s and is most commonly used for domestic windows. The soft coat is applied to pre-cut sheets of glass in a vacuum chamber at room temperature. Metal oxides are applied in a microscopically thin layer to standard glass via a magnetically enhanced cathodic sputtering process.

Because the coating is not bonded to the glass it's applied to the internal panes of double-glazed and triple-glazed units to avoid damage.

Soft coat low-e glass has a higher level of energy efficiency compared to hard coat low-e glass.

Low-E Glass Benefits

Improved Insulation and Energy Efficiency

The main benefit of low-e glass is energy efficiency and its ability to reduce the thermal transfer of energy. In winter heat is retained in a room and in summer the heat from outside is left outside and not absorbed into the room.

In the UK, low-e glass is predominantly used in colder rooms for its superior insulation and heat retention in a room. For north-facing rooms that don't benefit from natural direct sunlight and solar gain, low-e glass insulates a room to avoid the loss of warmth.

Reducing the amount of energy used to heat a home contributes to reducing climate change. With high energy prices, it also helps to keep energy bills as low as possible.

Reduced Overheating from Direct Sunlight

Low-e glass will also reduce the amount of infrared thermal energy from direct sunlight. In reverse to retaining heat generated in a room, in summer months we need to reduce the amount of heat that transfers from the outside to the inside.

The emissivity of the glass works exactly the same way for the infrared energy from the sun as for heat generated in the room.

Think about silver reflective sunscreens that are used in car windscreens - they are designed to reflect the most amount of heat from the sun. Low-e glass works in exactly the same way to reduce overheating in a room.

Some types of solar control low-e coating are also designed to reduce the amount of ultraviolet light that can pass through the glass. But laminated glass combined with low-e glass provides the best results to reduce excessive UV photodegradation.

Disadvantages of Low-E Glass

Natural Light Reduction

You should be aware that the microscopic low-e coating does minimally reduce the amount of natural light that can pass through the glass.

The benefits you get with increased energy efficiency offset any subtle decrease in the brightness of light.

On the flip side, the glare reduction from natural light is why many people prefer low-e glass, especially for conservatories or in large south facing windows. So, natural light reduction is both a benefit and a disadvantage.

If you want to maximise the amount of light that does pass through your window then low iron glass would be your best option. But low iron glass does not offer any thermal advantages and has an emissivity rating the same as standard glass.