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.
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.