Thermal Low-E glass (short for low-emissivity), is a type of special energy efficient glass which is designed to prevent heat loss through windows. A coating on the glass creates an invisible layer which reduces the amount of heat transferred out of the window, as well as by reflecting the heat back into the room.
These panes can contribute dramatically towards savings on the heating bill, as we already discussed, and could offer the average homeowner savings of around £170 on their annual costs of heating.
Typically, most old glazing units will not contain this glass and areas such as leaking money, not to mention their contribution to your carbon footprint. The installation of Low-E glass can go a long way towards bringing down the amount of carbon you use in a year, in some cases causing a reduction of over 600kg.
So how does Low-E Glass Work?
First, we need to understand what thermal emissivity (TE) is. Consider a spectrum; on one end we have a blackbody which has a TE of 1 and on the other a perfect reflector with a TE of 0. Everything on this planet will fall into this spectrum, dependent upon the amount of heat it either reflects, absorbs or emits. Take a brick, for instance, whose TE is 0.9. We consider that it absorbs and emits about 90% of the heat that it comes into contact with, whereas it reflects only 10%. We then consider the brick to have a thermal reflectance (TR) of 0.1 – as the combined value of the TE and TR must combine to make 1.
So from this, we can see that a brick would make a terrible window, not only because you can’t see through it, but because it will mean that most of your heating will be lost. Surprisingly enough, the average uncoated window has a TE of 0.91, even more than a brick, which translates to a 91% loss of heat that reaches the window.
Low-E glazing combats the naturally high TE of glass by utilising one of two methods.
The fluorinated tin oxide is deposited on the glass during the float glass stage of manufacture, and whilst the product is at a high temperature. This is also known as an online or hard coating.
This process involves the depositing of thin layers of silver and metal oxides such as tin and zinc within the glass, by way of a very large vacuum. Also known as offline, soft or vacuum coating.
Whilst the actual creation process is incredibly complex, the science behind it is fairly simple. The inclusion of these oxides within the glass panes reduces the size of the wavelength allowed to travel through the glass. Infrared energy, which is the main transmission of thermal energy, is kept on its side of the glass, and so the room remains warm, whilst the smaller wavelength of visible light is able to pass through without any problems.
Image by Emyln