Energy efficient windows are one of the essentials when updating your home. Windows have improved over time in terms of thermal and solar performance. One of these improvements is the low-e glass coating. But what is this coating? Let’s find out.
The Different Types of Light
The solar energy spectrum can be divided into ultraviolet light, infrared light, and visible light. Ultraviolet is the light responsible for the fading which occurs on fabrics, walls, and other interior materials. As the name suggests, visible light is the light that we are able to see with our own eyes. Infrared light is invisible light. We are not able to see it with our human eyes, but we can sometimes feel it on our skin as heat. Infrared is emitted by both the sun and warm objects.
Generally, people don’t want to block out the visible light that comes through the window, but we do want to block ultraviolet and infrared lights. Low-e glass is designed for just this purpose. The coatings on low-e glass reduce the amount of infrared and ultraviolet light that can pass through a window without reducing the amount of visible light.
The ability of a material to emit infrared light (heat) is called emissivity. Shiny, reflective materials have low emissivity, while matte, dark colored materials have a high emissivity. Uncoated glass will absorb heat. This heat is then either radiated further or carried by air that it comes in contact with. This is where low-e coatings come in. Uncoated glass will have a higher emissivity, which means it will radiate heat. Low-e coated glass has a lower emissivity. The lower the emissivity, the less heat will pass through the window.
Low-e coatings are really thin metal or metallic oxide layers applied to the surface of the glass. Generally much thinner than a human hair, they are able to reflect both longwave and shortwave infrared energy. This makes them effective for use in both warm and cold climates. In warm climates, it protects against the sun’s infrared energy. In cold climates, it reflects the heat from the the interior of the home back inside.
Two Types of Coatings
There are two different types of low-e coatings, pyrolytic and sputtered coatings. Pyrolytic low-e coatings are applied to the glass during the glass production. This is also known as a hard-coat because it is fused to the glass surface and very durable. Pyrolytic coatings are able to allow some of the sun’s infrared energy to pass through the window while reflecting the radiant energy back into the home.
Sputtered low-e glass coatings, on the other hand, are applied after the glass is made. With this process, the coating is applied to the glass in a vacuum chamber. The metal is vaporized and then deposited onto the surface of the glass to form a thin layer. There can be multiple layers of sputtered coatings but they are still practically invisible to the naked eye. This process results in glass that has superior solar control performance and a lower emissivity.
Low-e glass can have coatings applied to both sides of the glass. In the case of double or triple pane windows, all of the surfaces of glass can be coated to increase the energy efficiency. For example, in a double-pane window, the coating can be applied to the surface of the glass which faces the outdoors, the two glass surfaces which face each other and to the glass surface which faces indoors.
Measuring The Effectiveness of Low-E Glass
There are four units which are used to measure the performance of a low-e coating. U-factor measures how much heat loss the window allows. The lower the U-factor, the better the coating. Visible Light Transmittance (VT) measures how much visible light can pass through the window. You want the VT measurement to be high. Solar Heat Gain Coefficient (SHGC) is a fraction which measures the solar radiation that passes through the window. This number measures both the heat that is transmitted directly and the heat that is absorbed and radiated into the home. As with the U-factor, the lower this coefficient is, the better.
Is Low-E Glass Worth The Investment?
Low-e glass coatings can dramatically improve the performance of a window. They can have a significant impact on the cooling, heating and lighting costs of a building, which is why we recommend this extra investment when purchasing insulated windows.