Low E Glass Types Explained

There may be no building material more versatile than glass. It can truly do extraordinary things, and the glass being manufactured today is infinitely more advanced than glass that was made just a few short years ago. Of all the glass options on the market today, low-e glass has perhaps the greatest potential to transform your home's performance.  

If you've thought about investing in new windows, then you've probably heard a lot about low-e glass. Chances are, you have some questions about it as well. So let's take a deep dive into the world of low-e glass—how it works, what types are available, and how it can dramatically improve the energy efficiency of your home. 

What Is Low-E Glass?

From the beginning: the "E" is for emissivity. When heat or light energy is absorbed by a surface, such as glass, it's either reflected off the surface, or radiated through the surface. A material's ability to radiate energy is known as emissivity. What does that have to do with your windows? If you've ever been in a greenhouse, you probably already know the answer to that question. 

Radiating energy is one of the leading causes of heat transfer in window glass. emissivity (or low-e) windows radiate less energy, and therefore transmit less heat, which gives them greatly enhanced insulating properties. When heat energy—either from the sun or from your HVAC—strikes a low-e window pane, it is reflected back into the space it came from instead of being transferred through the window. 

How Low-E Glass Works

The secret to the remarkable insulating qualities of low-e windows is the thin metallic coating on the surface of the glass. Low-e windows often appear tinted, but low-e glass is not the same as tinted glass. Whereas tinted glass is made by adding alloying materials into the glass itself,  low-e glass has a microscopically thin layer—often multiple layers—of various metallic particles on the surface. These layers turn the glass into something like a filter, or a sieve. 

You see, different types of energy have different wavelengths. Infrared, for example, has a long wavelength, while the visible light spectrum has a relatively short wavelength. Different metals will "filter out" different wavelengths, so by adding thin layers of various metals onto the surface of the glass, it's possible to choose what types of energy get through.

If you want to filter out long wavelengths (infrared, i.e. heat) while allowing shorter wavelengths (light) to pass right through, low-e glass can do that. Most low-e glass also limits the amount of ultraviolet radiation that is allowed to pass through, which is beneficial both for the health of those who live in the home, as well as for the longevity of your furnishings. 

Types of Low-E Glass

Several different metals are used to create the micro-thin layer on the surface of low-e glass. Tin, silver and zinc are some of the most common, and many types of low-e glass utilize a combination of different metal layers. Still, with all the possible combinations, there are two basic types of low-e glass coatings: passive low-e coatings, and solar control low-e coatings. There's a crucial difference between the two. 

Passive low-e coatings are designed with the purpose of maximizing solar heat that passes into a building. This creates what's known as a “passive” heating effect, which reduces the amount you have to rely on artificial heating. Windows with passive low-e coatings are ideal for colder climates, as they can significantly reduce the cost of heating your home.
Solar control low-e coatings limit the amount of solar gain into a building. The purpose of of solar control low-e windows is to keep buildings cooler, and to reduce the amount of energy used by air conditioning. They are ideal for hot climates, and for homes and buildings that receive a lot of direct sunlight.

How Low-E Glass is Made

You may see low-e windows categorized as either "hard coat" or "soft coat." This is another way to differentiate between different types of low-e glass, but it has more to do with how the glass is made. Both passive and solar control low-e glass can be made through either a hard coat or soft coat process. Here's how they're different:

Hard coat low-e glass is manufactured trough what is known as a pyrolytic process. With this method, a thin layer of molten metal is applied to a sheet of glass during the manufacturing process, while the glass is still slightly molten. This causes the metal to essentially become welded to the glass, creating a very strong (i.e. "hard") bond. 
Soft coat low-e glass is made through a process known as Magnetron Sputter Vacuum Deposition (MSVD). That's a complicated way of saying that with this method, the particles that make up the metal layer are applied to pre-cut sheets of glass, at room temperature. This is done in a vacuum-sealed chamber filled with an electrically-charged inert gas. The resulting metal coating is extremely thin and somewhat delicate (i.e. "soft").

Choosing Low-E Glass

Homeowners have a lot to consider when choosing new low-e replacement windows for their home. The most crucial consideration is choosing the right windows for your climate, depending on whether you're most concerned about keeping your home cool during warm weather, or keeping it warm in the winter. The right combination of windows, when installed correctly, can do both.

While glass with a passive low-e coating is great for northern homes, and solar control low-e windows are great for hot southern locations, there's still a lot of grey area in which it can be challenging to choose. In these cases, it's helpful to have a knowledgeable glass professional to help guide you through the selection process. It's also important to know some of the terminology that manufacturers often use to grade their low-e windows:

U-Factor Values represent how well a window prevents heat from escaping a building (a lower u-factor indicates the glass is better at keeping heat in).
Solar Heat Gain Coefficient (SHGC) measures how well the glass blocks heat from the sun from entering the building (a lower SHGC indicates that a window is better at blocking heat gain).
Visible Transmittance (VT) measures how much light the glass allows to pass through (higher visible transmittance means more light gets through).

Low-E Glass Manufacturers

The glass industry is dominated by two cutting-edge manufacturers, Cardinal Glass Industries and Guardian Glass North America. These two companies supply the majority of window companies with high-performing glass packages.

Ready to learn more about low-e window glass? Contact us today to talk to our team about finding the perfect windows to maximize the comfort and energy-efficiency of your home. 