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Understanding Low-E Glass for Windows

On this page:
  • How does Low-E glass work?
  • Understand UV rays, infrared and visible light spectrum
  • Know about types of Low-E glass

What is Low-E Glass?

Low-E is the short-hand term for “low emissivity.” Low emissivity glass helps reduce the amount of infrared and ultraviolet light that flows through your windows. They do this through a microscopic, transparent coating of metallic oxides that are applied to a glass pane. Natural light still comes through the windows, unimpeded, so you never notice a difference in the aesthetic value of all that light. 

double pane windows

However, that tiny coating protects your furniture and carpets from fading and helps protect you from the harmful UV rays. It also helps reflect the interior temperatures back into your home, instead of letting the warm air flow out through the window. It does this by controlling the infrared light; it keeps your house warmer in the winter and cooler in the summer.

About The Different Types of Light

To fully understand how low-e glass works, it is important to understand the different types of light that come through those windows.

Low E Glass reflects UV and infrared rays - Mo

Ultraviolet, or UV light

This is what causes sunburn on our skin. It also affects materials in the home, such as carpets, furniture, and window treatments. Over time, it can fade the colors. You can not actually see this light. It occupies wavelengths of 310-380 nanometers on the light spectrum.

Visible light

This is what you can actually see, occupying the 380-780 nanometers of the spectrum. This is the light you want to let into the house.

Infrared light

This is another type of light that we can’t see. It begins at 780 nanometers. This is the part of the light spectrum that brings heat with it. Because of this, it might be referred to as “heat energy.” 

To go even further, the heat energy can be either long-wave or short-wave. While visible light is considered short-wave (or short wavelength) which is the radiant sunlight we see in the daytime, the infrared light is the long-wave which consists of heat radiation formed in Earth’s atmosphere.

Low-e glass is designed to block the ultraviolet and infrared light, but allow the visible light to shine through unimpeded.

Low-E Glass Cost

Low-e glass will cost you more than single and doubled paned windows as it adds the microscopic layer to the glass. You can expect to pay $350 to $850 per window for low-e glass installations. Compare the cost of different glass types here.

Types of Low-E Glass

There are different types of coatings on the glass, creating a few different types of low-e glass for windows.

Solar Control Low-E

The solar control or “soft” coating is always on the interior, as it can not withstand the elements and needs the controlled environment of an interior to work properly. It blocks the solar radiation coming from the outside to reduce cooling costs of your home.

Passive Low-E

The passive or “hard” coating is applied to the outside surface of the window, as it can withstand the elements. A very thin coating, more like a sheet of metallic oxide, can also be found between the window panes. The sheet does not just serve to create a low-e window; it also serves as thermal insulation to further protect against heat loss.

In addition, adding argon gas into the space with the coating can improve the R-value of the window. (The R-value gives you an idea of how energy-efficient the window is. The higher the value, the better the insulation.)

Which Type of Low-E Windows Are Right for You?

Remember, low-e windows reflect the interior heat back inside during the winter while also reflecting the long-wave solar energy from the outside. Some  low-e coatings reflect the short-wave energy, too. How well it does all this depends upon the climate you’re in.

If you are in a cold climate, the hard-coat glass is a better option, because it does allow the short-wave infrared to pass through. This allows for better heating in the winter while still reflecting the interior heat back inside.

If you live in a warm or hot climate, the soft-coat glass is great. It has better UV protection. It still reflects the heat back into the interior, keeping you warmer during the winter, but it also reflects the cool air, leaving you cooler in the hotter parts of the year.

Low-E Coatings Matter, But That’s Not All

According to Energy.gov, up to 30% of heat in your home during the winter flows out through the windows, and during the summer, about 76% of the sunlight that falls on the windows enters in the form of heat. 

Though the low-e coating you choose for your windows matters a great deal, keep in mind that it’s only part of the larger system that makes up your windows. The coatings don’t matter one bit if the window is drafty, does not have enough insulation, or is installed improperly. When it comes to energy-efficiency and comfort, double-pane windows are better than single-pane, and triple-pane windows are even better than double-pane. There should be argon or krypton gas between the panes to further insulate. The windows frames themselves should be sturdy, well-constructed, and created with techniques that take thermal efficiency into account.

Do You Already Have Low-E Coatings on Your Windows?

Though it might be a selling point for a home on the market, sometimes homeowners have no idea whether their windows are low-e or not, and so they don’t necessarily put it on the disclosure. To be sure, try this trick from Vitro Architectural Glass.

Let’s assume you have double-pane windows. Stand in front of your window at night. Hold a lit match or a pen light up to the window. You  should see four reflections of the light, as there are four glass surfaces of the double-pane window. Look at the reflections carefully. If you have low-e glass, one of those four reflected images will be a different color. It might be slight, but noticeable.

Find the Right Contractor

When you are looking for a window contractor, make sure they are well-vetted, with great references and a strong track record. They should be quite knowledgeable about low-e windows – and any other window concerns, for that matter. Modernize can put you in contact with the window professionals who can make your home look great as they install the best energy-efficient products.