As a homeowner shopping for replacement windows, you might find yourself overwhelmed by a flood of customization choices. From style, to frame color, to grids, it can be challenging to figure out which decisions are the most critical. For most homeowners, the most important decision is that of glass type. This is especially true if you’re concerned about energy efficiency (and who isn’t?), as 70% of energy loss in the home is due to windows and doors. Low-E is an innovative solution that improves energy efficiency in the home. 

To understand the benefits of Low-E glass windows and what makes them energy efficient, we’ll cover:

  • What Low-E glass is and how it works
  • Low-E coating performance indicators
  • Types of Low-E glass coating
  • Which Low-E glass window coating is best for your climate
  • How to tell if your existing windows have Low-E glass

 

 

What are Low-E glass windows & what makes them energy efficient?

Put simply, Low-E glass keeps unwanted light rays from entering the home, while preventing interior heat and air conditioning from exiting. It does all of this without blocking natural light from passing through.

To really understand how this energy efficient window coating works, we need to first explore the solar energy spectrum and the types of light involved. The three important light rays we’ll discuss are ultraviolet (UV) light, visible light, and infrared light. These light rays all occupy different parts of the solar energy spectrum, and are differentiated by their respective wavelengths. 

  • UV light (300-380 nanometers)
    • UV light fades interior fabrics and wallpaper
  • Visible light (380-780 nanometers)
    • The natural light you want in your home
  • Infrared light (780+ nanometers)
    • Solar heat that enters a building

While this may seem like an unasked for science lesson, the knowledge of these three light types is critical to understanding the benefits of Low-E windows. Another key concept is emissivity, which is how well a material radiates energy. When it comes to windows, the lower the emissivity, the higher the energy efficiency. That’s where low emissivity (or Low-E) coating factors in.

This coating, which is thinner than a strand of hair, limits the emissivity of the glass. Low-E glass prevents ultraviolet and infrared light from passing through the glass, into the home. At the same time, it maintains a stable temperature in your home by reflecting interior heat energy (whether it’s cool or warm), back inside. 

How Low-E glass works

All of these factors make these windows extremely energy efficient. For one, they prevent solar heat from entering the home, which means less of a need for air conditioning in the warmer months. Additionally, the reflective insulation properties cause them to radiate interior temperatures, whether cool or hot, back into the home. This means your house stays cool in the summer, and there is no need to blast your heater in the winter months. 

In summary, the benefits of Low-E glass windows are:
  • Keep UV rays from entering your home, which prevents damage to fabrics, furniture and your skin
  • Prevent solar heat from passing through, keeping your home cool in the warm months
  • Maintain a stable interior temperature by reflecting heat back into your home

 

Low-E Coating Performance Indicators

There are four different measures that indicate the effectiveness of Low-E glass, according to Vitro Architectural Glass:

  • Visible Light Transmittance: measures the amount of visible light that passes through the glass.
  • U-Value: rating based on a measure of how much heat loss the glass allows.
  • Solar Heat Gain Coefficient: measures how much solar heat the glass transmits. The higher the solar heat gain coefficient, the more solar heat it allows through. 
  • Light to Solar Gain: ratio measure comparing the Solar Heat Gain Coefficient to the Visible Light Transmittance of the glass.

 

Types of Low-E Glass Coating

There are two variations of Low-E glass coating, known as passive Low-E and solar control Low-E:

  • Passive Low-E: This type is also known as a hard-coat, due to the manufacturing process which fuses the coating to the glass surface. This coating actually maximizes the amount of solar heat (or infrared light) that enters the home, thereby passively heating the home. This allows homeowners to rely less on artificial interior heating. 
  • Solar Control Low-E: Also known as soft-coat, solar control achieves almost the exact opposite goal as compared to the passive coating. This coating prevents solar heat from passing through the glass, keeping spaces cooler and limiting the need for artificial cooling. 

 

Which Low-E glass coating type is best for my climate?

After covering the two types of Low-E coating, you’re likely wondering which is the right choice for your home. The most influential factor in making this decision is your local climate. 

Let’s explore which coating is best for each climate:

  • Solar Control Low-E glass is best you live in an area which experiences both cold and hot temperatures. This includes the majority of the United States, including the Northeast. This coating typically has a lower, and therefore better, U-Value and provides better UV protection as compared to Passive Low-E. Overall, Solar Control will help keep your home cool throughout the warmer months, whereas Passive will overheat your home throughout the summer. 
  • If you live in an area that experiences year-round cold temperatures, then Passive Low-E glass coating is the right choice. As it allowed infrared light to pass through, this glass type will passively heat your home, limiting the need for costly artificial heating. 

 

Do my windows have Low-E glass coating?

It’s been over 35 years since this coating revolutionized the window industry, so there’s a chance that your home already has Low-E windows. To determine if this is the case, you can do the following:

  • Hold a light or match up to the window. If you have double pane windows, you will see four reflections of the light.
  • If all four light reflections are the same color, then your window does not have a Low-E coating.
  • If one of the reflections is a different color, then your window does have a Low-E glass coating.

 

If you’re starting to explore a window replacement, and the many options available to you, it’s important to recognize the important of the glass type you choose. Window class has a major impact on the energy efficiency on your entire home. While Low-E coated windows are often slightly more expensive, the potential energy savings that accompany them make the investment worthwhile.