How Does a Window Qualify for an ENERGY STAR Rating?

ENERGY STAR-certified windows are considered the industry’s highest rating for top-performing energy-efficient windows. When you choose ENERGY STAR certified windows for your replacement windows, you’ll see benefits, from saving on energy bills and preserving your home’s interiors to making your home more valuable on the market. The benefits might be obvious, but how is an ENERGY STAR rating determined? What is the ENERGY STAR rating based on for replacement windows?

What Is ENERGY STAR?

ENERGY STAR ratings are determined by the government and to become certified, a window must meet a list of requirements. Requirements vary depending on the region; so, a window in the South will have different requirements than a window in the North. When a window has the ENERGY STAR label on a product, this means that the window is certified energy-efficient. 

ENERGY STAR determines its ratings based on information from the National Fenestration Rating Council (NFRC), which tests windows in five different categories across four different U.S. regions. The performance ratings are based on U-factor, a window’s solar heat gain coefficient (SHGC), air leakage, visible transmittance, and condensation resistance. The value of the scores depends on your region (Northern, North Central, South Central, and Southern). 

Put simply, having an ENERGY STAR label on a window tells you that it's energy-efficient, while the NFRC label breaks down why giving you the ratings across all categories. It’s important to note that ENERGY STAR only uses the U-factor and SHGC to make its decision but you should still consider all of the ratings to choose the best windows for your home. 

What Are the Different Performance Ratings and What Do They Measure?

U-Factor

What is the U-factor in windows? U-factor tells you how well the window performs as an insulator by measuring how much heat the window allows to escape. Values will typically range from 0.25 to 1.25. The lower the score, the better the U-factor and the less heat that will escape your home. Ideally, your window will have a rating below 0.3 to perform well. 

Solar Heat Gain Coefficient (SHGC)

The SHGC measures how well a window blocks heat from entering your home. This number is measured on a scale from 0 to 1 and is usually between 0.25 and 0.80. 

What is considered a good SHGC score? It depends on your region. In the two Southern regions, a lower SHGC score is better for keeping out heat and reducing the amount of air conditioning you’ll need to keep your home cool. 

In the Northern regions, you’ll likely want a slightly higher score to allow heat from the sun to help warm your home during the winter, as you can see in the table below from the NFRC. In both cases, the goal is to reduce the amount of energy your HVAC system will have to use to keep your home comfortable.

Air Leakage

The Air Leakage (AL) score is exactly what it sounds like: it measures the rate of air that is allowed to leak through the window. A passable score is equal to or less than 0.3. Again, the lower the score, the better for keeping your HVAC system from working too hard and using more energy. 

Visible Transmittance

Visible Transmittance (VT) measures how much light is allowed to pass through the window and is reported using a scale of 0 to 1. Scores usually range from 0.2 to 0.8. Higher scores are better when it comes to Visible Transmittance. Better VT scores mean you could depend on natural light more during the day, reducing the amount of electricity you’ll use to artificially light your home. 

Condensation Resistance

Condensation Resistance isn’t a required performance score for window makers, but is important nonetheless, so the NFRC includes it in their testing when possible. It measures how well a window keeps the build-up of condensation at bay and reports the score on a scale from 1-100, with a higher score being more desirable. 

How Are the Most Energy-Efficient Windows Made?

Certain features in windows make them more energy-efficient. The best energy-efficient windows will have multiple panes, a Low E coating, gas fill, and ideal framing materials. But what makes those features effective?

Multiple Glass Panes

A double-pane window consists of two sheets of glass separated by a spacer and a triple-pane window consists of three. Between the layers of glass is airspace that’s filled with air or glass, which acts as insulation. Because more is better, to a degree, triple-pane windows are better insulated. Of course, this is contingent on the window being built well and not having leaks. 

Low E Coating

The Low E coating on a window is a microscopically thin layer of metallic oxide that is added to the glass surface to reduce the amount of heat that is emitted through a window. This goes for letting heat in and letting heat out. It effectively reduces the amount of energy your HVAC system uses to keep your house cool or warm, compared to windows without this coating. 

It also reduces the number of UV rays, which can discolor and fade fabrics and flooring, preserving your home’s interiors. 

Gas Fill

Gas fill-in multi-pane windows is a better insulator than air alone because gas works as a better insulator to reduce heat loss. Krypton and argon are the standard gasses used in energy-efficient windows. Krypton performs better but is more expensive, which is reflected in the price of the window. Some manufacturers will use a blend of krypton and argon to create a non-toxic combination that is effective but affordable. 

Window Framing Materials

Different framing materials will affect how energy-efficient a window will be. The best material for energy efficiency is fiberglass. Fiberglass window frames are more stable, making them structurally more likely to reduce air leakage. They also have insulated air cavities, making them thermally superior to other materials. 

Vinyl and wood are also good insulators, depending on how they’re made. Composite wood tends to be a better performer when it comes to energy efficiency. Metals, like aluminum, conduct heat and cold, making them least efficient, if at all. 

Which Window Manufacturers Produce ENERGY STAR Windows?

Different manufacturers will make windows that are best suited for different regions. Marvin is a well-known maker of highly efficient windows with different collections for different styles and functions. Marvin is also consistently listed on the ENERGY STAR “Most Efficient” list, year after year for their superior energy efficiency.  

Window and Door Showplace is a proud carrier of the Marvin windows lines. Operating in Northern Virginia, we know which windows best suit homes in the North Central region. We are also highly experienced in flawless window installation, which is a major factor in making sure these high-quality windows retain their energy efficiency. Contact Window and Door Showplace to schedule a consultation to discuss how we can make your home more energy-efficient today.