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Window Energy Performance Ratings Scale

energy scale for home energy performance ratings

Basics: Window Energy Performance Ratings In St. Louis Metro

Here in the St. Louis Metro area, window energy performance ratings tell you how energy efficient one window model is from another. The ratings give you, a homeowner, the ability to shop around and invest in products engineered for our region.

The stickers you see on certified windows are there to enable apple-to-apple comparisons of a window’s energy properties. Considering the VAST majority of us don’t know how to measure something as complex as residential energy efficiency, they really come in handy.

window energy performance rating

Who Certifies Windows?

There are two bodies in America that certify window energy performance in conjunction with other agencies like the Environmental Protection Agency (EPA).

Let’s take a look.

#1: The National Fenestration Rating Council (NFRC)

Wondering what fenestration is? When used in the context of architecture, it simply refers to the arrangement of windows and doors. The word itself stems from the Latin term ‘Fenestra,’ which stands for “opening.”

The NFRC is a non-profit that created a fair, accurate, and credible way to rate windows beginning in 1989. Their goal is to help the public become more energy efficient.

So far, it’s helping BIG TIME.

According to the Residential Energy Consumption Survey (RECS), in 2017, America reached a milestone of around 60% of all single-family homes, apartments, and mobile homes being outfitted with more efficient double or triple-pane options.

In 2020, Americans invested $4.1 billion in energy-efficient windows!

But we still have a long way to go as a nation and that’s where you come into the picture. To help you,  there’s the NFRC. How could you trust window manufacturers to tell you which is the best? The NFRC is that third-party you need to make a fair decision.

The following video was updated in the summer of 2020. It’s under two minutes long and explains the basics of who they are and why they’re important.

Their window energy performance ratings are objective, based on clear science, and independent.

That said, the NFRC system isn’t going to tell you what windows are “good” and which are “bad. That’s not what they do. They test. They set standards. They measure performance. They’re quality control set apart from window and door manufacturers.

NOTE: Under the NFRC, there is also the Efficient Windows Collaborative (EWC) who has extra tools, resources, and helpful information.

So who is responsible for communicating all the complex energy science and testing to consumers?


This is a joint program with the EPA and the U.S. Department of Energy (DOE). ENERGY STAR was created to help St. Louis Metro homeowners save money and become greener through the adoption of high-quality energy-efficient products.

Their job is to make it easy for you to identify NFRC-certified windows through their own sticker or label.

The ENERGY STAR label identifies top-performing, cost-effective products, homes, and buildings.

energy star logo

They base their window qualification primarily on U-Factor and Solar Heat Gain Coefficient ratings. When these are combined with their climate zones (where the climate is most severe), you start to get a clear picture of what these two organizations are doing.

Energy Star Climate Zones

Here in the St. Louis Metro area, we’re right on the border between the Northern and North-Central zones… which is why XteriorPRO / Window Depot USA of St. Louis windows are the absolute best!

That said, you need a thorough understanding of what the different numbers (measurements) on ENERGY STAR and NFRC ratings mean. There are five you should be aware of as a homeowner.

U-Factor (Transfer Coefficient) | Window Insulation Value

  • Base Range:                                       0.20 to 1.20
  • ENERGY STAR Minimum:             0.35
  • Recommended for St. Louis:     0.30 (equal to or less than)
  • OUR WINDOW’S U-FACTOR:       0.19 — better than the most efficient standards! 

ENERGY STAR focuses on U-Factor. This indicates the window’s effectiveness as part of your home’s overall insulation system. Rather than measuring the amount of resistance the window has to thermal energy (this would be the R-value), the U-Factor measures thermal transmission — the #1 measurement because while glass is great when it comes to insulating, it cannot resist heat.

So, if you have a window with a poor U-Factor rating, more heat escapes quicker in the winter and gets in quicker during the summer. Not good! The lower the number, the better, and the longer it takes for heat to transmit.

As an installer of ENERGY STAR windows, XteriorPRO / Window Depot USA of St. Louis understands the importance of energy efficiency. We understand that to meet our St. Louis Metro area’s criteria, your entire window’s U-Factor must be comprehensively tested: glass, frame, sashes, and over 15 other points. Less reputable companies try to skirt the rules by basing their U-Factor on testing specific components – which ENERGY STAR doesn’t certify.

Like ENERGY STAR, XteriorPRO / Window Depot USA of St. Louis only works with windows in St. Louis Metro that have proper “Whole Unit” U-Factor measurements approved by the NFRC.

A Note About Low-E Window Coatings

Simply put, modern Low-E (low emissivity) coatings are designed to further minimize the amount of UV and infrared light that can transmit through your windows. They improve U-Factor measurements.

While some coatings can impact the Visible Transmittance (VT) of light coming through your windows, they’re getting better as manufacturing improves.

There are two primary kinds of Low-E coatings:

  1. Passive Low-E: These maximize solar heat gain and thus create a passive form of heating for your home. They help reduce the amount of heat you need from other sources.
  2. Solar Control Low-E: Designed to limit solar heat gain, keeping your home cooler. Ideal for warmer climates.

Low-E also impacts the second window energy performance rating we need to discuss.

Solar Heat Gain Coefficient (SHGc) | Window Sun-Block Value

  • Base Range:                                       0 to 1.
  • ENERGY STAR Minimum:             0.42
  • Recommended for St. Louis:     0.40
  • OUR WINDOW’S SHGc:                 0.23 — better than the most efficient standards.

There’s no easier way to put this: the lower the rating, the better the window is able to block sun-generated heat. In our St. Louis Metro area, this number isn’t as critical as it is in the South-Central and Southern ENERGY STAR climate zones.

If you lived in Arizona, our windows would save a ton of money on cooling costs.

While in St. Louis, you might assume a higher SHGc number would be better for the hotter months; the U-Factor and Low-E Coatings all work together to create premium all-around windows for our area.

Let’s move on to our third important window energy performance rating.

VT (Visible Transmittance) | Window Light Value

  • Base Range:                                       0 to 1
  • ENERGY STAR Recommens:       0.51
  • Recommended for St. Louis:     0.5
  • OUR WINDOW’S VT:                       0.37 (can vary according to style & size)

A VT measurement is also easy to understand: the amount of light that comes through your window. Some windows you may want to be dimmer than others, so this measurement can be variable.

Generally, we don’t get as much sun in St. Louis Metro as in other climate zones. This is why our windows have lots of options and each can impact the VT.

Examples include:

  • Different window series; construction and design.
  • Type of glass (Low-E UV & HC) and the type of gas used between the panes – argon.
  • The level of glass strength: single, double, and triple.

If you’re the type of homeowner who wants as much natural light as possible, our mid-range VT windows are a good ‘best of both worlds’ option.

Now let’s look at our fourth important measurement. This one becomes a HUGE deal here in St. Louis Metro around Dec-Jan-Feb.

AI (Air Infiltration) & AL (Air Leakage) | Window Seal Value

  • Base Range:                                        0 to 1
  • ENERGY STAR Recommends:     0.3
  • Recommended for St. Louis:      0.3 is standard building code.
  • OUR WINDOW’S AI:                         0.05 — not 0.5, but 0.05, well below double-pane.

Obviously, you don’t want air passing through your windows when they aren’t open. You want your window’s framing and seals to remain air-tight. Did you know most air escapes at the meeting rail of your windows where the sashes meet the frame?

Unfortunately, in the vast majority of St. Louis Metro homes, a perfect seal is impossible. Even with our best-of-the-best windows on the market.

So if you go on vacation for a month and leave all the windows and doors shut tight, the air will still transfer in and out. The good news is that we can measure the amount of air that can pass through one square foot of your window area per minute.

The lower the rating the better. We improve this number through superior window construction, impeccable installation, and proper weather stripping.

As your air leakage rating gets better, your HVAC run time gets significantly less, and your home energy costs go down.

(CR) Condensation Resistance | Window Moisture Control Value

  • Base Range:                                        0-100
  • ENERGY STAR Recommends:     50
  • Recommended for St. Louis:      50
  • OUR WINDOW’S RATING:             70 — 56% better than average double-pane.

Here in our neck of the woods, condensation resistance is something you need to be aware of as a window consumer and homeowner.

The higher the better,  especially when you consider that an old wooden single-pane window might have a rating of only 15! Ratings of 70 and above are typically found on hyper-efficient, triple-pane windows like the ones we carry.

Wrap Up: Window Energy Performance Ratings Are CRITICAL!

Let’s briefly look at what we’ve covered so far. First, we talked about the two organizations that are responsible for informing you about window energy efficiency: The NFRC & ENERGY STAR in conjunction with government agencies like the EPA and DOE, along with other non-profits like the EWC.

Then we talked about the five major measurements that dictate how efficient a window is. In other words, how well it will perform for your home here in St Louis Metro:

  1. The U-Factor – Insulation Value
  2. The SHGc – Sun Block Value
  3. Visible Transmittance – Light Value
  4. Air Infiltration – Seal Value
  5. Condensation Resistance – Moisture Control

If you’d like to know more about Our Triple-Pane Windows which have some of the best ratings for our area, please visit our Window Energy Efficiency page. Thanks so much for your time today, and keep up the quality research! Your choices DO make a difference.