5988 Mid Rivers Mall Drive #239. St. Peters, MO 63304

Window Energy Efficiency

The St. Louis Metro Area’s Guide
To Window Energy Efficiency

What You Need To Know To Make An Informed Buying Decision.

Energy efficiency is one of the most important factors to consider when buying replacement windows in St. Louis, St. Charles, and the surrounding areas. Not only will a window’s efficiency affect your energy costs, but it will also impact the comfort of your home.

But buying energy efficient windows can be a little trickier than most homeowners think. Energy efficiency can vary wildly from window to window. If you don’t know exactly what to look for, you may end up buying windows with subpar energy performance that skyrockets your utility costs. 

So how do you know if a window is truly energy efficient? By looking at its NFRC label.

NFRC stands for National Fenestration Rating Council. The NFRC is a third-party non-profit organization that educates consumers about energy efficiency in windows. They break down window energy efficiency into the following categories: U-Factor, Solar Heat Gain Coefficient, Visible Transmittance, and Air Leakage. 

The NFRC program is voluntary for window manufacturers and the only objective rating system for window energy efficiency. By being in the program, window manufacturers allow the NFRC to perform unplanned quality inspections at any time. The NFRC can pull a window straight off the production line and test it on the spot to confirm it performs as advertised. 

Many window manufacturers dislike the idea of the NFRC’s random quality inspections, so they choose not to participate in the program. Windows that aren’t NFRC certified may perform as the manufacturer says, but there is no third-party inspection to verify it. 

NFRC-certified windows have the certification seal on their energy label. Like “window stickers” on new vehicles, this energy label is attached to a window to tell you what kind of efficiency to expect. Here is what the energy label looks like for our windows (notice the NFRC seal in the upper-left corner):

We’ll explain what these ratings mean in a moment. First, let’s look at another type of efficiency assurance you want your new windows to have: ENERGY STAR certification.

ENERGY STAR is a program run by the U.S. Environmental Protection Agency and U.S. Department of Energy that promotes energy efficiency. Windows that meet or exceed NFRC energy ratings for a certain climate are awarded ENERGY STAR certification. Windows that really go above and beyond are awarded the rare distinction of ENERGY STAR Most Efficient. (We’ll get to exact specifications in a moment.)

All ENERGY STAR-certified windows in the St. Louis metro area have ENERGY STAR decals attached to them. Here’s what the standard decal looks like:

And here is what the Most Efficient decal looks like:

ENERGY STAR estimates Missouri homeowners can save up to $352 per year when replacing single pane windows with ENERGY STAR-certified double pane windows. These average savings increase when you choose to upgrade to Most Efficient triple pane windows.  

Specific savings depend on a variety of factors. But if you’re replacing your old windows, the bottom line is you will save… and make your home more comfortable in the process.

While the ratings on the NFRC label might seem confusing, they’re actually simple to understand. Let’s look at them individually… 

U-Factor: This measures the amount of heat your windows let out of your home. U-Factor is rated on a scale between 0 and 1. The lower the number, the better the window keeps heat inside. To qualify for ENERGY STAR certification, windows in the St. Louis metro area must have a U-Factor of 0.30 or lower. To earn ENERGY STAR’s Most Efficient status, windows must have a U-Factor of 0.20 or lower.

Solar Heat Gain Coefficient (SHGC): SHGC rates how much of the sun’s heat your windows let in on a scale between 0 and 1. As with U-Factor, lower SHGC numbers are better. For the St. Louis metro area, ENERGY STAR requires an SHGC of at least 0.42.

Air Leakage: This is how much air enters a room by Cubic Feet Per Minute (CFM). Air Leakage is measured on a scale between 0 and 1, with lower numbers once again being better. The industry standard for windows is 0.30 CFM.

Visible Transmittance: Measures how much light passes through a window expressed as a number between 0 and 1. With Visible Transmittance, higher numbers mean more visible light gets through. 

Condensation Resistance: Another efficiency measurement is Condensation Resistance. You won’t find this rating on some energy efficiency labels, but it measures a window’s ability to resist condensation on its interior. This is rated on a scale of 0 to 100 with higher numbers are better. You want a window with a Condensation Resistance rating of at least 50 (60 and up is preferable).

ENERGY STAR has awarded our triple pane windows Most Efficient for 2019. As ENERGY STAR states, the Most Efficient honor is “a distinction recognizing products that deliver cutting-edge energy efficiency along with the latest in technological innovation.”

To earn the Most Efficient award, windows in the St. Louis metro area and across the country must have a U-Factor of 0.20 or less. Our windows have a U-Factor of 0.17—even greater than Most Efficient standards!


Our windows pack all the latest energy-saving technology: triple pane glass, insulated foam-filled frames, Intercept® Ultra Warm Edge Spacers… the works. 

But those features alone would achieve a U-Factor of about 0.20—still enough to qualify for the Most Efficient award, but not sufficient for the 0.17 U-Factor our windows have.

So… what is our secret, energy-saving “sauce”? Krypton gas.

Most window companies in St. Louis and St. Charles Counties sell windows filled with Argon gas. Argon is relatively dense and does a solid job of insulating. Plus, it’s cheap because it’s abundant. 

Krypton gas is more than twice as dense, but it’s much rarer. So while Krypton gas is the superior insulator, it’s also more expensive. Because of the added costs, many companies choose not to offer it. 

We do. 

If you’ve read our Zero Compromises page, you know we don’t bat an eyelash when it comes to paying more to ensure the highest quality. That’s why we’re one of the few St. Louis metro area window companies that offer Krypton-infused windows.

When you consider just how much higher Krypton performs than Argon, you’ll see why it’s worth the extra investment…

Fits Triple Pane Like A Glove: Krypton particles are smaller than Argon particles. This allows Krypton to better fill every nook and cranny of the tight spaces between the panes of a triple pane window. 

Thermal Conductivity: This is the rate at which heat passes through a material. Argon’s thermal conductivity is 0.0094btu/hr/ft-F, while Krypton’s thermal conductivity is 0.0050btu/hr-ft-F. In plain English, Krypton’s thermal conductivity is 47% better than Argon’s. 

Gas Density: The denser the gas, the better it insulates. Krypton is 110% denser than Argon!

Atomic Mass (Density)39.983.8
Thermal ConductivityAverageExceptional
Insulation PropertiesAverageExceptional
  • ENERGY STAR requires a SHGC of 0.42. Our windows have a SHGC of 0.23.
  • ENERGY STAR recommends an Air Leakage rating of 0.3 CFM. Our windows have an Air Leakage rating of 0.05 CFM. (Not 0.5—0.05!)
  • ENERGY STAR recommends a Condensation Resistance rating of 50. Our windows have a Condensation Resistance rating of 70—a 56% improvement over the average double pane window.

Bottom line… 

Our windows don’t just meet ENERGY STAR standards. Or even ENERGY STAR’s rigid Most Efficient standards. They exceed both. 

The result? Windows in the St. Louis metro area that maximize your energy savings and keep your home more comfortable.

For more information about the energy efficiency of our windows, visit this page: Double Pane Window Alternatives