Window energy ratings: How they’re determined and what they mean

Key points

• Window energy ratings can help you find energy-efficient windows that will save you money on your heating and cooling bills.

• The NFRC tests windows to see if they’re eligible for the Energy Star label.

• The right type of energy-efficient window for your home depends on your climate.

Windows, like appliances, can be Energy Star certified. It’s the job of the National Fenestration Rating Council (NFRC) — a nonprofit organization that rates the energy efficiency of windows, doors and skylights — to test windows and determine if they qualify for the Energy Star program. To do this, the organization developed a rating system that quantifies a window’s performance, including the amount of solar heat it lets in and the amount of air it lets out.

Why do these ratings matter? They help you determine the type of windows you need to make your home as energy efficient as possible. And when buying windows, spending more upfront on a certified energy-efficient model that suits your climate can mean savings over time on heating and cooling bills.

We spoke to Michelle Blackstone, senior director of development and engagement at the NFRC, to help you understand window energy-efficiency ratings and how they can inform your next window purchase.

What are NFRC window ratings?

The NFRC is the only organization that tests windows to determine if they’re eligible for the Energy Star program. The NFRC works with university researchers, the U.S. Department of Energy (DOE), the U.S. Environmental Protection Agency (EPA), window manufacturing companies and window glassmakers to develop a set of tests that quantify how energy efficient a window is.

A network of accredited labs across the country conducts testing. The NFRC oversees the physical testing and computer simulations, then forms an overall picture of a window’s heat transfer and airflow characteristics.

These efficiency ratings provide information about several window characteristics that vary in favorability depending on your home and location. For example, an extremely airtight window is great at preventing heat loss through airflow but might not be good at limiting solar heating in the summer. In other words, the ratings don’t determine which window is the best overall — the right type for your home depends on factors such as your climate and the HVAC system you use.

The following sections cover the most important window efficiency ratings in detail and explain how they factor into Energy Star ratings.

U-Factor

The U-factor or U-value determines how much heat a window lets out of a home. The lower the U-factor, the more insulating a window.

“Essentially, it tells you how much heat inside your home is escaping through your window, and this is important in, say, cold climates during the winter,” said Blackston. “You don’t want your heater working overtime to warm your house, and then all the heat is escaping through the window.”

You don’t want your heater working overtime to warm your house, and then all the heat is escaping through the window.

Michelle Blackstone

If you live in a cold climate — somewhere like Wyoming, Montana or Maine — you should prioritize windows with lower U-factors. The U-factor is less important in warmer places, like Southern California or Florida, where it’s easier to keep your house warm.

To qualify for Energy Star certification, a window must have a U-factor below a certain threshold that depends on where you live. The table below shows the threshold U-factor requirements by climate zone:

Climate Zone	U-Factor
Northern	≤ 0.27
North-Central	≤ 0.30
South-Central	≤ 0.30
Southern	≤ 0.40

Solar heat gain coefficient

Another factor the NFRC uses to rate windows is the solar heat gain coefficient (SHGC). This measures the ratio of heat entering your home to the total amount of solar energy hitting the window. Simply put, it’s the amount of energy from the sun that turns into heat when light passes through a window.

“[SHGC] is important for warmer climates like Arizona, Florida, Texas [and] parts of Southern California,” said Blackston. Like U-factor ratings, the lower a window’s SHGC, the better it is at keeping your home cool when the sun is beating on it.

The Energy Star program also has SHGC requirements, and, like U-factors, the thresholds for eligibility depend on where you live. The following table shows SHGC requirements for windows per climate zone:

Climate Zone	U-Factor
Northern	Any
North-Central	≤ 0.40
South-Central	≤ 0.25
Southern	≤ 0.25

Visible transmittance

The amount of visible light that passes through your window is called its visible transmittance (VT). The VT rating doesn’t account for heat; it only measures the amount of light transmitted through the window.

“The reason [visible transmittance] is important is because windows that allow more natural light into your home…can reduce your use of artificial lights,” said Blackston. “That cuts down on your electricity use and your utility bills.” 

Of course, having windows with high visible transmittance only helps if they get a lot of sunlight on them. If your windows are shaded for most of the day, even a high VT rating won’t help improve your home’s lighting much.

The NFRC gives windows VT ratings, but they don’t factor into Energy Star eligibility.

Condensation rating

Condensation ratings measure how quickly condensation forms on the inside of a window when it’s hot inside and cold outside. Warm air inside your house condenses when it hits the cold surface of the window, but some windows form condensation more quickly than others. Ideally, you want a low condensation rate — the more condensation you have, the more likely you’ll run into issues such as mold growth and warping in the window frame.

There’s no condensation rate requirement for a window to be eligible for an Energy Star label. However, most Energy Star-rated windows will show the condensation rate.

Air leakage

Finally, the amount of air that flows through your window also helps determine whether a window qualifies for the Energy Star program. Unlike the U-factor and the SHGC, minimal air leakage is beneficial in all climates. When it’s hot out, low air leakage reduces the amount of cool air that escapes through the window. Conversely, in the winter, low-air-leakage windows keep more warm air in your home.

Air leakage is measured in a complicated-sounding unit — cubic feet per minute per square foot or cfm/ft² —  but the idea is simple. It measures the cubic footage of air passing through a window’s surface area each minute. The less air that passes through your window, the better, so a low cfm/ft² is optimal.

A window must have air leakage of less than or equal to 0.3 cfm/ft² to qualify for the Energy Star program.

How do you find energy ratings for windows?

Now that you understand how the NFRC establishes its ratings and how those ratings translate to Energy Star certification, we can talk about how to find windows with the right ratings for your home.

The good news is you don’t have to parse through all the individual U-factor, SHGC and VT ratings yourself. Instead, you can use the online Efficient Windows tool to find high-quality, efficient windows based on your location.

All you have to do is enter your ZIP code and a few details about your home, and it returns a list of a few types of windows in order of suitability. You can click on each window type to see products from leading brands and links to websites where you can buy them.

If you prefer to shop in local stores, you can look for Energy Star stickers and NFRC labels on the packaging to guide your search.

Should your window be Energy Star certified?

Yes, any new windows you purchase should be Energy Star certified. If you’re doing home improvement projects to make your home more energy efficient, buying Energy Star-certified products will qualify you for tax credits and save you money on energy bills in the long run.

Energy Star appliances and windows are more expensive than their uncertified counterparts because they’re of higher quality; however, federal incentive programs allow you to claim 30% of the cost of Energy Star-certified windows and skylights (up to $600) on your income taxes in the year you install them. These incentives can effectively close the cost gap between windows that qualify for the certification and those that don’t.

You can check out the Database of State Incentives for Renewables and Efficiency (DSIRE) for more information about local, state and federal incentives for energy-efficient windows.

Installing Energy Star-certified windows will also save you money by lowering your monthly heating and cooling bills. Energy-efficient windows make your HVAC system more effective by reducing how hard it has to work to keep your home at a comfortable temperature. That means replacement windows with the Energy Star label are more cost-effective overall, even if they look more expensive on the price tag.

What’s next?

Contact window installers and balance your budget with long-term savings from the optimal energy-efficient window features. Have a rough idea of what you’re looking for before you contact a window company; it will be much easier for a pro to help you. With that said, a good window installation professional will work with you to determine the best window type for your home.

Don’t be afraid to shop around, and consider buying new windows from one of our top-rated window brands. If you want to maximize your long-term savings and get the most energy-efficient windows you can afford, have patience to find the windows with the right performance ratings for your home.

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