How ENERGY STAR windows become stars

Energy Star-certified products are trusted for their quality and energy efficiency.

Today, we’re taking you inside the rigorous process that awards that valuable designation.

When the National Association of Realtors surveyed Americans in 2021 on what they expect to find in an energy-efficient house, the most popular answer was efficient windows – 83 percent of respondents identified ENERGY STAR-certified windows as the most obvious ingredient. The ENERGY STAR program, overseen by the U.S. Environmental Protection Agency and the Department of Energy, rates windows, computer monitors, refrigerators, and many other common products. According to the program’s calculations, people who replace old windows with ENERGY STAR-certified versions can expect their monthly electricity bill to drop by about 11 percent. How can they be sure? That certainty is the result of a testing system developed by our parent organization, the National Fenestration Rating Council.

Energy Star - NFRC label

The NFRC tests roughly 80 percent of the windows in the U.S. and Canada for their energy performance, and most of the windows you see for sale have an NFRC label affixed to them that displays the results. It’s a voluntary program for window manufacturers to have their products tested, and most choose to do so because of the chance to build trust with customers by giving them more information. To receive the ENERGY STAR certification, companies must opt for some extra steps in that process, and together, there are about 2,500 window options that make the grade.

That testing process begins at one of 10 laboratories across the U.S. that NFRC has accredited to perform the tests. Each lab contains a small house inside it – a structure about 12 feet wide, 12 feet deep, and 16 feet high, with its own heating and cooling systems. “The interior is like a little living room inside the lab,” NFRC Residential Program Manager and former window inspector Steve McDowell told me. The windows are installed in this mock living room for testing just as they would be in a home, but with sensors around it to measure heat flow.

Technicians then blow hot and cold air in cycles for 48 hours and monitor factors such as the amount of heat that can escape the house through the window (its u-Factor), the amount of heat that can enter through it (the solar heat gain coefficient) and the amount of light that can pass through (visible transmittance). Windows must meet specific standards for each of these three efficiency factors, plus one more – the amount of air that can pass through any leaks, as no window provides a perfect seal. Those that don’t qualify for ENERGY STAR still get an NFRC label detailing the results.

NFRC also recognizes Inspection Agencies that evaluate product construction, consistency, and quality through yearly in-plant inspections. These inspections and life cycle testing every five years are the basis for our product certification program. For manufacturers participating in the ENERGY STAR program, EPA requires that they submit to random product testing as an added level of scrutiny.

Finally, the ENERGY STAR process requires manufacturers to provide detailed installation instructions, which are used in testing as well as by window installers.

For now, about 80 percent of windows are efficient enough to meet ENERGY STAR requirements, but that figure will drop later this year. That’s because ENERGY STAR periodically raises its standards, and the seventh version of them takes effect in October 2023. In doing this, the program, over time, plays a key role in making windows more efficient. You can read more about your fellow window shoppers’ trust in the ENERGY STAR program and what they want in a window in this blog post on window trends we wrote in September. Try the Window Ratings page of this site for a quick guide to u-Factor, visible transmittance, and the other ratings that NFRC provides to consumers and to the ENERGY STAR program.


[1] NOTE – pulled that stat from the newsletter article we wrote last summer after the NAHM/NAR webinar