Will replacement windows lower your energy bills? What ENERGY STAR and the FTC say

Updated November 10, 2026 · KeystoneProGuide editorial team

How much will new windows save?

Less than many sales pitches suggest. ENERGY STAR, the federal program that certifies efficient windows, says replacing old windows with certified ones lowers household energy bills by an average of up to 13 percent nationwide when the windows being replaced are single-pane. Every qualifier in that sentence matters. It is an average, so some homes save less. It is "up to", so it is a ceiling for the average rather than a promise. And it is measured against single-pane glass; replacing double-pane windows starts from a better baseline. The page puts the same figure a second way, as up to 13% of heating and cooling costs compared with non-certified products, and says its estimates come from calculations made for the program.

Why do the savings vary so much?

Because windows do not use energy themselves. The Department of Energy's purchasing guidance says windows, doors and skylights do not consume energy directly; they add to the heating and cooling loads of the building they sit in, and an efficient window reduces that load. How much it reduces depends on the climate: the department says features that make windows efficient in one climate may offer little benefit in another.

It also depends on how the energy is being lost. The National Park Service points out that heat escapes both by conduction through the glass and by air leaking around the sash and frame. A new window addresses both, but so, in part, do cheaper measures. The Park Service says caulking, weatherstripping and replacing deteriorated glazing compound should always be done before considering replacing historic windows.

What claims should you doubt?

Large percentages and guarantees. In 2012 the Federal Trade Commission settled charges against five replacement-window sellers, two of them based in Pennsylvania, over exaggerated and unsupported claims of energy savings. The claims it cited included a "40% Energy Savings Pledge", "50% Energy Savings Guaranteed", heating and cooling cuts of "up to 49%", and sales scripts saying windows would "cut energy bills in half" and pay for themselves in eight years. The FTC said the companies lacked a reasonable basis for them.

The orders that followed set a useful test for any claim you hear. A seller covered by them may not claim a specific or "up to" level of savings, or pledge one, unless it has competent and reliable scientific evidence that all or almost all customers are likely to achieve the maximum savings claimed, and it must disclose any conditions, such as the type of window replaced or the region, next to the claim. You can apply the same test yourself: ask the seller what evidence supports the number, for a house like yours.

The Park Service is blunter. It calls the common idea that replacing windows saves as much as 50% in energy costs "simply not true", and says a replacement window does not generally pay for itself in a reasonable length of time.

How do you compare windows on efficiency?

By the label, not the brochure. ENERGY STAR says certified windows are independently tested and certified by the National Fenestration Rating Council (NFRC), and that you should look for the ENERGY STAR label for your climate zone next to the NFRC label on the glass. The two ratings that matter most, according to the Department of Energy, are the U-factor, which measures heat loss, and the solar heat gain coefficient (SHGC), which measures how much of the sun's heat comes through. Choose products whose ratings meet ENERGY STAR's for your climate zone; the NFRC directory lists certified products by zone.

ENERGY STAR adds orientation advice for colder climates: a south-facing window ideally has a higher SHGC and a low U-factor, east and west windows a low SHGC or shade, and north windows the lowest U-factor you can afford. The current criteria took effect on October 23, 2023. Ask your installer for both labels and keep them.

Installation counts as much as the rating. ENERGY STAR says even the best windows can be drafty if they are poorly installed.

Are storm windows an alternative?

For some houses, yes. The Park Service reports a 2002 study by Lawrence Berkeley National Laboratory which found that a storm window over a historic window can achieve thermal performance similar to a new low-E vinyl replacement. The Department of Energy says storm windows with a low-e coating reflect heat back inside in winter and outside in summer, and that ENERGY STAR now certifies storm windows. Whether storms suit your windows depends on their condition; the comparison is set out in repair, add storms or replace.

What about comfort and condensation?

Comfort is a real benefit that does not show up on a bill. ENERGY STAR says the cold inner surface of an inefficient window pulls heat from your body, so you can feel chilly by a window at 70 degrees, and that certified windows keep the interior glass warmer.

Condensation is a different matter. The EPA says moisture on windows can be a sign of high humidity, and recommends keeping indoor humidity below 60 percent, ideally 30 to 50 percent. Treat condensation as a humidity question first and a window question second.

Is there still a tax credit?

According to the IRS, the energy efficient home improvement credit covered improvements made through December 31, 2025. It paid 30% of the cost of qualifying windows and skylights up to $600 a year, required ENERGY STAR Most Efficient windows, and did not count labor. ENERGY STAR's tax credit page also describes the credits as available through the end of 2025, although its window page still mentions claiming them. Utility rebates are separate; ENERGY STAR says many utilities offer them.

So are replacement windows worth it?

That depends on what you are replacing and why. If your windows are single-pane, failing or beyond repair, ENERGY STAR's figures describe the kind of saving to expect, and the comfort gain is real. If the main goal is a lower bill, the sources point first to sealing, weatherstripping and, for sound older windows, storms. Prices are in the cost guide,. Listed businesses are on the exterior remodeling directory page.

Sources

1. Residential Windows, Doors, & Skylights, ENERGY STAR (U.S. Environmental Protection Agency)
2. What makes it ENERGY STAR? (Residential windows, doors and skylights), ENERGY STAR (U.S. Environmental Protection Agency)
3. Purchasing Energy-Efficient Residential Windows, Doors, and Skylights, U.S. Department of Energy (Federal Energy Management Program)
4. Purchasing Energy-Efficient Residential Storm Windows, U.S. Department of Energy (Federal Energy Management Program)
5. Weatherization of Historic Buildings: Repair and Upgrade Windows and Doors, National Park Service
6. Window Marketers Settle FTC Charges That They Made Deceptive Energy Efficiency and Cost Savings Claims, Federal Trade Commission
7. Energy Efficient Home Improvement Credit, Internal Revenue Service
8. Federal Tax Credits for Energy Efficiency, ENERGY STAR (U.S. Environmental Protection Agency)
9. A Brief Guide to Mold, Moisture and Your Home, U.S. Environmental Protection Agency