Energy-Efficient Windows Explained: NFRC Labels and ENERGY STAR Zones

An energy-efficient window is one that slows the flow of heat through the glass, the frame and the gaps around them, and lets in the right amount of the sun's heat for your climate. You cannot judge that by looking at a window. You judge it by the numbers on its National Fenestration Rating Council (NFRC) label, mainly the U-factor (how fast heat escapes; lower is better) and the solar heat gain coefficient, or SHGC (how much of the sun's heat gets in) ³.

ENERGY STAR turns those numbers into a pass mark that changes by region. Its current table sets a maximum U-factor of 0.22 in the Northern zone, 0.25 in the North-Central zone, 0.28 in the South-Central zone and 0.32 in the Southern zone, plus an SHGC limit for each zone and an air leakage limit of 0.3 cfm per square foot ¹. A window that meets the numbers for your zone earns the label there; the same window may not qualify in another zone. The rest of this article explains what each number measures, why the right glass differs between Minnesota and Texas, and how to use the label to compare quotes.

How heat moves through a window

A window loses or gains heat in three ways. Heat passes through the glass and frame from the warm side to the cold side. Heat radiates from warm surfaces toward cold ones. And air leaks in and out around the sash and frame. On top of that, sunlight carries heat into the room through the glass.

Windows matter more than their size suggests. The U.S. Department of Energy (DOE) notes that windows make up about 10% of an average home's surface area, yet approximately 35% to 45% of the heat a home loses in winter passes through them ⁶. Across all buildings, DOE's Building Technologies Office says windows are responsible for about 10% of energy use ⁷. These are national figures from DOE, not a prediction for any one house: a home's climate, orientation, insulation and the windows it already has all change the picture.

What makes a window efficient

DOE's consumer guide lists the features that ENERGY STAR windows typically use ⁵:

• Two or three panes of glass with a sealed space between them. Each layer of trapped gas slows heat flow.
• Gas fills. Argon or krypton replaces air between the panes; NFRC describes argon as an inert, nontoxic gas used to reduce heat transfer ⁸.
• Low-E coatings. Microscopically thin metal or metal-oxide layers on the glass ⁸ that reflect heat energy into or out of the house ⁵. Depending on the coating, they keep winter heat in or summer sun out. More in low-E glass explained.
• Insulating spacers that hold the panes apart and reduce heat flow at the edge of the glass, which also helps limit condensation ⁵.
• Frames made of materials designed to insulate better ⁵.

A third pane is the biggest single step. DOE says newer "thin-triple" windows improve window energy performance by 40% or more and weigh less than conventional triple-pane units ⁶. Whether that is worth the cost depends heavily on climate, which triple-pane windows covers in detail.

The NFRC label, rating by rating

The NFRC label is the standard way to compare windows from different manufacturers. It shows two required ratings and up to three optional ones. NFRC describes them like this ³⁴:

The ranges come from NFRC's label page, except condensation resistance, whose 1 to 100 scale is on NFRC's consumer site, which also notes that a window rated above 0.3 for air leakage cannot qualify for ENERGY STAR ³⁴. The U-factor is the one most people ask about; window U-factor explained answers "is 0.27 good?" for each zone.

SHGC is the number people most often get wrong. A low SHGC blocks solar heat, which helps in a hot climate. A higher SHGC lets winter sun warm the house, which helps in a cold one. Neither is "better" on its own; it depends on the climate and which way the window faces ⁴⁵.

ENERGY STAR criteria by climate zone

ENERGY STAR divides the country into four zones and sets different limits for each, because "some windows, doors and skylights are better at keeping you warm, others excel at keeping you cool" ². DOE describes the specification behind these limits as Version 7.0 ⁶. The values below were read from ENERGY STAR's own criteria table ¹:

All zones: air leakage of 0.3 cfm per square foot or less. U-factor is in Btu per hour per square foot per degree Fahrenheit ¹.

Two things stand out. The Northern zone sets a floor on SHGC rather than a ceiling, and it lets a window with a slightly higher U-factor qualify if it lets in more winter sun: the "equivalent energy performance" rows ¹. And the two southern zones share the same SHGC limit of 0.23 or lower, because blocking the sun's heat matters most there ¹. ENERGY STAR's criteria page links a lookup that tells you which zone your address is in ².

Which numbers matter where you live

DOE's consumer guide adds a practical layer: the right glass can differ by the side of the house a window faces ⁵. Its guidance, summarized:

In short: in cold places, chase a low U-factor first; in hot places, chase a low SHGC first; in mixed climates, look at both. VT is a matter of taste, but a very low VT may make rooms darker than you expect, so check it against the glass you have now.

Do energy-efficient windows make a difference?

They change how heat moves, and that is measurable on the label. How much that shows up in comfort or in a utility bill varies from house to house, and the honest answer depends on what you are replacing. A single-pane window and a 15-year-old double-pane window with low-E glass are very different starting points.

Two cautions from DOE's guide are worth repeating. Even the most efficient window can leave a drafty house and cause condensation if it is not installed properly, so the installer matters as much as the product ⁵. And DOE suggests insulating and air sealing the house before replacing windows ⁵. If your windows are sound but leaky, storm windows are another route worth reading about, and if the problem is fog or wet glass, start with window condensation.

What the glass package costs

Better glass usually costs more, but surveys report it differently. Fixr, in a guide updated January 21, 2026, lists $450 to $2,000 per window for double-pane glass, $1,000 to $4,000 for triple-pane, and $300 to $1,100 for a window with low-E glass ⁹. Those ranges overlap because frame material, size and brand move the price as much as the glass does. For whole-window prices and why surveys disagree, see window replacement cost.

How to compare quotes on efficiency

When two installers quote different windows, the NFRC label lets you compare like for like:

1. Ask for the NFRC numbers of the exact unit quoted, not a brochure range for the product line. U-factor, SHGC and VT at minimum.
2. Check them against your ENERGY STAR zone. If a quote says "ENERGY STAR," ask which zone the product qualifies for.
3. Compare the glass package line by line: number of panes, gas fill, which low-E coating, spacer type.
4. Ask how the window will be installed and sealed, since installation affects drafts and condensation ⁵.
5. Look at air leakage if it is shown. It is optional on the label, so a missing number is worth a question rather than a conclusion ⁴.

If you are ready to see what windows in your area cost with the glass your climate calls for, you can compare quotes from window installers. FindAWindowPro is a lead-generation service and is paid by window companies when you request quotes.

Frequently asked questions

Are ENERGY STAR windows good?

The label means a window met EPA's U-factor, SHGC and air leakage limits for a given climate zone ¹. It does not rate build quality, durability or installation. Two windows that both qualify can still differ a lot on the label, so the NFRC numbers are the better comparison.

How can I tell if my windows are rated?

Look for the NFRC label on a new window ³, and ask for the same numbers in writing on your quote or order. If the label is long gone, the manufacturer or installer may be able to identify the product and its ratings from the model and series.

Which brand of window is the most energy efficient?

No brand is efficient across its whole range; efficiency belongs to a specific window with a specific glass package. Compare the NFRC numbers of the units you are actually quoted.

Should I always buy the lowest U-factor?

Not always. In the Northern zone, ENERGY STAR accepts a slightly higher U-factor when SHGC is higher, because winter sun helps heat the house ¹. In hot climates, SHGC usually matters more ⁵.

Related articles

• Triple-pane windows
• Low-E glass explained
• Window U-factor explained
• Storm windows
• Window condensation

About these figures

• ¹ Official record: ENERGY STAR criteria table for residential windows (U-factor and SHGC by climate zone, the Northern equivalent-performance rows, air leakage of 0.3 cfm per square foot, U-factor units), read in the official PDF, checked October 22, 2026.
• ² Official record: ENERGY STAR statement that criteria vary by climate zone and its link to a zone lookup, checked October 22, 2026.
• ³ Official record: NFRC descriptions and ranges for U-factor (0.20 to 1.20), SHGC (0 to 1), VT (0 to 1), air leakage (0.3 or less) and condensation resistance, checked October 22, 2026.
• ⁴ Official record: NFRC consumer site on which label ratings are mandatory or optional, the 1 to 100 condensation resistance scale, and that air leakage above 0.3 does not qualify for ENERGY STAR, checked October 22, 2026.
• ⁵ Official record: DOE consumer guide (2021 PDF) on efficient-window features, installation, air sealing first, and glass choice by orientation and climate, checked October 22, 2026.
• ⁶ Official record: DOE article (November 1, 2022) on ENERGY STAR Version 7.0, windows as about 10% of a home's surface area, 35% to 45% of winter heat loss, and thin-triple windows improving performance by 40% or more, checked October 22, 2026.
• ⁷ Official record: DOE Building Technologies Office statement that windows are responsible for about 10% of energy use in buildings, checked October 22, 2026.
• ⁸ Official record: NFRC glossary definitions of gas fills, argon, krypton and low-E coatings, checked October 22, 2026.
• ⁹ Reported: Fixr per-window ranges by glass type (double-pane $450 to $2,000, triple-pane $1,000 to $4,000, low-E $300 to $1,100), updated January 21, 2026, checked October 22, 2026.