There is no best window, and any list claiming to name one for a whole country is answering a question that does not have a national answer. What exists instead is a label, produced under an independent testing procedure, carrying numbers that mean the same thing on every product you look at. Learning to read it is the entire skill.
The label is issued under the National Fenestration Rating Council programme, and ENERGY STAR criteria are set by the EPA on properties independently verified according to NFRC procedures. Two organisations, two jobs: NFRC measures, ENERGY STAR decides what counts as good enough where you live.
The two numbers that carry the decision
| Rating | What it measures | How to read it |
|---|---|---|
| U-factor | The rate of heat transfer through the whole unit, in Btu per hour per square foot per degree F. It tells you how well the window insulates. | Values generally run from 0.15 to 1.1. Lower is better. A U-factor of 0.20 or lower likely indicates a triple-pane unit. |
| Solar heat gain coefficient | The fraction of solar energy the product transmits, so how well it blocks heat from sunlight falling on it. | A 0 to 1 scale, with values typically between 0.15 and 0.80. Lower transmits less solar heat. Lower is not automatically better; it depends on climate. |
The NFRC label carries five categories in total, and the other three matter less to most buying decisions than these two. Visible transmittance describes how much light comes through, air leakage describes how much air passes the assembly, and condensation resistance describes how well the unit resists condensation forming.
The trap: centre-of-glass numbers
A U-factor or SHGC figure can describe the glass alone or the whole product. NFRC-certified ratings assess the whole unit, meaning centre of glass, edge of glass, sash and frame together. A centre-of-glass number will always look better because it excludes the parts of the assembly that conduct heat worst.
So a quote quoting a flattering number without the NFRC label behind it is not necessarily wrong, it is describing something narrower than what gets installed in your wall. Ask for the NFRC-certified whole-unit rating for the exact product and configuration being proposed. It is a documented fact, not an opinion, and any manufacturer selling into this market can produce it.
Why the qualifying numbers change by climate
ENERGY STAR sets its criteria against four climate zones, Northern, North-Central, South-Central and Southern, which align with the International Energy Conservation Code zones. Windows must meet a U-factor requirement and, where applicable, an SHGC requirement for the zone.
The direction of travel is the useful part. Colder zones permit higher SHGCs, because sunlight arriving through a window in January is free heat you want. Hotter zones require lower SHGCs, because the same sunlight in July is a cooling load you are paying to remove. U-factor tightens everywhere as you go north, since heat leaving the building is the dominant loss.
Two structural features are worth knowing. The Northern zone offers two routes to qualification, a prescriptive path and an equivalent energy performance path that allows a higher U-factor paired with a minimum SHGC. And Version 7.0 introduced, for the first time in the Northern zone, a minimum SHGC for windows rather than only a maximum, which formalises the free-winter-heat logic above.
We are not reprinting the threshold table here, deliberately. Version 7.0 took effect on 23 October 2023 and the numbers have been revised several times across previous versions, so a table copied into an article ages badly and there are plenty of pages still quoting superseded figures. The current thresholds live on the ENERGY STAR residential windows, doors and skylights page, which is the only place worth reading them from.
Which package suits your particular house, once you know your zone, is a separate question covered in our note on choosing windows for your climate and orientation.
Keep the labels
ENERGY STAR advises saving the ENERGY STAR and NFRC labels as documentation. That is worth doing for reasons beyond tidiness: they are the evidence of what was actually installed, they support warranty claims, and where a utility or tax incentive is involved they are usually what substantiates it.
Photograph them before the installer peels them off, because that is the last moment the number and the physical window are provably connected.
Questions readers actually ask
Is a lower SHGC always better?
No, and this is the most common misreading of the label. SHGC is the fraction of solar energy the window transmits. In a cooling-dominated climate you want it low. In a heating-dominated one, solar gain offsets heating load, which is why ENERGY STAR permits higher SHGCs in colder zones and, in Version 7.0, introduced a Northern minimum.
What U-factor should I be looking for?
It depends on your climate zone, and the qualifying threshold is published by ENERGY STAR rather than by us. For orientation on the scale: values generally run 0.15 to 1.1, lower insulates better, and 0.20 or below likely indicates triple glazing. Check your zone against the current specification rather than a number from an article.
The salesperson quoted a much better number than the label. Why?
Most likely a centre-of-glass figure rather than a whole-unit one. NFRC-certified ratings cover the whole product including edge of glass, sash and frame, which conduct heat more readily than the middle of the pane. Ask for the NFRC whole-unit rating for the specific product and configuration quoted.
Does ENERGY STAR certification mean it is the best window available?
It means the product meets the EPA threshold for your climate zone on independently verified NFRC ratings. It is a floor, not a ranking, and many products exceed it. It also says nothing about installation quality, which is where a large share of real-world performance is won or lost.
Sources
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Every figure in this article traces to a government record or to a named independent, non-commercial research body. We do not cite insurance marketplaces or affiliate comparison sites for data.
- ENERGY STAR (U.S. EPA) Residential Windows, Doors and Skylights Published 2023-10-23Supports: Windows, doors and skylights must meet U-Factor and, where applicable, Solar Heat Gain Coefficient requirements based on climate zone; the current criteria were finalized in October 2022 with new performance levels effective 23 October 2023; save the ENERGY STAR and NFRC label as documentation; windows, doors and skylights originally qualified for the ENERGY STAR label in March 1998.
- U.S. Department of Energy, Building America Solution Center (PNNL) High-Performance (ENERGY STAR) Windows Published 2026-01-01Supports: ENERGY STAR window criteria are set by the EPA based on properties independently verified according to procedures established by the National Fenestration Rating Council; the NFRC label shows performance ratings in five categories; U-factor measures the rate of heat transfer and indicates how well the window insulates, values generally range from 0.15 to 1.1 measured in Btu/h per square foot per degree F, lower is better, and a U-factor of 0.20 or lower likely indicates a triple-pane window; SHGC measures the fraction of solar energy transmitted and indicates how well the product blocks heat from incident sunlight, measured 0 to 1 with values typically 0.15 to 0.80, lower transmits less solar heat; ENERGY STAR uses a four-zone climate map equating to IECC climate zones, with higher SHGCs permitted in colder climates and lower SHGCs required in hotter climates.
- ENERGY STAR (U.S. EPA) ENERGY STAR Version 7.0 Residential Windows, Doors and Skylights Specification Published 2022-10-01Supports: Windows may qualify in the Northern zone by a prescriptive path or by an equivalent energy performance path using NFRC-certified U-Factor and SHGC ratings; EPA adjusted the greater-than-half-lite South-Central and Southern zone door U-factor requirements from 0.25 to 0.28 to allow greater product selection alongside gains in SHGC.
Figures last verified August 30, 2026.