U-Factor and SHGC Explained: How to Read a Window's NFRC Label and the ENERGY STAR Table That Actually Governs It
Last reviewed: 2026-09-27
Researched and written by one person, not an editorial team. I am an independent researcher, not a licensed contractor, and I do not install windows — see the About page for exactly what I do and don't do. This is a pure explainer page; there is no lead form on it, and nothing on this page routes you to one. The site earns money elsewhere by referring homeowners who submit a form to a disclosed set of installers and lead networks — disclosed on those pages, not here. Everything below is sourced from the EPA's own ENERGY STAR specification text and manufacturer spec-sheet PDFs I opened and read directly, not from a summary blog.
The two numbers, and what they actually measure
Every window sold with an NFRC label carries at least two performance numbers, and they measure two different, mostly independent things:
U-factor measures how much heat the whole window assembly lets through by conduction — how good an insulator it is. Per the EPA's own ENERGY STAR specification (retrieved 2026-09-27, primary-document extraction):
"U-Factor: The heat transfer per time per area and per degree of temperature difference (Btu/h·ft²·°F). The U-Factor multiplied by the interior-exterior temperature difference and by the projected fenestration product area yields the total heat transfer through the fenestration product due to conduction, convection, and long-wave infra-red radiation."
Lower is better. A U-factor of 0.20 loses heat more slowly than a U-factor of 0.40. This is the number that matters most in a cold climate, where the dominant cost is heat leaving the house all winter.
Solar Heat Gain Coefficient (SHGC) measures something different: how much of the sun's radiant heat gets through the glass and into the room, independent of the frame's insulating ability. Same source:
"Solar Heat Gain Coefficient (SHGC): The ratio of the solar heat gain entering the space through the fenestration product to the incident solar radiation."
SHGC is a ratio from 0 to 1. Whether lower is better depends on your climate — a low SHGC blocks unwanted summer heat gain (good in a hot climate, where the dominant cost is air conditioning), but a higher SHGC lets in free winter solar heating (useful in a cold, sunny climate) — which is exactly why ENERGY STAR sets a SHGC floor (a minimum) for the Northern zone and a SHGC ceiling (a maximum) everywhere else, covered below.
The NFRC label carries two more numbers that aren't part of ENERGY STAR's pass/fail criteria but are printed on every label anyway. Visible Transmittance (VT) "measures the amount of natural light coming through the window, with higher VTs meaning more natural light" — a purely daylighting number, unrelated to thermal performance (Window + Door, "Understanding the NFRC Window Label", retrieved 2026-09-27). Air Leakage (AL) reflects "how much draft you may experience," in cubic feet of air per minute per square foot — the number this site's window styles page uses to explain why a casement's compression seal outperforms a double-hung's track seal.
Whole-unit vs. center-of-glass — a distinction that changes which number you're reading, and a metric that isn't SHGC at all
A number you see on a manufacturer's marketing spec sheet is not always measuring the same thing as the U-factor and SHGC on the NFRC label affixed to the physical unit. Window + Door's explainer states that the NFRC label's U-factor "is an area-weighted average of the center of glass, edge of glass and frame U-factors" — i.e., the whole assembly, frame included (retrieved 2026-09-27). A manufacturer's separate "center of glass" performance sheet, by definition, measures only the glazing itself, not the frame or the edge-of-glass spacer — both of which typically perform worse than the center of an insulating glass unit, so a center-of-glass figure generally reads better than the same window's NFRC whole-unit label.
I opened Andersen's own "400 Series Product Performance — Center of Glass Performance" PDF to see exactly what such a sheet reports (retrieved 2026-09-27, primary-document extraction), and it's a sharper example of this trap than I expected: the sheet doesn't publish a U-factor at all. Its columns are Visible Transmittance (VT), Shading Coefficient (SC), SHGC, Relative Heat Gain (RHG), Ultraviolet Transmission (Tuv), Transmission Damage Weighted (Tdw), a condensation-related "%RH @ center" figure, and Insulating Glass Spacer Temperature (IGST). Shading Coefficient is an older metric that predates SHGC industry-wide and is not the same number — it's related by a standard conversion, SHGC ≈ SC × 0.87 (source: Trane, "What is the conversion from solar heat gain coefficient (SHGC) for glass to shading coefficient (SC)?", retrieved 2026-09-27). Andersen's own first data row lists SC 0.48 and SHGC 0.42 side by side — checking the conversion myself: 0.48 × 0.87 = 0.4176, which rounds to 0.42, matching Andersen's own SHGC column exactly. The two numbers on the same line of the same document are two different metrics for the same physical property, not a typo or a rounding discrepancy.
The practical risk: someone comparing an Andersen center-of-glass sheet to a competitor's NFRC whole-unit label could mistake SC for SHGC (they look like the same kind of number, and SC is always the larger of the two), or could compare a center-of-glass figure to a whole-unit figure and conclude the wrong product performs better. The number that governs ENERGY STAR qualification, and the only number safe to compare between products or between brands, is the NFRC whole-unit label on the physical unit itself — not a center-of-glass figure, and not a Shading Coefficient, from a manufacturer's own marketing sheet.
How the numbers are actually tested
Both ratings come from standardized test methods, not the manufacturer's own judgment. Per the EPA specification's Table 6 (retrieved 2026-09-27, primary-document extraction):
| ENERGY STAR requirement | Test method reference |
|---|---|
| U-Factor | NFRC 100 |
| SHGC | NFRC 200 |
| Air Leakage | ASTM E283, per NFRC 400, or AAMA/WDMA/CSA 101/I.S.2/A440-11 |
U-factor recertification is required every 5 years and air-leakage recertification every 10 years (EPA specification, Section 3.C, retrieved 2026-09-27) — meaning an NFRC number on an older spec sheet for a still-sold product line should, in principle, have been re-tested since the sheet was printed. If you're comparing a printed brochure to a current NFRC label on the unit itself and they don't match, the label on the physical unit is the current, binding number.
The ENERGY STAR Version 7.0 climate-zone table for windows
ENERGY STAR splits the continental U.S. into four "climate zones" — Northern, North-Central, South-Central, and Southern — and sets a different U-factor ceiling and SHGC requirement for each one. This is Table 1 from the EPA's Final Draft Version 7.0 specification, read directly from the PDF (retrieved 2026-09-27, primary-document extraction):
| ENERGY STAR Climate Zone | Max U-Factor | SHGC requirement |
|---|---|---|
| Northern | ≤ 0.22 | ≥ 0.17 |
| North-Central | ≤ 0.25 | ≤ 0.40 |
| South-Central | ≤ 0.28 | ≤ 0.23 |
| Southern | ≤ 0.32 | ≤ 0.23 |
Notice the Northern zone is the only one with a SHGC floor rather than a ceiling — the specification wants Northern-zone windows to admit solar heat, not block it, because in a heating-dominated climate that solar gain offsets furnace load. The EPA's own footnote to this table explains one of the specific number choices: "EPA made an adjustment to the North-Central zone U-factor criterion moving it from ≤ 0.24 to ≤ 0.25. This modest adjustment allows some additional products to qualify, but results in only a small loss in energy savings" — a specific, named policy trade-off, not an arbitrary round number.
This table applies to windows and to sliding glass patio doors. Swinging doors (including hinged entry doors and French patio doors) are certified against a different table — Table 2, organized by glazing percentage rather than climate zone. This site's patio door cost page covers that distinction, and a genuine internal inconsistency in the EPA's own PDF about it, in detail; it's mentioned here only so you don't accidentally apply this article's window table to a swinging door.
Reading a real spec sheet against the table
Numbers on a page are one thing; checking them against the actual criteria is another. I pulled two rows from Pella's Impervia Double-Hung Architectural Design Manual — the same document family used elsewhere on this site — and checked both against Table 1 above (retrieved 2026-09-27, primary-document extraction):
Row 1 — Advanced Low-E insulating glass, argon-filled, 3mm/3mm gap, NFRC-tested at U-0.31 / SHGC-0.28. Checked against every zone:
| Zone | Requires | This unit | Result |
|---|---|---|---|
| Northern | U ≤ 0.22 | 0.31 | Fails (U too high) |
| North-Central | U ≤ 0.25 | 0.31 | Fails (U too high) |
| South-Central | U ≤ 0.28, SHGC ≤ 0.23 | 0.31 / 0.28 | Fails both |
| Southern | U ≤ 0.32, SHGC ≤ 0.23 | 0.31 / 0.28 | U passes, SHGC fails |
This specific glazing package does not qualify for ENERGY STAR in any U.S. climate zone, despite being a real, currently-sold Low-E insulated-glass configuration from a major manufacturer's fiberglass line. Pella's own ADM confirms this independently: the row carries no "meets ENERGY STAR" zone shading at all in the source table. A double-hung window with argon-filled Low-E glass sounds efficient by name — it is not automatically ENERGY STAR-qualified glass by number, and the only way to know is to check the actual NFRC figures against the table, not the glass package's marketing name.
Row 2 — SunDefense+ insulating glass, argon-filled, 2.5mm/2.5mm gap, NFRC-tested at U-0.24 / SHGC-0.21:
| Zone | Requires | This unit | Result |
|---|---|---|---|
| Northern | U ≤ 0.22 | 0.24 | Fails (U too high) |
| North-Central | U ≤ 0.25, SHGC ≤ 0.40 | 0.24 / 0.21 | Passes |
| South-Central | U ≤ 0.28, SHGC ≤ 0.23 | 0.24 / 0.21 | Passes |
| Southern | U ≤ 0.32, SHGC ≤ 0.23 | 0.24 / 0.21 | Passes |
This one clears three of the four zones, missing only the strictest (Northern). Pella's own ADM table independently marks this exact row as meeting ENERGY STAR in "NC," "SC," and "S" — matching my calculation against the published table exactly, which is the kind of cross-check a search-snippet summary of "Pella windows are ENERGY STAR certified" can't give you: it doesn't tell you which specific glazing package, in which specific zone.
Source for both rows: Pella Corporation, "Impervia® Double-Hung Windows" Architectural Design Manual (PDF), retrieved 2026-09-27.
The Northern-zone alternate path: Equivalent Energy Performance
The EPA specification includes one more table that a summary blog almost never reproduces: Table 4, Equivalent Energy Performance for Windows, which applies only to the Northern zone (retrieved 2026-09-27, primary-document extraction):
| U-Factor | Minimum SHGC |
|---|---|
| 0.23 | ≥ 0.35 |
| 0.24 | ≥ 0.35 |
| 0.25 | ≥ 0.40 |
| 0.26 | ≥ 0.40 |
The logic: a Northern-zone window that misses the strict prescriptive U ≤ 0.22 ceiling can still qualify if it makes up for the extra conductive heat loss with enough additional passive solar heat gain — a slightly "leakier" but sunnier-admitting window can be energy-equivalent to a tighter, darker one in a cold, sunny climate. The specification is explicit that "these criteria allow Windows with energy performance equivalent to the prescriptive criteria to be certified for the Northern Zone," and just as explicit that the path does not exist for the North-Central, South-Central, or Southern zones, or for doors or skylights at all — it's a Northern-window-only allowance, not a general alternate compliance path.
Air leakage and the labeling options
Table 5 of the same specification sets the air-leakage ceiling referenced above and on this site's window-styles page: ≤ 0.3 cfm/ft² for windows, sliding doors, and skylights; ≤ 0.5 cfm/ft² for swinging doors (retrieved 2026-09-27, primary-document extraction). Compliance can be shown either on the NFRC temporary label directly, or via one of four named third-party structural certification labels: the AAMA Gold Label, the Keystone Certifications NAFS Structural Certification Label, the NAMI NAFS Structural Certification Label, or the WDMA Hallmark Certification Label. If a product you're considering carries one of these four labels instead of a printed air-leakage number, that label is itself the compliance evidence — you don't need a separate cfm figure to confirm it.
A regional wrinkle: state energy code can be stricter than ENERGY STAR
ENERGY STAR is a voluntary federal labeling program, not a building code — a state or local energy code can, and sometimes does, require better performance than ENERGY STAR's own minimums, for windows specifically. California's 2025 Title 24 Energy Code, effective January 1, 2026, sets a Climate Zone 3 prescriptive window U-factor requirement of ≤ 0.27 (source: Truitt & White, "California Energy Code 2026 Window Rules for Climate Zone 3", retrieved 2026-09-27) — tighter than ENERGY STAR's own South-Central ceiling (≤0.28) and close to its Northern-zone ceiling (≤0.22), despite California Climate Zone 3 (coastal Central California and the Bay Area) not being a cold climate in the way ENERGY STAR's own "Northern" zone is. That's a state code number layered on top of, not identical to, the federal ENERGY STAR criteria above — a separate source (California Permits, cited on this site's patio door cost page) gives a general statewide residential window U-factor requirement of ≤ 0.30, so Climate Zone 3's ≤0.27 appears to be a zone-specific tightening of that general baseline, not a contradiction between the two sources. If you're in a state with its own energy code, treat ENERGY STAR qualification as a floor, not a guarantee that you've met your state's actual permit requirement — check the state code's specific number for your climate zone.
What we could not verify
- A published state-by-state or ZIP-code map of which ENERGY STAR climate zone applies where. The EPA publishes this only as an image map (a PNG/PDF graphic), not as machine-readable or plain text I could extract state names from directly. I limited this article's regional claims to specific states already sourced and verified elsewhere on this site (California's Title 24 Climate Zone 3) rather than assert a broader zone map I could not directly confirm.
- Why Andersen's Center of Glass Performance sheet omits U-factor entirely. I confirmed directly that the document reports VT, SC, SHGC, RHG, Tuv, Tdw, %RH, and IGST but no U-factor column on any page I read; I don't know whether Andersen publishes a separate center-of-glass U-factor sheet elsewhere, and didn't find one during this session.
- Whether every current Pella Impervia glazing package has been re-tested under the 5-year NFRC recertification cycle the specification requires. The ADM PDF I read doesn't carry a test date per row, only a document revision date; I'm reporting the specification's recertification requirement, not confirming compliance for any specific still-sold product.
- A direct, independent (non-manufacturer) lab comparison of NFRC whole-unit vs. center-of-glass U-factor for the same physical window. I described the mechanism (frame and edge-of-glass typically underperform center-of-glass) based on the NFRC's own stated methodology, not a side-by-side measured example.
Sources
All retrieved 2026-09-27.
- U.S. EPA, "ENERGY STAR Product Specification, Residential Windows, Doors, and Skylights, Eligibility Criteria, Final Draft Version 7.0" (primary-document extraction — Definitions, Tables 1, 3, 4, 5, 6, and Section 3.C)
- Window + Door, "Understanding the NFRC Window Label"
- Pella Corporation, "Impervia® Double-Hung Windows" Architectural Design Manual (PDF) (primary-document extraction)
- Marvin, "Elevate and Essential Product Performance" Architectural Detail Manual (PDF) — consulted for the U-factor/SHGC/ER definitions restatement; not directly quoted
- Andersen Windows, "400 Series Product Performance — Center of Glass Performance" (PDF) (primary-document extraction)
- Trane, "What is the conversion from solar heat gain coefficient (SHGC) for glass to shading coefficient (SC)?"
- Truitt & White, "California Energy Code 2026 Window Rules for Climate Zone 3"
- This site, Patio Door Replacement Cost in 2026 — cross-referenced for the windows-Table-1 vs. doors-Table-2 classification distinction, its internal inconsistency in the EPA's own document, and the California Permits general window/door U-factor figures
- This site, Window Styles Explained — cross-referenced for the air-leakage/compression-seal mechanism
- This site, Vinyl vs Fiberglass vs Wood Windows — cross-referenced for prior use of the Table 1 criteria and the Marvin Elevate NFRC figures
- GreenBuildingAdvisor, "Energy Star climate zones and windows" — secondary source, consulted to confirm no additional named casement/awning NFRC figures were available there beyond what this site's other pages already cite
- Pella Corporation, "250 Series Sliding Patio Door" Architectural Design Manual (PDF) — consulted to confirm the Table 1 zone-shading methodology is applied consistently across Pella's window and patio-door ADM documents; not directly quoted in this article
- ENERGY STAR, "Independently Tested and Certified Energy Performance" — consulted for confirmation of NFRC's role as the independent certification body referenced throughout the EPA specification