Where to find U-factor and SHGC on your windows.

Short answer: every NFRC-certified window carries a black-and-white NFRC energy performance label on the glass or the frame. U-Factor is the top-left rating, Solar Heat Gain Coefficient (SHGC) is top-right. Those two are the numbers your energy code compliance report and your load calculation need. Write them down per window type before the installers peel the stickers off.

What the two numbers mean

The National Fenestration Rating Council is the independent, non-profit certification body for the energy performance of windows, doors, and skylights in the U.S. Its label reports ratings from a standardized test, so a 0.30 from one manufacturer means the same thing as a 0.30 from another.¹

• U-Factor - how fast heat moves through the whole window assembly, in Btu/h·ft²·°F. Lower is better. NFRC's published range is about 0.20 to 1.20.¹ This is the number the IECC limits by climate zone and the number REScheck multiplies by window area in its UA calculation.
• SHGC - the fraction of solar energy that gets through, 0 to 1. Lower blocks more sun. The IECC limits it in hot climates and marks it "NR" (no requirement) in cold ones. Manual J uses it in every climate to size cooling, because a west-facing room full of 0.60 glass has a very different afternoon load than the same room at 0.25.

The label also lists Air Leakage (most codes cap it at 0.3 cfm/ft²), Visible Transmittance, and sometimes Condensation Resistance.² Those matter for comfort and for some product standards; they are not inputs to REScheck's UA path or to Manual J.

Reading the label

The layout is standardized. Top band: manufacturer, product line, frame material, glazing description (for example "Vinyl-clad wood frame · Double glazing · Argon fill · Low-E"). Below that, a grid of ratings, U-Factor and SHGC in the top row, always in that order. The NFRC certification mark and a ratings ID sit at the bottom.¹

Two things trip people up:

1. The label may show two U-factors - one in U.S. units (Btu/h·ft²·°F) and one metric (W/m²·K), the metric one roughly 5.7 times larger. Use the U.S. one. A "1.70" on a residential window is almost always the metric number for a 0.30 window, not a terrible window.
2. One label per product, not per opening. If your window schedule has twelve casements of the same series and glazing, they share a rating. What changes between openings is area, not U-factor. Record the rating once per window type and the size per opening.

Whole-window versus center-of-glass: the substitution that gets reports rejected

The NFRC U-factor is a whole-product rating: glass, spacer, and frame, tested as an assembly. That is the number the energy code means when it says "fenestration U-factor," and it is what a plan reviewer compares against the climate zone table.

Glass suppliers also publish center-of-glass U-factors, measured on the glazing alone. Those are always lower - often dramatically so, because the frame and edge-of-glass are the leaky parts. Putting a center-of-glass 0.24 on a REScheck for a window whose NFRC label says 0.31 is not a rounding error; it is the wrong quantity, and it is one of the first things a reviewer checks when a UA result passes by a hair. If the report gets kicked back, this is a common reason.

When the sticker is already gone

Installers remove the labels; homeowners remove them; sun fades them. In descending order of usefulness:

1. Manufacturer's spec sheet or order acknowledgment. The order paperwork from the window dealer lists each line item's series and glazing package, and the manufacturer's published performance sheet for that combination carries the NFRC ratings. This is the best substitute because it ties a rating to your specific order.
2. NFRC Certified Products Directory. NFRC publishes a searchable directory of every certified product by manufacturer and series. Search by the series name printed on the sash or the frame's inside edge (most manufacturers stamp it).¹
3. The frame itself. Many manufacturers etch or print a series code and sometimes a glazing code on the spacer bar between the panes (look at the edge of the glass) or on the top of the sash. That code plus the manufacturer's website usually gets you to the rating.
4. Default tables, as a last resort. The IECC provides default U-factor and SHGC values for fenestration that has no certified rating (Table R303.1.3 in the 2015, 2018, and 2021 editions), keyed to frame material and number of panes. The defaults are intentionally pessimistic - a double-pane metal frame defaults to a U-factor most real labeled windows beat by a wide margin - so using them costs you in a UA analysis. Use them only when nothing else exists, and say so on the report.

What number does your code require?

The IECC sets a maximum fenestration U-factor and, in warm zones, a maximum SHGC, by climate zone, in the prescriptive insulation and fenestration table (R402.1.2 in the 2015 through 2021 editions; the 2024 edition renumbers it). The values step down as the zone number goes up: warm Zones 1 and 2 allow a relatively high U-factor but cap SHGC; cold Zones 5 through 8 demand a low U-factor and leave SHGC as "NR." Your state's amended code may tighten these further, and a few states (Washington, California, Oregon) use their own tables entirely. Look up your state for the edition in force.

Two practical points:

• Prescriptive means every window meets the number. If one window type is over the limit, the prescriptive path fails for the whole house - but the UA alternative (R402.1.5) lets a better ceiling or wall offset it. That trade-off is exactly what a REScheck report documents, and it is why the U-factor on the label matters even when a window is "close enough."
• A better U-factor is currency. If your windows beat the requirement, that surplus can rescue a wall assembly that misses its R-value. It only counts if the label backs it up.

Skylights and doors

Skylights have their own, higher U-factor limit in the same table (skylights are tested at a slope and always rate worse). Exterior doors with glass are rated as fenestration; opaque doors are rated by U-factor alone. NFRC door labels show a small grid of ratings by glass configuration with the applicable one circled.¹ One opaque door per dwelling is typically exempt from the U-factor requirement under the IECC (R402.3.4 in the 2015 through 2021 editions), which is worth knowing when a decorative front door does not have a rating.

What we need from you

For a REScheck or a Manual J, send the window schedule from the plan set and, for each window type, either the NFRC label values or the manufacturer's spec sheet. If the schedule only says "Low-E double pane," we will tell you exactly which sheet to get from the dealer rather than default the whole house. Every rating we use is annotated with where it came from, so the plan reviewer can verify it without calling anyone.

Frequently asked questions

Is the NFRC U-factor the whole window or just the glass?

Whole window, frame included. That is the code value. Center-of-glass numbers from a glass supplier are lower and are not what the code or REScheck means.

What is a good U-factor?

Lower is better. "Good enough" is set by your climate zone in the IECC fenestration table; a lower value than required becomes trade-off credit in a UA analysis.

What if there is no label?

Spec sheet, then the NFRC Certified Products Directory, then the code's default table. Defaults are punitive by design.

Do I need SHGC in a cold climate?

Not for compliance where the table says NR, but Manual J uses it in every climate to size cooling room by room.

Can I use ENERGY STAR instead of the NFRC label?

ENERGY STAR windows are NFRC-certified by definition - NFRC is ENERGY STAR's certification body - so an ENERGY STAR window has an NFRC label with the numbers on it.² "ENERGY STAR" alone is not a U-factor; the report needs the value.

Sources

1. National Fenestration Rating Council, "What does the NFRC label tell you?" - label layout, U-Factor range 0.20-1.20, SHGC range 0-1, air leakage ≤ 0.3, door label format, Certified Products Directory. nfrc.org. Read 2026-09-02.
2. U.S. EPA ENERGY STAR, "Independently Tested and Certified Energy Performance" - the five label ratings, typical ranges, 0.3 cfm/ft² air leakage norm, NFRC as ENERGY STAR's certification body. energystar.gov. Read 2026-09-02.
3. International Energy Conservation Code, residential provisions: fenestration U-factor and SHGC by climate zone (Table R402.1.2, 2015-2021 editions), UA alternative (R402.1.5), default fenestration values (Table R303.1.3), one-door exemption (R402.3.4). Section numbers as they appear in the 2015 through 2021 editions; verify against the edition your jurisdiction has adopted.

This page explains where the values come from and how the code uses them. It does not quote per-zone numeric limits because those differ by edition and by state amendment; the state finder tells you which edition applies. Final authority rests with your building official.