Updated September 2026. Definitions and rating scales from the National Fenestration Rating Council (NFRC) and ENERGY STAR Version 7.0. How we research this
Window salespeople tend to talk in labels like “Low-E,” “ENERGY STAR,” “argon-filled,” and “triple-pane.” None of these tells you how the specific window on your quote will perform. Two windows can both be “Low-E” and ENERGY STAR certified and still behave very differently in your house.
Watch for this.
The NFRC label is the only place U-factor, SHGC, VT and Air Leakage appear together, measured the same way for every product. Whichever brand a contractor pitches, ask for the label of the exact configuration on your quote. Without it, every energy claim is generic.
The numbers that let you compare windows directly are on the NFRC label, the temporary sticker on certified windows. To compare two quotes, compare the whole-window NFRC ratings for the exact units quoted: U-factor, SHGC, visible transmittance, and air leakage, plus condensation resistance if it is listed. Then match those numbers to your climate zone and what each room needs. There is no single “best” set of numbers. The right numbers for a north-facing bedroom in Minnesota are not the right numbers for a west-facing kitchen in Phoenix.
If a quote cannot give you the label numbers, it is not comparable.
What the NFRC label is—and isn’t
The National Fenestration Rating Council is a nonprofit that runs a rating and certification program for windows, doors, and skylights. According to NFRC, its ratings describe the whole product, meaning the glass, frame, sash, and spacer together, tested and simulated under standardized conditions.
This matters because the frame and the edge of the glass usually perform worse than the center of the glass.
The label is:
- A standardized comparison tool. Every certified product is rated the same way, so a 0.27 U-factor from one manufacturer means the same thing as a 0.27 from another.
- Specific to a product configuration. Change the glass package, gas fill, spacer, or frame, and the ratings change.
- The basis for ENERGY STAR. ENERGY STAR states that windows must be NFRC-certified to qualify, and it sets its criteria using NFRC-rated values.
The label is not:
- A rating of installation quality. A great window that is poorly installed can still leak.
- A prediction of your energy bill or payback period.
- A brand ranking.
Center-of-glass versus whole-window
Brochures and sales sheets often show center-of-glass numbers, which describe only the middle of the glass unit. These almost always look better than the whole-window NFRC figure because they leave out the frame and edge.
If one quote shows a center-of-glass U-factor and another shows an NFRC whole-window U-factor, you are comparing different measurements. Always ask for the NFRC whole-product rating for the specific configuration being quoted.
The NFRC label decoder
Here is what each field means and what to look for depending on your goal. The ranges below come from the rating scales NFRC defines and the thresholds ENERGY STAR uses. They are not manufacturer product-line ranges.
| Metric | What it means | Scale / typical range | What to prefer by goal |
|---|---|---|---|
| U-factor | How quickly the whole window lets non-solar heat pass through it. This is the insulation number. | NFRC describes the typical range for windows as roughly 0.20–1.20. ENERGY STAR V7.0 caps it between 0.22 and 0.32 depending on zone. Lower is better insulation. | For winter heat loss, cold rooms, and comfort near windows, prefer lower U-factor. At minimum, meet the ENERGY STAR cap for your zone. |
| Solar Heat Gain Coefficient (SHGC) | The fraction of solar radiation admitted through the window, either passed directly or absorbed and released inside. | 0 to 1. Lower means less solar heat enters. | For overheating, air-conditioning load, and west- or south-facing glass in hot climates, lower is usually preferable. In cold climates or rooms where you want winter sun, lower is not necessarily better. |
| Visible Transmittance (VT) | How much visible light comes through the window. | 0 to 1. Higher means more daylight. | For dark rooms, views, and plants, prefer higher VT. In glare-prone rooms, moderate VT may be preferable. Check VT together with SHGC. |
| Air Leakage (AL) | How much air passes through joints in the window assembly under test pressure, measured in cubic feet per minute per square foot of window. | ENERGY STAR V7.0 requires ≤ 0.3 cfm/ft². Lower means tighter. AL is optional on the NFRC label, so it may not always be shown. | For drafts, windy sites, and comparison between sliders and casements, prefer lower AL. If it is not on the label, ask for it. |
| Condensation Resistance (CR) | How well the window resists interior condensation forming on its surfaces. | 1 to 100. Higher means more resistant. CR is optional. | For cold climates, humid homes, bathrooms, and kitchens, prefer higher CR. It does not guarantee that a window will never condense in a very humid home. |
Other items on the label worth checking
- Manufacturer, series/model, and product description: Confirm that the frame material, number of panes, Low-E coating, and gas fill match the product line and options on your quote.
- Certified product identifier: NFRC maintains a Certified Products Directory where rated products can be looked up. If your installer names a product, you or the installer should be able to find its ratings there.
Shareable takeaway: “Low-E” is a coating, not a performance number. Two Low-E windows can differ widely in SHGC. You are choosing numbers, not a sticker phrase.
Why “ENERGY STAR” isn’t a universal answer
ENERGY STAR certification is useful, but it is climate-zone specific. A window certified for the Southern zone may not meet the Northern criteria, and the reverse is also true. ENERGY STAR divides the United States into four window climate zones and sets different U-factor and SHGC requirements for each.
According to ENERGY STAR Version 7.0, Table 1 of the final specification (effective October 23, 2023), the window criteria are:
| ENERGY STAR V7.0 zone | U-factor | SHGC |
|---|---|---|
| Northern | ≤ 0.22 | ≥ 0.17 |
| North-Central | ≤ 0.25 | ≤ 0.40 |
| South-Central | ≤ 0.28 | ≤ 0.23 |
| Southern | ≤ 0.32 | ≤ 0.23 |
Three points matter:
- In the Northern zone, SHGC has a minimum, not a maximum. ENERGY STAR’s logic is that in heating-dominated climates, some winter solar gain is useful. Lower SHGC is not automatically better.
- The Northern zone also allows equivalent-energy-performance pathways. ENERGY STAR V7.0 lets some products qualify with a different U-factor/SHGC combination that achieves similar overall performance. If a Northern-zone quote shows an SHGC below 0.17 but says “ENERGY STAR,” ask which pathway it qualifies under.
- “ENERGY STAR” on a quote tells you nothing about the zone. Ask: “Certified for which zone?” Check your location against the ENERGY STAR zone map rather than relying only on the salesperson.
The matrix: numbers to demand on your quote
Use this as a starting point. The baseline in each cell is the ENERGY STAR V7.0 criterion for that zone. The room-goal adjustments are directional: they tell you which way to push when comparing two quotes, not a precise target to hit.
| Room goal ↓ / Zone → | Northern | North-Central | South-Central | Southern |
|---|---|---|---|---|
| Baseline | U ≤ 0.22; SHGC ≥ 0.17, or a documented equivalent pathway | U ≤ 0.25; SHGC ≤ 0.40 | U ≤ 0.28; SHGC ≤ 0.23 | U ≤ 0.32; SHGC ≤ 0.23 |
| Cold room / winter comfort | Prioritize the lowest U-factor available; compare triple versus double on U-factor, not pane count | Prioritize lower U-factor | Meet the U baseline; lower U helps less than SHGC here | Meet the U baseline |
| Overheating / high AC load | A lower SHGC may be justified on large west-facing glass, but confirm that the product still qualifies | Push SHGC toward the lower end on west- or south-facing exposures | Prioritize lower SHGC | Prioritize the lowest SHGC offered, then check VT |
| Free winter sun | Ask for higher-SHGC options that still meet the U baseline | Higher SHGC within the ≤ 0.40 limit may help on south-facing glass | Usually not a priority because cooling dominates | Usually not a priority |
| Daylight / views | Compare VT between quotes; do not accept a large VT drop without a reason | Compare VT | Check that low-SHGC glass has not reduced VT too much | Look for low SHGC and adequate VT together |
| Drafts / windy exposure | Demand AL and prefer lower | Demand AL | Demand AL | Demand AL |
| Condensation | Demand CR if available; prefer higher CR. Lower U-factor also helps | Prefer higher CR | It is less common, but still worth asking about in bathrooms | It is less common |
How to use the matrix
You do not need identical numbers for every window in the house. Many manufacturers offer different glass packages by elevation, such as a lower-SHGC coating on west-facing glass.
If your quote uses one glass package throughout, ask whether a split package is available and what it would cost. Do not assume the split package is automatically worth it.
For a deeper look at when the extra U-factor improvement from triple-pane glass matters, see Triple-Pane vs. Double-Pane Windows.
What people get wrong when comparing window numbers
1. Comparing center-of-glass to whole-window
This is the fastest way to make one quote look better than it is. Ask both companies for the NFRC whole-window value for the exact configuration.
2. Assuming lower SHGC is always better
In cooling-dominated climates, lower SHGC usually helps reduce solar heat gain. In the Northern zone, ENERGY STAR sets a minimum SHGC because winter solar gain has value. The right SHGC depends on climate and orientation.
3. Treating “triple-pane” as a rating
Pane count is a construction detail. The U-factor on the label shows what that construction actually achieves for the whole unit.
4. Ignoring air leakage because it is not on the sticker
AL is optional on the NFRC label, but it is part of ENERGY STAR V7.0 requirements. If it is not shown, ask for it.
5. Converting numbers into payback years
A U-factor difference does not translate into a reliable number of years without knowing your house, fuel prices, window area, orientation, and installation quality. Be skeptical of any quote that gives a precise payback figure without showing its assumptions.
Our Are New Windows Worth It? guide covers how to think about value without invented numbers.
6. Confusing condensation on the glass with fog between the panes
A low CR rating relates to condensation on interior surfaces. Fog between panes usually indicates a failed seal, which is a different problem. See Fog Between Window Panes.
A note on homeowner forums
Home-improvement forums are full of posts where someone photographs an NFRC sticker and asks, “Is this good?” That tells us many homeowners can see the numbers but lack a framework for reading them.
We treat those threads as a signal of what questions people have, not as a technical source. The answers vary widely in accuracy. The definitions in this guide come from NFRC and ENERGY STAR.
The quote glass-line checklist: 8 items
Before you sign, make sure the glass and performance section of each quote answers all eight questions. If a line is missing, ask for it in writing.
- Exact product line and series. Not just the brand. The label ratings are tied to a specific product.
- Full glass package. Number of panes, Low-E coating name or type, gas fill, and spacer type. Changes to any of these can change the ratings. For the gas-fill fork, see argon vs krypton.
- Whole-window NFRC U-factor. Specifically the NFRC whole-product value, not center-of-glass.
- Whole-window NFRC SHGC and VT. Both together, so you can see the trade-off between heat gain and daylight.
- Air leakage (AL). Ask even if it will not appear on the sticker.
- Condensation resistance (CR). If the manufacturer rates it, include it. This matters most in cold or humid homes.
- ENERGY STAR zone. Ask, “Certified for which zone?” Also ask whether a Northern-zone product qualifies through an equivalent-energy pathway.
- Whether ratings differ by window. If different elevations or window types—such as casements, sliders, or picture windows—use different glass or ratings, each should be listed separately.
Once you have these numbers for both quotes:
- Enter them in the Window Quote Analyzer to find gaps in a single quote.
- Use Compare Two Window Quotes to put two bids side by side on matching terms.
- For the full scope of a quote, including installation, warranty, and removal, use the Window Quote Checklist and our guide to comparing window quotes.
- For the broader decision, including frame material and when replacement makes sense, start at the Windows hub.
How we researched this
This guide is based on primary sources, not manufacturer marketing or product testing. We did not test windows ourselves.
- National Fenestration Rating Council (NFRC): Consumer explanations of U-factor, SHGC, visible transmittance, air leakage, and condensation resistance, and of NFRC ratings as whole-product values. NFRC’s Certified Products Directory is where rated products can be looked up. Link: nfrc.org
- ENERGY STAR: The Version 7.0 Residential Windows, Doors, and Skylights specification, including Table 1 criteria by climate zone, the air leakage requirement, Northern-zone equivalent-energy pathways, and ENERGY STAR’s statement that NFRC certification is required for qualification. Spec PDF: ENERGY STAR V7.0 Final Specification — product page: energystar.gov
- U.S. Department of Energy: Consumer guidance on window energy-performance ratings and how climate and orientation affect window selection.
We do not publish manufacturer U-factor ranges because they vary by configuration and change over time. Check the label and the NFRC directory for the product you are quoted. More on our process: How we research this. See also the site disclaimer.
FAQ
Where do I find the NFRC label if the windows aren’t installed yet?
The temporary NFRC label is applied to certified windows at the factory, so you usually will not see it until delivery. Before signing, ask the installer for the NFRC ratings for the exact configuration quoted, or ask for the product identifier so you can look it up in NFRC’s Certified Products Directory. Check the stickers against the quote when the windows arrive, before installation.
Is a lower U-factor always worth paying for?
Not automatically. A lower U-factor means better insulation, and it matters most in heating-dominated climates and cold rooms. How much it is worth depends on your climate, window area, energy prices, and how long you will stay. Compare the U-factor difference between quotes against the price difference, and be wary of any payback figure that does not show its assumptions.
My quote says “ENERGY STAR” but lists no numbers. Is that enough?
No. ENERGY STAR criteria are different for each climate zone, so the label alone does not tell you whether the product qualifies for your zone or how it compares with another quote. Ask for the NFRC U-factor, SHGC, VT, and air leakage, plus the ENERGY STAR zone it is certified for.
Should every window in my house have the same numbers?
It does not have to. Many manufacturers offer different glass packages for different orientations. A lower SHGC on west-facing glass in a hot climate, or a higher SHGC on south-facing glass in a cold climate, can make sense. Ask whether a split glass package is available and have each variant listed with its own NFRC ratings on the quote.