A window’s U-factor is the rate at which it lets heat through, and ENERGY STAR sets a maximum for each of its four climate zones. This page gives the current requirements (ENERGY STAR Version 7.0, in effect since 23 October 2023) for windows, doors and skylights, a lookup that finds your zone from your ZIP code using the 41,697 ZIP codes in the ENERGY STAR list, and a calculator that turns two U-factors into heat loss so you can see what the difference between, say, 0.27 and 0.30 is worth.
Find your ENERGY STAR window zone by ZIP code
ENERGY STAR window requirements by climate zone
| ENERGY STAR zone | U-factor (max) | SHGC | Equivalent R-value of the window | ZIP codes in the zone |
|---|---|---|---|---|
| Northern | ≤ 0.22 | ≥ 0.17 | R-4.5 or better | 18,439 (44%) |
| North-Central | ≤ 0.25 | ≤ 0.40 | R-4.0 or better | 9,713 (23%) |
| South-Central | ≤ 0.28 | ≤ 0.23 | R-3.6 or better | 9,017 (22%) |
| Southern | ≤ 0.32 | ≤ 0.23 | R-3.1 or better | 4,528 (11%) |
Every certified window must also have an NFRC-certified air leakage rating of 0.3 cfm per square foot or less. The ratings are for the whole unit, frame included, as tested and certified under the National Fenestration Rating Council (NFRC) program, not the glass alone. Lower U-factor is better everywhere. SHGC works the other way round depending on climate: the three warmer zones cap it, because sun through the glass adds to the cooling load, while the Northern zone sets a minimum, because winter sun is free heat there.
The Northern zone trade-off
| Northern zone path | U-factor | SHGC |
|---|---|---|
| Northern (prescriptive) | ≤ 0.22 | ≥ 0.17 |
| Northern (equivalent performance) | = 0.23 | ≥ 0.35 |
| Northern (equivalent performance) | = 0.24 | ≥ 0.35 |
| Northern (equivalent performance) | = 0.25 | ≥ 0.40 |
| Northern (equivalent performance) | = 0.26 | ≥ 0.40 |
In the Northern zone a window can qualify in two ways. The prescriptive path is a U-factor of 0.22 or lower with an SHGC of at least 0.17. The equivalent-performance path accepts a slightly higher U-factor, 0.23 to 0.26, as long as the glass lets in more sun: SHGC 0.35 or more at 0.23 and 0.24, and 0.40 or more at 0.25 and 0.26. ENERGY STAR treats the extra solar gain in a heating climate as making up for the extra conduction. That only works if the window actually gets sun, so a north-facing window, or one shaded by a porch or a neighbouring house, gains little from the higher SHGC and is better served by the lowest U-factor you can buy.
Doors and skylights
| Door glazing level | Zone | U-factor (max) | SHGC |
|---|---|---|---|
| Opaque (no glass) | All zones | ≤ 0.17 | not rated |
| Up to half glass (≤ ½-lite) | All zones | ≤ 0.23 | ≤ 0.23 |
| More than half glass (> ½-lite) | Northern and North-Central | ≤ 0.26 | ≤ 0.40 |
| More than half glass (> ½-lite) | South-Central and Southern | ≤ 0.28 | ≤ 0.23 |
| Skylight zone | U-factor (max) | SHGC |
|---|---|---|
| Northern | ≤ 0.45 | any |
| North-Central, South-Central and Southern | ≤ 0.50 | ≤ 0.25 |
Doors are rated by how much of the slab is glass, because glass is the weak spot. A solid insulated door has to reach U-0.17 in every zone, which is better than any qualifying window. A door that is more than half glass, such as a full-lite patio door, is held to roughly window-level numbers. Skylights get the most lenient U-factor limits (0.45 and 0.50), partly because sloped glazing loses more heat than the same glass standing upright, as warm air circulates more freely between the panes; on the other hand, a skylight takes the full overhead sun, so outside the Northern zone its SHGC is capped at 0.25.
U-factor and R-value conversion
| U-factor | R-value (1/U) | Btu/h per 100 sq ft per °F | Million Btu per 100 sq ft per season at 6,000 HDD |
|---|---|---|---|
| 0.15 | R-6.7 | 15 | 2.16 |
| 0.17 | R-5.9 | 17 | 2.45 |
| 0.20 | R-5.0 | 20 | 2.88 |
| 0.22 | R-4.5 | 22 | 3.17 |
| 0.25 | R-4.0 | 25 | 3.60 |
| 0.27 | R-3.7 | 27 | 3.89 |
| 0.28 | R-3.6 | 28 | 4.03 |
| 0.30 | R-3.3 | 30 | 4.32 |
| 0.32 | R-3.1 | 32 | 4.61 |
| 0.35 | R-2.9 | 35 | 5.04 |
| 0.40 | R-2.5 | 40 | 5.76 |
| 0.45 | R-2.2 | 45 | 6.48 |
| 0.50 | R-2.0 | 50 | 7.20 |
U-factor and R-value describe the same property from opposite ends. R-value is resistance, so you add R-values of layers; U-factor is conductance, the heat in Btu per hour that passes through one square foot for each degree Fahrenheit of difference between inside and outside. One is the reciprocal of the other, which makes the scale of a window easy to see. The best ENERGY STAR windows, at U-0.22, are about R-4.5. A 2×4 wall with R-13 batts works out at about R-9 through the framing layer once the studs are counted (the worked example on our R-value of wood page), twice the best window, before sheathing and drywall are added. Windows are always the weakest part of the envelope; what U-factor buys you is a smaller weak spot.
The scale is not linear in the way people expect. Going from U-0.50 to U-0.30 removes 0.20 Btu/h per square foot per degree; going from 0.30 to 0.22 removes only 0.08 more, even though both steps sound like “better windows”. Each further step saves less heat than the one before, and the saving is worth most where winters are long and cold, which is where ENERGY STAR sets its strictest limit.
Compare two windows: heat loss calculator
Worked example: U-0.27 vs U-0.30
A house has 150 sq ft of windows, about twelve 3 x 4 ft units. On a cold night with 70°F inside and 10°F outside the difference is 60°F. At U-0.30 the glass passes 0.30 x 150 x 60 = 2,700 Btu/h; at U-0.27 it passes 2,430 Btu/h, 270 Btu/h less. For the season, use heating degree days: a place with 6,000 heating degree days (inside the 5,400 to 9,000 range of IECC zones 5 and 6) gives a difference of 0.03 x 150 x 24 x 6,000 = 648,000 Btu, or about 6.5 therms of heat delivered. A furnace that is 90% efficient burns about 7.2 therms of gas to deliver that.
So the step from 0.30 to 0.27 is real but small for the whole house: a few therms a year. It matters more when the windows are large, the climate is cold or the energy is expensive, and it matters most when you are choosing between two otherwise identical windows at the same price. For comparison, the same 150 sq ft at the Northern limit (U-0.22) passes 1,980 Btu/h on that night; at the Southern limit (U-0.32) it would pass 2,880 Btu/h. In a cooling climate the bigger lever is usually SHGC, not U-factor, because summer sun through the glass can add more heat than conduction does.
How the ENERGY STAR zones compare with IECC climate zones
ENERGY STAR assigns each ZIP code to one of four zones. The International Energy Conservation Code (IECC) uses eight numbered zones with moisture letters instead, which is what our climate zone by county page lists. In the ENERGY STAR list every county falls in a single zone (all 3,144 state and county names in the file), so we matched it county by county against the 2015 DOE county table. Of the 3,133 counties that matched by name, 2,905 (93%) follow a simple pattern: IECC zones 1 and 2 are Southern, zone 3 is South-Central, zones 4A and 4B are North-Central, and the marine zone 4C plus zones 5 to 8 are Northern. Of the rest, 205 counties sit in a warmer ENERGY STAR zone than that pattern predicts and 23 in a colder one. Most of the warmer ones are 4A counties now in South-Central and 5A counties now in North-Central, the same direction in which the 2021 IECC map moved some counties, but the ENERGY STAR specification does not say which map its zones follow, so use the ZIP lookup rather than converting your IECC zone.
| IECC zone (2015 DOE table) | Northern | North-Central | South-Central | Southern |
|---|---|---|---|---|
| 1A | – | – | – | 8 |
| 2A | – | – | – | 221 |
| 2B | – | – | – | 20 |
| 3A | – | 16 | 539 | 22 |
| 3B | – | 1 | 107 | – |
| 3C | – | – | 14 | – |
| 4A | 5 | 632 | 96 | – |
| 4B | 1 | 54 | 2 | – |
| 4C | 38 | – | – | – |
| 5A | 543 | 74 | – | – |
| 5B | 140 | 9 | – | – |
| 5C | 1 | – | – | – |
| 6A | 337 | – | – | – |
| 6B | 122 | 2 | – | – |
| 7 | 19 | – | – | – |
| 7A | 85 | – | – | – |
| 7B | 16 | – | – | – |
| 8 | 9 | – | – | – |
ENERGY STAR window zones by state
Of the 50 states and DC, 24 sit entirely in one zone and 27 are split. All of Alaska, Connecticut, Idaho, Iowa, Maine, Massachusetts, Michigan, Minnesota, Montana, Nebraska, New Hampshire, North Dakota, Oregon, Rhode Island, South Dakota, Vermont, Washington, Wisconsin and Wyoming are Northern; Delaware, District of Columbia and Kentucky are entirely North-Central; Florida and Hawaii are entirely Southern. Arizona and California each have all four zones, from desert lowlands to the mountains. Percentages are shares of each state’s ZIP codes in the ENERGY STAR list, not of its area or population.
| State | ENERGY STAR zones (share of ZIP codes) |
|---|---|
| Alabama | South-Central 83%, Southern 17% |
| Alaska | Northern 100% |
| Arizona | Northern 14%, North-Central 9%, South-Central 14%, Southern 63% |
| Arkansas | North-Central 19%, South-Central 81% |
| California | Northern 5%, North-Central 6%, South-Central 89%, Southern 1% |
| Colorado | Northern 95%, North-Central 5% |
| Connecticut | Northern 100% |
| Delaware | North-Central 100% |
| District of Columbia | North-Central 100% |
| Florida | Southern 100% |
| Georgia | South-Central 78%, Southern 22% |
| Hawaii | Southern 100% |
| Idaho | Northern 100% |
| Illinois | Northern 73%, North-Central 27% |
| Indiana | Northern 53%, North-Central 47% |
| Iowa | Northern 100% |
| Kansas | Northern 9%, North-Central 91% |
| Kentucky | North-Central 100% |
| Louisiana | South-Central 32%, Southern 68% |
| Maine | Northern 100% |
| Maryland | Northern 5%, North-Central 95% |
| Massachusetts | Northern 100% |
| Michigan | Northern 100% |
| Minnesota | Northern 100% |
| Mississippi | South-Central 91%, Southern 9% |
| Missouri | Northern 15%, North-Central 83%, South-Central 2% |
| Montana | Northern 100% |
| Nebraska | Northern 100% |
| Nevada | Northern 33%, North-Central 19%, South-Central 48% |
| New Hampshire | Northern 100% |
| New Jersey | Northern 37%, North-Central 63% |
| New Mexico | Northern 44%, North-Central 35%, South-Central 21% |
| New York | Northern 75%, North-Central 25% |
| North Carolina | North-Central 17%, South-Central 83% |
| North Dakota | Northern 100% |
| Ohio | Northern 73%, North-Central 27% |
| Oklahoma | North-Central 17%, South-Central 83% |
| Oregon | Northern 100% |
| Pennsylvania | Northern 69%, North-Central 31% |
| Rhode Island | Northern 100% |
| South Carolina | South-Central 95%, Southern 5% |
| South Dakota | Northern 100% |
| Tennessee | North-Central 51%, South-Central 49% |
| Texas | North-Central 5%, South-Central 35%, Southern 60% |
| Utah | Northern 94%, South-Central 6% |
| Vermont | Northern 100% |
| Virginia | Northern 1%, North-Central 84%, South-Central 15% |
| Washington | Northern 100% |
| West Virginia | Northern 24%, North-Central 76% |
| Wisconsin | Northern 100% |
| Wyoming | Northern 100% |
How to read the NFRC label
Every ENERGY STAR window, door and skylight carries a temporary NFRC label, usually stuck to the glass, and the numbers on it are the ones to compare with the tables above. The two main ratings are U-factor and solar heat gain coefficient. Visible transmittance, a number between 0 and 1 for how much daylight gets through, is printed beside them; low-SHGC glass for hot climates can have a lower visible transmittance too, which you may notice as a slight tint. Air leakage and condensation resistance may also appear. For ENERGY STAR, the air leakage box shows “≤ 0.3” (or “≤ 0.5” for a swinging door), unless the maker shows compliance with one of the structural certification labels the spec allows instead.
Two things trip people up. First, compare whole-unit ratings with whole-unit ratings: brochures sometimes quote a “centre of glass” figure, which is better than the rating of the finished window because it leaves out the frame and the spacer at the edge of the glass. Second, the label rates the product as built. The same window in a different size, frame colour or glass package can have a different rating, so check the label on the unit you are actually buying.
Before you replace windows
- Seal first. Gaps around the frame and between sash and frame leak air, and air leakage can cost more than the difference in U-factor between two good windows. Caulk and weatherstrip existing windows before deciding they are the problem.
- Install it right. A low U-factor does little if the rough opening is left open around the frame. Fill the gap with low-expansion foam or backer rod and sealant, and flash the opening so water drains out.
- Match SHGC to the side of the house. In a heating climate a south-facing window with higher SHGC gains useful winter sun; in a cooling climate west-facing glass gets the hottest afternoon sun and benefits most from a low SHGC.
- Insulate the walls and attic too. Windows are a small share of the envelope area. The insulation R-value lookup gives the ENERGY STAR recommendation for your county, and the insulation calculator converts it to inches and bags.
- Energy codes are separate. A permit for new construction or replacement windows is checked against the energy code your state or city has adopted, which sets its own maximum U-factor by IECC zone. ENERGY STAR is a voluntary label and may be stricter or looser than your local code.
Frequently asked questions
What U-factor do I need for ENERGY STAR windows?
Under Version 7.0: 0.22 or lower in the Northern zone (or 0.23 to 0.26 with a higher SHGC), 0.25 or lower in North-Central, 0.28 or lower in South-Central and 0.32 or lower in the Southern zone.
What ENERGY STAR zone am I in?
Enter your ZIP code in the lookup above. It uses the ENERGY STAR “V7 ES Zones by Zip Code” list, which places every ZIP code, by its county, in one of the four zones.
Is a lower U-factor always better?
For heat loss, yes: lower U means less heat passes through. But it is only one rating. SHGC matters as much in hot climates, and air leakage and installation quality matter everywhere.
Is U-0.27 much better than U-0.30?
It passes 10% less heat by conduction. For 150 sq ft of windows in a 6,000 heating degree day climate, that is about 6.5 therms of heat a season; the calculator above works it out for your own numbers.
What is the R-value of a window?
It is 1 divided by the U-factor. A U-0.25 window is about R-4, a U-0.30 window about R-3.3, far less than an insulated wall.
When did the current ENERGY STAR window criteria take effect?
Version 7.0 took effect on 23 October 2023, and its tables were still the ones on the ENERGY STAR website when we checked on 25 October 2026.
Sources and method
The criteria tables are copied from the ENERGY STAR Version 7.0 specification and checked against the tables on the ENERGY STAR website. The ZIP lookup uses the ENERGY STAR ZIP code file unchanged: 41,697 ZIP codes, 3,136 county FIPS codes, none listed twice. The state shares and county comparison were counted by script from that file and the DOE county table.
- US EPA, ENERGY STAR, Program Requirements for Residential Windows, Doors, and Skylights, Eligibility Criteria Version 7.0 (effective 23 October 2023), Tables 1 to 5.
- ENERGY STAR, What makes it ENERGY STAR? Residential windows, doors and skylights, with the zone map and criteria tables (read 25 October 2026).
- ENERGY STAR, V7 ES Zones by Zip Code (41,697 ZIP codes with state, primary county and zone), linked from the ENERGY STAR climate zone search.
- US Department of Energy, Guide to Determining Climate Regions by County (PNNL-17211 Rev. 3, 2015), for the IECC comparison.
- All are US Government publications. The R-value conversion, heat-loss arithmetic, ZIP lookup, state shares and the county comparison are ours.
More insulation tools
- Insulation R-value by climate zone (county lookup)
- Climate zone by county: all US counties
- Window U-factor by climate zone: ENERGY STAR limits for windows, doors and skylights, with a ZIP code lookup
- Insulation calculator: R-value from thickness, or thickness for a target R, and how many bags or batts
- All construction calculators
Last reviewed: 25 October 2026. How we build and check our data: methodology and corrections. Spotted an error? Please let us know.