Window U-Factor Explained: What Is a Good Number for Your Climate?

A window's U-factor measures how fast heat escapes through it. The lower the number, the better the window keeps heat in. The National Fenestration Rating Council (NFRC), which runs the rating label on new windows, gives a typical range of 0.20 to 1.20 for windows ¹.

What counts as a good U-factor depends on where you live. ENERGY STAR's current limits are 0.22 or lower in its Northern zone, 0.25 or lower in North-Central, 0.28 or lower in South-Central and 0.32 or lower in the Southern zone ³. So a window rated 0.27 meets the U-factor limit in the two southern zones but not in the two northern ones. The rest of this article explains the number, the other four ratings on the label, and how to compare two windows.

What U-factor actually measures

NFRC's glossary defines U-factor as the rate of non-solar heat loss or gain through a material or assembly ⁶. "Non-solar" is the key word: U-factor covers heat moving because it is warmer on one side of the glass than the other, not heat from sunlight. Sunlight is covered by a separate number, the solar heat gain coefficient (SHGC).

ENERGY STAR states U-factor in Btu per hour per square foot per degree Fahrenheit ³. You will never need the unit to shop, but it explains the scale: the number is the amount of heat that flows through each square foot of window for each degree of temperature difference. Lower means less heat flow.

If you have seen R-values on insulation, U-factor works the other way around. R-value measures resistance, so higher is better; U-factor measures flow, so lower is better.

The five ratings on the NFRC label

The label always shows U-factor, SHGC and visible transmittance. Air leakage and condensation resistance are optional ². NFRC describes them like this ¹²:

NFRC's consumer site gives a wider typical U-factor range, 0.10 to 2.00, and notes that a window rated above 0.3 for air leakage cannot qualify for ENERGY STAR ². The 1 to 100 condensation resistance scale also comes from that site ².

What is a good U-factor?

Use ENERGY STAR's zone limits as the benchmark. ENERGY STAR's criteria page links a lookup that tells you which zone your address falls in ⁴. The current limits ³:

All zones also have an air leakage limit of 0.3 cfm per square foot ³.

Is 0.27 a good U-factor?

It meets the U-factor limit for the South-Central (0.28) and Southern (0.32) zones ³. It does not meet North-Central (0.25) or Northern, where the limit is 0.22, or 0.26 at most on the alternate path with an SHGC of 0.40 or higher ³.

Is 0.28 or 0.30 good?

A U-factor of 0.28 meets the South-Central and Southern limits; 0.30 meets only the Southern limit of 0.32 ³. Both sit toward the low end of NFRC's typical 0.20 to 1.20 range, but neither earns the label in the two colder zones ¹³.

Is lower always better?

For U-factor alone, yes. But in the Northern zone ENERGY STAR deliberately accepts a slightly higher U-factor, 0.23 to 0.26, if the window has a higher SHGC ³, meaning it lets in more of the sun's heat ¹. A window should be judged on U-factor and SHGC together.

SHGC, the other big number

SHGC runs from 0 to 1. Low numbers resist solar heat and high numbers allow it ¹. In the South-Central and Southern zones ENERGY STAR requires 0.23 or lower, which limits the sun's heat ³. In the Northern zone it sets a floor instead: at least 0.17 ³. DOE's consumer guide adds that orientation matters: south-facing windows in a cold climate benefit from a high SHGC, while west-facing windows in most climates do better with a low SHGC or shade ⁵.

How to compare two window labels

1. Confirm the zone. Look up your ENERGY STAR climate zone ⁴.
2. Compare U-factor first in cold climates and SHGC first in hot ones. In mixed climates, look at both.
3. Check VT if you care about daylight; a lower SHGC may come with a lower VT.
4. Look for air leakage, and ask about it if it is missing; it is optional on the label ².
5. Compare the same size and type. Ratings are for a specific product, so compare a casement with a casement, not with a picture window.
6. Get the numbers in writing on the quote for the exact unit, not the product line in general.

For how the glass package moves these numbers, see low-E glass explained and triple-pane windows. The full picture is in energy-efficient windows explained.

If you want prices for windows that meet your zone's numbers, you can compare quotes from window installers and ask each for the NFRC ratings of the units they quote.

About these figures

• ¹ Official record: NFRC label descriptions and ranges for U-factor (0.20 to 1.20), SHGC (0 to 1), VT (0 to 1), air leakage (0.3 or less) and condensation resistance, checked October 22, 2026.
• ² Official record: NFRC consumer site on mandatory and optional ratings, the 0.10 to 2.00 typical U-factor range, the 1 to 100 condensation resistance scale, and the 0.3 air leakage limit for ENERGY STAR, checked October 22, 2026.
• ³ Official record: ENERGY STAR U-factor and SHGC limits by climate zone, the Northern alternate paths, air leakage of 0.3 cfm per square foot and U-factor units, read in the official criteria PDF, checked October 22, 2026.
• ⁴ Official record: ENERGY STAR statement that criteria vary by climate zone and its zone lookup, checked October 22, 2026.
• ⁵ Official record: DOE consumer guide (2021 PDF) on choosing SHGC and U-factor by window orientation and climate, checked October 22, 2026.
• ⁶ Official record: NFRC glossary definition of U-factor, checked October 22, 2026.