What U-Factor Do I Need? Code Minimum vs. ENERGY STAR by Climate Zone

The U-factor you need depends on your climate zone and on which bar you are trying to clear. The building code sets the legal minimum for a permit. ENERGY STAR is a voluntary EPA label with stricter limits. Under the 2024 International Energy Conservation Code (IECC), the maximum window U-factor is 0.30 in climate zones 3 and 4, 0.28 in zones 5 and 6, and 0.27 in zones 7 and 8. ENERGY STAR Version 7.0 asks for 0.22 or lower in its Northern zone. In both systems, lower is better. The number that applies to your house comes from the code edition your state or city actually enforces, and that is often not the newest one.

Updated: October 2026.

Code minimums by climate zone: 2021 and 2024 IECC

The table shows the residential IECC's prescriptive limits for vertical fenestration, meaning windows and glazed doors. U-factor and SHGC values are maximums. "NR" means the code sets no requirement.

A few rules sit around the table. The 2024 edition allows a U-factor of 0.30 in Marine zone 4 and zones 5 through 8 for buildings above 4,000 feet or in wind-borne debris regions where opening protection is required. Up to 15 square feet of glazed fenestration per dwelling unit is exempt from the U-factor and SHGC limits (2024 IECC, Section R402.4.3).

The limits also cover replacements. When some or all of an existing window is replaced, "including sash and glazing," the new unit has to meet the same U-factor and SHGC table (2024 IECC, Section R503.1.1.1). If you replace several windows, an area-weighted average of the new units can be used to comply.

The code is specific about where the numbers come from. U-factor is determined under NFRC 100 and SHGC under NFRC 200, by an accredited independent laboratory, and the product is labeled and certified by the manufacturer. A product without a labeled rating is assigned a default value from the code's tables (2021 IECC, Section R303.1.3).

ENERGY STAR is a separate, stricter bar

ENERGY STAR uses its own four zones, which have names instead of numbers, and assigns them by county and ZIP code. Its map is not the IECC map, so look up both. Version 7.0 took effect on October 23, 2023. A window qualifies only with NFRC-certified ratings that meet its zone's limits, plus air leakage of 0.3 cfm/ft² or less.

In the Northern zone, ENERGY STAR also accepts U-factors from 0.23 to 0.26 when the SHGC is high enough, which trades a little insulation for more winter sun. ENERGY STAR's own summary is short: in the North, "you are generally looking for windows with a U-factor that is 0.22 or lower," and in the South, an SHGC of 0.23 or lower.

The gap between the two bars is real. A window rated 0.28 passes the 2024 IECC in zones 5 and 6, but it misses the ENERGY STAR limit in the Northern and North-Central zones. For more on the label itself, see our page on ENERGY STAR windows.

Example: Chicago

Cook County is in IECC climate zone 5A. Illinois has enforced the 2024 IECC, as amended by the state, for residential buildings since November 30, 2025, according to the Illinois Capital Development Board. In the base 2024 table, zone 5 allows a U-factor of 0.28.

Inside city limits, Chicago runs its own energy code. The City's Department of Buildings says the 2022 Chicago Energy Transformation Code is based on the 2021 IECC and applies to projects whose first permit application started on or after November 1, 2022. The base 2021 table allows 0.30 in zone 5. Neither agency's page lists window amendments, so confirm the figure with the building department before you order. A window rated 0.22 or lower clears both code tables and the ENERGY STAR Northern limit, whichever applies.

Example: Texas

Texas has one statewide energy code for one- and two-family homes of three stories or less. The State Energy Conservation Office (SECO) adopted Chapter 11 of the 2015 International Residential Code, effective September 1, 2016. The DOE Building Energy Codes Program lists the state's residential code as the 2015 IECC with amendments. Those 2015 limits are looser than the newer tables:

The 2015 code also applies to replacement windows, including sash and glazing (2015 IRC, Section N1109.1.1.1). The code's 0.25 SHGC limit sits close to ENERGY STAR's 0.23 for the South-Central and Southern zones, while the U-factor gap is wider: 0.40 under the Texas code against 0.28 or 0.32 for ENERGY STAR. Ask your city's building department which edition and local amendments it enforces for your permit.

How to check a window against your number

1. Find your IECC zone in the code's county table, and your ENERGY STAR zone with the ENERGY STAR climate zone finder.
2. Ask the building department which code edition and amendments apply to your permit date.
3. Read the NFRC label. NFRC says U-factor "measures how well a product keeps heat from escaping a room," and "the lower the number, the better it is at keeping heat in." SHGC measures how much solar heat the product lets in.
4. Confirm the exact product and glass package in the NFRC Certified Products Directory or the ENERGY STAR Product Finder.
5. Model the difference. In the GlassBase window energy calculator, enter your ZIP code and wall direction, then compare two glazing options in kWh of heating and cooling energy. The results are engineering estimates, not NFRC ratings. See how the simulator works.

DOE's window guide lists low-E coatings and spacers among the features that cut heat flow through a window. See low-E glass and spacers, and look up terms in the glossary. To find window companies, browse Chicago, Houston, Dallas, Austin or San Antonio.

Compare U-factors for your ZIP code

Sources

• International Code Council, 2024 IECC, Chapter 4 [RE] (Tables R402.1.2 and R402.1.3, footnote on 0.30 above 4,000 feet or in wind-borne debris regions, Section R402.4.3) and Chapter 5 [RE] (Section R503.1.1.1)
• International Code Council, 2021 IECC, Chapter 4 [RE] (Table R402.1.2) and Chapter 3 [RE] (Table R301.1, Cook County 5A; Section R303.1.3)
• International Code Council, 2015 IECC, Chapter 3 [RE] (Table R301.1, Texas counties) and Chapter 4 [RE] (Table R402.1.2)
• International Code Council, 2015 IRC, Chapter 11 (Table N1102.1.2; Section N1109.1.1.1)
• US EPA, ENERGY STAR, Residential Windows, Doors, and Skylights Version 7.0 (Tables 1, 4 and 5; effective October 23, 2023) and climate zone finder
• US EPA, ENERGY STAR, High Performing Windows or Storm Windows (0.22 in the North, SHGC 0.23 in the South)
• Illinois Capital Development Board, Illinois Energy Conservation Code (2024 IECC as amended, effective 11/30/25)
• City of Chicago, Department of Buildings, Chicago Energy Transformation Code
• Texas Comptroller, SECO, Single-Family Construction (2015 IRC Chapter 11, effective September 1, 2016)
• US DOE, Building Energy Codes Program, Texas and Illinois code status
• NFRC, What does the NFRC label tell you? (U-factor and SHGC definitions) and Certified Products Directory
• US DOE, Guide to Energy-Efficient Windows (spacers and low-E coatings)