What a window here actually has to meet
Six different legal requirements across one metro, two places with none at all, and one number that clears all of them.
Ask a window salesman what efficiency rating you need and you will get a product recommendation. The actual answer is a legal one, it changes at city limits, and in a couple of places here the answer is that there is no requirement at all. We found 8 different requirements across this one metro.
The climate half has one answer
St. Louis City, St. Louis, St. Charles, Jefferson and Franklin counties in Missouri and Madison and St. Clair in Illinois all sit in IECC Climate Zone 4A, mixed-humid, confirmed one county at a time. The 2021 revision moved about a tenth of American counties between zones and none of these moved; the 4A/5A line in Illinois sits north of Madison County. Monroe County, Illinois — Columbia and Waterloo — was not in the county table we read, so we are not going to assert an IECC zone for it, though ENERGY STAR does place it with the rest of the metro. It is the legal half of the question that fragments.
What each code edition requires
| Edition | Max fenestration U-factor, Zone 4 | Max SHGC |
|---|---|---|
| 2009 IECC | U-0.35 | None |
| 2012 IECC | U-0.35 | — |
| 2015 IECC | U-0.35 | 0.40 |
| 2018 IECC | U-0.32 | 0.40 |
| 2021 IECC | U-0.30 | 0.40 |
| 2024 IECC | U-0.28 | — |
A widely circulated PNNL summary table appears to be shifted by one code cycle against DOE's own edition documents. The values here follow the four DOE and NGA documents that agree with each other. A dash means the SHGC figure for that edition could not be settled between sources, so none is printed. No jurisdiction in this metro is on the 2009 or 2012 edition in any case.
And what each place here has actually adopted
Grouped by what a replacement window has to meet:
| Requirement | Places | Which ones |
|---|---|---|
| U-0.28 or lower | 2 | Brentwood, Richmond Heights |
| U-0.30 or lower | 7 | Arnold, Clayton, Cottleville, Florissant, Kirkwood, O'Fallon, MO, St. Peters |
| U-0.32 or lower | 3 | Des Peres, St. Louis City, University City |
| U-0.34 or lower | 7 | Affton, Chesterfield, Manchester, Mehlville, Oakville, Unincorporated St. Louis County, Wildwood |
| U-0.35 or lower | 6 | Creve Coeur, Imperial, Lake St. Louis, Maryland Heights, St. Charles, Webster Groves |
| U-0.40 or lower | 1 | Wentzville |
| No energy code adopted | 8 | Ballwin, Eureka, Granite City, Pacific, Town and Country, Union, Washington, Waterloo |
| Not published | 11 | Alton, Belleville, Collinsville, Columbia, Edwardsville, Ellisville, Fairview Heights, Festus, Glen Carbon, O'Fallon, IL, Swansea |
Two of these are the jurisdiction's own number, not a code table
St. Louis County enforces U-0.34, which is the figure it wrote into the condition on its own window permit exemption. Wentzville enforces U-0.40, which the city wrote into its adopted version of the energy table — looser than the model 2021 code it otherwise adopted, and the loosest number anywhere in this metro. Both are published, both are real, and neither appears in any national table.
And two places have no requirement at all
Ballwin adopted the 2015 Residential Code and then wrote “Delete Chapter 11 Energy Efficiency.” It has adopted no energy conservation code either. Pacific deleted the same chapter and the corresponding building code section “in its entirety.” In those cities there is no codified U-factor or SHGC for a replacement window. That is not a loophole to exploit — it just means nobody will stop you buying badly.
Illinois works completely differently
Cross the river and the patchwork disappears. The Illinois Energy Conservation Code is adopted statewide by the Capital Development Board, and the Energy Efficient Building Act preempts local variation in both directions:
“No unit of local government, including any home rule unit, may regulate energy efficient building standards for residential buildings in a manner that is either less or more stringent than the standards established pursuant to this Act.”
20 ILCS 3125/15
The current edition is the 2024 IECC as amended, in force since 30 November 2025. The narrow exceptions that let a local government go stronger are: municipalities over a million people, which means Chicago and nobody in the Metro East; units of local government that had already adopted an equivalent or stricter code before 15 May 2009; and units adopting the Illinois Stretch Energy Code. Nothing lets one go weaker.
What we are not going to tell you
The unamended 2024 figure for this climate zone is U-0.28, and you will find sites stating that flatly as the Illinois requirement. But Illinois publishes its own amendments to the code it adopts, and it amended that same table in the previous cycle — for ceilings rather than windows, but it amended it. We have not been able to open the current amendment document, so we are not going to print U-0.28 as the number enforced in Madison, St. Clair or Monroe County. Buy to the ENERGY STAR figure below and it does not matter which way that lands.
A live discrepancy worth knowing about
The state's own technical-assistance site still names the 2021 edition. The Capital Development Board, which adopts the code, and the US Department of Energy both say 2024, effective 30 November 2025. If you are relying on this, the Capital Development Board's energy code office is on 217-685-4079.
One more Illinois point: the state code hangs off the local permit requirement. It applies to work “for which a municipality or county requires a building permit,” and there is no exemption for replacement windows as such. But in a village like Swansea, which exempts window and siding replacement from permitting altogether, there is no local permit for the state energy code to attach to on a same-size window swap.
When does the code actually apply to a replacement?
There is a clean line and almost nobody explains it. The energy code has a dedicated section for this:
“Where some or all of an existing fenestration unit is replaced with a new fenestration product, including sash and glazing, the replacement fenestration unit shall meet the applicable requirements for U-factor and SHGC.”
IECC R503.1.1.1
Replace the sash and the glazing and the new unit has to meet the code. Replace only the glass in an existing sash, or add a storm window over the existing unit, and it does not.
ENERGY STAR is stricter than every code in force here
All eight counties we work in are in ENERGY STAR's North-Central climate zone, confirmed one county at a time against ENERGY STAR's own finder. Under Version 7.0, in force since October 2023, that means:
- U-factor 0.25 or lower — stricter than every IECC edition adopted anywhere in this metro, including the 2024 edition Illinois enforces
- SHGC 0.40 or lower
- Air leakage 0.30 cfm/ft² or lower, which is the number manufacturers most often leave off the label because it is optional. If it is missing, ask why.
- The Most Efficient tier sits at U-0.20
The practical upshot: buy to the ENERGY STAR number and you clear every code requirement in this metro without having to know which one applies to you. That is the simple version of this whole page.
The federal tax credit for windows has ended
The 25C Energy Efficient Home Improvement Credit was terminated early. The IRS states that the credit “will not be allowed for any property placed in service after December 31, 2025,” and its main page now describes the credit as applying to property placed in service before that date. Any contractor quoting you a $600 window credit in 2026 is working from stale material. Neither the IRS page nor the ENERGY STAR credit page mentions siding at all.
Do not chase the lowest SHGC
In a mixed climate, solar gain is not purely a cost. The Department of Energy's guidance for Zones 3 and 4 is a moderate-solar-gain low-e with an SHGC between 0.25 and 0.40, which cuts heat loss and keeps light transmittance up while still letting some free winter heat in. Buying the lowest SHGC on the shelf is right for Houston and wrong here. In practice: west-facing glass takes the worst afternoon cooling load and wants the lowest number you can get; north-facing glass barely cares about SHGC at all and should be optimised on U-factor and visible transmittance instead.
Sources: glass.org · energycodes.gov · energycodes.gov · energycodes.gov · cdb.illinois.gov · energycodes.gov · law.justia.com · energystar.gov · energystar.gov
Common questions
What is a U-factor, in plain terms?
How fast heat escapes through the window. Lower is better, and the scale runs from about 0.10 to 2.00. A single-pane wood window is around 0.88. A decent modern double-pane is around 0.30. It is the number that matters most in a heating-dominated mixed climate like this one.
And SHGC?
Solar heat gain coefficient — how much of the sun's heat comes through, on a scale of 0 to 1. Lower keeps summer heat out; higher lets free winter heat in. In St. Louis you want it in the middle, roughly 0.25 to 0.40, and you want it lower on the west side of the house than the north.
If my city has no energy code, can I buy any window I like?
Legally, in Ballwin and Pacific, yes. Practically it is a bad idea: you will feel the difference in January, and the next owner's inspector will not care what the ordinance said. Buy to the ENERGY STAR figure and the question goes away.
Does replacing just the glass trigger the energy code?
No. The code catches you when the sash and the glazing are replaced together as a unit. Glass replaced in an existing sash and frame is a repair, and a storm window added over an existing window is expressly excepted.
Want the number for your address?
We look up the adopted edition and the U-factor it puts on the window before we quote.
Tell us about the house
We answer the phone. If we miss you, we call back.