The St. Louis energy code and rebate guide

Missouri has no statewide code. Illinois has one, and it is a ceiling as well as a floor.

Missouri has no statewide residential building or energy code. What insulation a house here has to have is decided entirely by whatever code the city, village or county has adopted by local ordinance, and by whether that jurisdiction runs a permit and inspection programme at all. In unincorporated areas the county's code applies; inside a city the municipal ordinance does, and a municipality may contract its permitting back to the county. There is no state floor and no state pre-emption.

Each jurisdiction adopts a model code (almost always an ICC International Residential Code edition) by ordinance, usually with local amendments on file with the building official. There is no state floor and no state pre-emption.

The practical result on the Missouri side of this metro is 7 different energy code editions in force across the 34 jurisdictions on this site, 1 with no residential energy code at all, and 10 more that publish nothing we could verify. The oldest and the newest adopted editions here are 9 years apart, on the same question, in the same metro.

Illinois runs the opposite system. The Capital Development Board's Illinois Energy Conservation Code page states that the 2024 edition of the IECC as amended went into effect on 11/30/25 for both commercial and residential buildings. The 2021 edition applied to permit applications filed on or after 1/1/2024, and the 2018 edition took effect 7/1/2019.

20 ILCS 3125/20 provides that where a structure is being added to, altered, renovated or repaired, the Code adopted under the Act applies only to the portions of that structure that are being added, altered, renovated or repaired. The University of Illinois EnergySense center, which delivers the state's energy code training, states that renovations, alterations, additions, and repairs to most existing commercial and residential buildings must follow the Illinois Energy Conservation Code.

What the adopted editions actually require

These are the Missouri-side jurisdictions where the adopted edition and its amendments were readable. Where a city amended the R-value table down — 2 of them did — the amended number is the one shown, because that is the one an inspector applies.

Every Illinois jurisdiction on this site answers to the same state code, so they are left out of this table rather than repeated eleven times: R-49 attic, R-20 + R-5 continuous, or R-13 + R-10 continuous walls, climate zone 4.

The code number is a floor. This is the number worth building to.

Code minimums are written for new construction and they are the least you may legally do. The Department of Energy publishes a separate recommendation for what actually pays for itself in this climate zone, and it is higher.

DOE's page states it uses the 2021 International Energy Conservation Code climate zone map. The whole metro — all eight counties on both sides of the river — is all eight counties are iecc 2021 climate zone 4, moisture regime a - zone 4a, mixed-humid..

The climate the numbers are built for

Those last two lines are the ones worth sitting with. NOAA publishes both figures; we have deliberately not subtracted one from the other to produce a “freeze-thaw cycles per year” number, because that would be our arithmetic rather than NOAA's. The published pair says enough on its own: most St. Louis freezes thaw the same day, and it is that daily cycling, not the depth of the cold, that drives moisture up into a cold attic and leaves it there.

Who pays you to do it

On the Missouri side

This side of the metro is on the Missouri side, which for most addresses means Ameren Missouri — and Ameren Missouri does not hand out a straight per-square-foot insulation rebate the way its Illinois sister utility does. What it runs instead is Pay As You Save: a free home energy assessment, then the work installed by a programme partner and paid back through a fixed monthly charge on the electric bill, which cannot exceed 80% of the estimated monthly saving, so the bill still comes down while it is being repaid. No loan and no credit check.

Published incentives inside that programme for the 2026 programme year are $0.20 per square foot on ceiling and attic insulation, $0.06 per square foot on air sealing, and 50% of costs up to $250 per home on duct sealing. Each one has a condition attached, and they matter:

• Attic: Beginning R-value must be R-20 or below and must be increased to a minimum of R-38. An attic already better than R-20 does not qualify, however much you add to it.
• Air sealing: Must achieve a minimum 20 percent air leakage reduction. That is measured, not asserted, which is why the blower door goes on the door twice.
• Duct sealing: Must achieve a minimum 20 percent duct leakage reduction.
• Who qualifies: the free assessment is open to Ameren Missouri residential electric customers generally, but the incentives above sit inside tier 4 of Pay As You Save and require that you have lived in the home for the past twelve months.

Ameren Missouri also runs CommunitySavers, which installs attic insulation at no charge for income-eligible households. The program operates through December 31, 2026; incentives are dependent on available funds.

Spire, the gas utility on this side, lists furnace, water heater and thermostat rebates on its Missouri rebate page rather than a standalone insulation one. Its published route to attic insulation and air sealing is the same on-bill Pay As You Save programme, which it co-delivers, plus income-based weatherization. If you are a Spire customer it is worth ringing them to ask what is open this year, because that page moves.

If the household income qualifies, the work can be free rather than discounted. St. Louis County is served by Community Action Agency of St. Louis County (CAASTLC) under the Missouri low-income weatherization program — 314-863-0015, details here. That is a different queue from ours, and we will point you at it rather than sell you something you could get for nothing.

On the Illinois side

This side of the metro is on the Illinois side, and the Illinois programme is the more generous of the two by a wide margin. Ameren Illinois publishes flat per-unit discounts applied by the contractor rather than mailed to you as a cheque. On Ameren Illinois' 2026 discount table: $1.10 per square foot for attic insulation and the same for wall insulation, $2.00 per linear foot for rim joist, $3.00 per linear foot for crawl space wall, and $0.70 per cubic foot per minute (CFM) of air leakage reduced for air sealing.

Two conditions decide whether any of that reaches you. Air sealing has to be pre-approved before the work starts — do it first and ask afterwards and the money is gone. And only a registered Ameren Illinois Program Ally can apply the discount at all: a contractor who is not on that list cannot get it for you, whatever they say on the doorstep. Ask which they are before you sign.

Compare that with the Missouri side of the metro, where the same attic gets an on-bill programme paying $0.20 per square foot instead, and only if the attic started at R-20 or below. Same weather, same houses, different utility regulator.

If the household income qualifies, the work can be free rather than discounted. Madison County is served by Madison County Community Development under the Illinois low-income weatherization program — 618-692-8940, details here.

One more, by its absence: the federal 25C credit, which covered 30% of insulation and air sealing up to $1,200 a year, ended for anything placed in service after 31 December 2025. Work finished on or before that date is still claimed on that year's return; nothing done in 2026 is. It should not be in your 2026 price, whoever tells you otherwise — the rebate guide has the detail.

The two things that stop an old attic job

Vermiculite

EPA describes vermiculite insulation as "a pebble-like, pour-in product" that is "usually gray-brown or silver-gold in color." It appears as shiny, expanded flakes poured between attic joists or into wall cavities, not as batts or blown fiber. EPA states the Libby, Montana mine "supplied over 70 percent of all vermiculite sold in the United States from 1919 to 1990" and that "the vermiculite from Libby was contaminated with asbestos."

Neither the federal asbestos rule nor Missouri's own abatement regulation forces a single-family homeowner to hire a licensed abatement contractor: both carve out residential buildings of four or fewer dwelling units, and Missouri's exemption is set out in the Department of Natural Resources' own publication. We were not able to read the equivalent definition in the Illinois rule, so we are not stating an Illinois threshold here — on that side of the river, ask the Illinois EPA before anyone touches it. Either way the exemption is a regulatory one, not a safety opinion. We do not blow insulation over vermiculite and we do not remove it.

Knob-and-tube wiring

This one catches people out because it reads like an insulation rule and is not: it is an electrical one, and it means the wiring gets dealt with first or the attic does not get insulated. In a pre-war St. Louis or Metro East house it is worth checking before you take a quote seriously.

Permits: what this metro actually publishes

Of the 45 jurisdictions on this site, only a handful say anything at all about whether insulation or air sealing needs a permit. The most useful document in the metro is St. Louis County's own list of work that does not require a building permit, which covers unincorporated St. Louis County and, by contract, several municipalities inside it: increasing attic insulation is exempt, and so is exterior wall insulation installed under new or replacement siding — but insulating, furring and drywalling the inside face of an exterior or basement wall does need a permit.

Everywhere else, the honest answer is that the city publishes no answer. We confirm it with the department before starting and pull whatever is required. Your city's page says exactly what that city publishes and nothing more.

Municipal research 2026-09-09; energy code, rebate and climate research 2026-09-10. Rebate amounts carry the programme year they were published under. Sources: stlpr.org · cdb.illinois.gov · cdb.illinois.gov · ameren.com · ameren.com · ameren.com · ameren.com · ameren.com · amerenillinoissavings.com · irs.gov · irs.gov · energy.gov · energy.gov · ncei.noaa.gov · epa.gov · berkeleyca.gov. Adopted code editions, department phone numbers and utility rebate amounts all change — confirm with the jurisdiction or the utility before relying on anything here.