How much insulation the law actually asks for on your street, who pays you to put it in, and the two things in an old St. Louis attic that stop a job before it starts. Of the 45 jurisdictions on this site, 34 publish an energy code edition and 32 a required attic R-value. Every claim below names its source.
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 program 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.
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.
| Jurisdiction | County | Energy code | Attic R required |
|---|---|---|---|
| Affton | St. Louis County | 2015 IECC | R-49 |
| Creve Coeur | St. Louis County | 2015 IECC | R-49 |
| Imperial | Jefferson County | 2015 IECC | R-49 |
| Lake St. Louis | St. Charles County | 2015 IECC | R-49 |
| Maryland Heights | St. Louis County | 2015 IECC | R-49 |
| Oakville | St. Louis County | 2015 IECC | R-49 |
| St. Charles | St. Charles County | 2015 IECC | R-49 |
| Unincorporated St. Louis County | St. Louis County | 2015 IECC | R-49 |
| Ellisville | St. Louis County | 2015 IRC ch. 11 | R-49 |
| St. Louis City | St. Louis City | 2018 IECC | R-49 |
| University City | St. Louis County | 2018 IECC | R-49 |
| Des Peres | St. Louis County | 2018 IRC ch. 11 | R-49 |
| Eureka | St. Louis County | 2018 IRC ch. 11 | R-49 |
| Town and Country | St. Louis County | 2018 IRC ch. 11 | R-49 |
| Arnold | Jefferson County | 2021 IECC | — |
| Clayton | St. Louis County | 2021 IECC | R-60 |
| Florissant | St. Louis County | 2021 IECC | R-60 |
| Kirkwood | St. Louis County | 2021 IECC | R-60 |
| O'Fallon | St. Charles County | 2021 IECC | — |
| Cottleville | St. Charles County | 2021 IRC ch. 11 | R-38 |
| St. Peters | St. Charles County | 2021 IRC ch. 11 | R-38 |
| Brentwood | St. Louis County | 2024 IECC | R-49 |
| Richmond Heights | St. Louis County | 2024 IECC | R-49 |
| Pacific | Franklin County | None adopted | — |
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.
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.
| Where | DOE recommendation, zone 4 | Note |
|---|---|---|
| Attic, uninsulated | R60 | The depth that reaches it depends on the material, which is why we mark it on a ruler rather than quote inches |
| Attic with 3–4 inches already there | R49 | The usual St. Louis case: something is up there, it is just not enough |
| Floor over a crawl space | R19 | Only if the crawl space is vented and stays that way |
| Wood frame wall, uninsulated | R20 + R5 CI, or R13 + R10 CI, or R0 + R15 CI | New construction figures; a dense-pack retrofit is judged on what fits the cavity |
| Wall already insulated, 2x4 | Add R10 CI | Continuous exterior board, which is a siding job, not an insulation-only one |
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 in IECC climate zone 4A, mixed-humid.
The code sets the number. These are the layers that deliver it, numbered from the top.
| Figure | Number | What it drives |
|---|---|---|
| Climate zone | Zone 4A, mixed-humid | Every R-value in the code table, on both sides of the river |
| Heating degree days | 4,433 | How hard the furnace works. Base 65°F, 1991–2020 normals at St. Louis Lambert International Airport |
| Cooling degree days | 1,713 | Why an under-insulated attic costs you in July as well as January |
| Nights at or below freezing | 84.9 | Condensation on the cold side of the insulation |
| Days that never rise above freezing | 19.3 | Most St. Louis freezes thaw the same day — the cycling is the problem, not the cold |
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.
On the Missouri side most addresses buy their electricity from 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 program partner, with the incentives below taking part of the cost off and the rest paid back through a fixed monthly charge on the electric bill. That charge 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 program for the 2026 program 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:
Ameren Missouri also runs CommunitySavers for income-eligible households: the products and services it installs can include attic insulation, and there is no charge for the assessment or the work. 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 program, which it co-delivers, plus income-based weatherization. If you are a Spire customer it is worth calling 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, through the Missouri low-income weatherization program. It is run county by county:
That is a different queue from a paid job, and worth checking before anyone quotes you.
On the Illinois side the program is the more generous of the two by a wide margin. Ameren Illinois publishes flat per-unit discounts taken off the contractor's price rather than mailed to you as a check. On Ameren Illinois' 2026 discount table: $1.10 per square foot for attic insulation and the same for wall insulation, $2.50 per linear foot for rim joist, $3.75 per linear foot for crawl space wall, $0.70 per cubic foot per minute (CFM) of air leakage reduced for air sealing, and $225.00 per residence toward ENERGY STAR exhaust ventilation.
The steps are Ameren Illinois' own. First, its Residential Eligibility Tool says whether the address qualifies. Second, the discount is offered only through a registered Ameren Illinois Program Ally — a contractor enrolled and trained by the program — listed in its directory under “Insulation, Air Sealing and Ventilation”. Third, the Program Ally does a home energy assessment and, where the house qualifies, hands over a project application showing the estimated discount and the out-of-pocket cost. Ameren says you can ask more than one Program Ally for a quote.
Compare that with the Missouri side of the metro, where the same attic gets an on-bill program 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, through the Illinois low-income weatherization program. It is run county by county:
That is a different queue from a paid job, and worth checking before anyone quotes you.
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.
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.
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.
Neither state licenses insulation contractors as a trade. In Missouri there is no statewide residential code at all, and registration is imposed city by city — several metro municipalities require a contractor to be registered with them before a permit is pulled.
Illinois does not license insulation contractors as a trade either, though it does run the residential energy code from Springfield rather than leaving it to each city. Municipal registration varies — some Metro East cities require a business license, others require none at all.
What matters more in practice, on either side of the river, is utility program enrollment, because it decides whether a program discount can be applied to the job: on the Illinois side only a registered Ameren Illinois Program Ally can offer it, and on the Missouri side Pay As You Save work is installed by the program's own partners.
The code and the rebates decide the target; how the work is done is on each service page: attic insulation, spray foam insulation, crawl space insulation, basement insulation, garage insulation, insulation removal, home energy audits, wall insulation and air sealing.
Municipal research 2026-09-09; energy code, rebate and climate research 2026-09-10. Rebate amounts carry the program year they were published under. Sources: stlpr.org · cdb.illinois.gov (2) · ameren.com (2, 3, 4, 5) · amerenillinoissavings.com · cdn.amerenillinoissavings.com · irs.gov (2) · energy.gov · energystar.gov · ncei.noaa.gov · epa.gov · berkeleyca.gov. These change; confirm with the office or the utility first.
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