The R-value question is settled in Springfield for O'Fallon. What is local is the permit desk, the contractor registration and the rebate paperwork — all of it below, with the housing stock that decides the job.
O'Fallon runs to about 16 square miles and roughly 32,289 people. It sits about 15 miles east of downtown St. Louis. There are around 13,225 housing units here, and the great majority of them are heated and cooled through a ceiling nobody has ever sealed. The community dates back to 1874. We cover ZIP codes 62269 and 62225.
We have not logged a completed job in O'Fallon yet. We would rather say so than put a number on this page that is not real.
The typical home in O'Fallon dates to around the late-1990s. Houses of this vintage usually got batts or blown fibre at whatever the standard of the day was, which is well short of what is recommended now. Most are one top-up away from being right.
Housing is spread across many named platted subdivisions (Thornbury Hill, Nolin Creek Estates, Fairwood Hills, Deer Creek, Forest Hills, Fairwood East), reflecting late-20th-century subdivision-driven growth after the city's 1854 founding.
We work across O'Fallon rather than one corner of it — Thornbury Hill, Nolin Creek Estates, Fairwood Hills, Deer Creek, Forest Hills and Fairwood East included. Construction and vintage change between them, sometimes by thirty years and a whole different framing method, which decides whether a wall can be dense-packed at all.
Founded in 1854 and incorporated as a village in January 1874 (city form of government adopted 1905), O'Fallon, IL sits near Scott Air Force Base, which has driven much of its subdivision growth.
Housing from this stretch in O'Fallon generally got a real attempt at insulation — a real layer in the attic, batts in the walls — so the argument here is rarely about starting from nothing. It is about the gap between what was standard then and what is recommended now, and about the parts nobody insulated at all. Recurring finds: batts stuffed into the rim joist rather than sealed, insulation missing entirely above a cathedral or tray ceiling, soffit vents blocked solid by insulation blown over them with no baffles, and can lights punched straight through the ceiling plane. A house from this era is usually a top-up plus a day of sealing, not a rebuild.
O'Fallon enforces the Illinois Energy Conservation Code (2024 IECC with Illinois amendments), by state law. What the state code requires is known independently: 20 ILCS 3125 makes the Illinois Energy Conservation Code, currently the 2024 IECC as amended effective 11/30/2025, the governing residential energy standard in every Illinois municipality of under 1,000,000 people.
Under that edition the required attic value for new work here is R-49, and walls R-20 + R-5 continuous, or R-13 + R-10 continuous. For comparison, the Department of Energy recommends R60 for an uninsulated attic in this climate zone and R49 where there are already a few inches up there.
Table R402.1.3 of the 2024 IECC as amended for Illinois, climate zone 4. St. Clair County is in climate zone 4 per the NAIMA IECC compliance guide for Illinois homes.
O'Fallon publishes no answer either way. Insulation and air sealing appear on neither the permit-required list nor the exempt list. That is the normal state of affairs across this metro — only a handful of the 45 jurisdictions on this site say anything about it at all. The city's economic development site states that O'Fallon issues building, plumbing and electrical permits for any new construction and most interior renovations, but does not name insulation. We confirm it with the department before we start and pull whatever is needed. We would rather write that here than guess on your behalf.
The published number is 618-624-4500. The department page is here.
Permitting is on us, not on you.
O'Fallon 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. St. Clair County is served by St. Clair County Community Action Agency under the Illinois low-income weatherization program — 618-277-6790, 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.
O'Fallon sits in all eight counties are iecc 2021 climate zone 4, mixed-humid, which is the zone that has to be defended in both directions: 4,433 heating degree days and 1,713 cooling degree days a year at Lambert. That is what every R-value on this page is sized for. The energy code and rebate guide works through the rest of the figures.
Elsewhere in St. Clair County we also cover:
Illinois sets the residential energy code from Springfield, so these figures agree across the county — what changes here is the permit desk, not the R-value.
| City | Energy code | Attic R required | Permit to insulate | Building dept. |
|---|---|---|---|---|
| Belleville | Illinois state code | R-49 | Not published | 618-233-6817 |
| Fairview Heights | Illinois state code | R-49 | Not published | 618-489-2060 |
| Swansea | Illinois state code | R-49 | Not published | 618-234-0044 |
Free assessment, a measured R-value, and photos you keep either way.
The code edition O'Fallon enforces sets R-49 for new work. The Department of Energy separately recommends R60 for an uninsulated attic in this climate zone, and R49 where a few inches are already there.
Nobody can tell you from a website, including this one. O'Fallon lists neither insulation nor air sealing as permit work, and does not exempt it either. We ring the department, get an answer for your specific job, and pull anything it needs.
Ameren Illinois publishes $1.10 per square foot for attic insulation and $0.70 per cubic foot per minute (CFM) of air leakage reduced for air sealing on its 2026 discount table, applied by the contractor rather than paid to you directly. Two conditions decide whether it reaches you: air sealing has to be pre-approved before the work starts, and only a registered Ameren Illinois Program Ally can apply the discount at all. The federal 25C credit ended for work placed in service after 31 December 2025.
The typical home in O'Fallon dates to around the late-1990s. Houses of this vintage usually got batts or blown fibre at whatever the standard of the day was, which is well short of what is recommended now. Most are one top-up away from being right. That figure is an ACS-derived median republished by housing-data services, not a survey of individual houses.
Not yet. We have no completed job logged in O'Fallon, and we would rather say so than put a number on this page that is not real.
Illinois does not license insulation contractors as a trade, 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 licence, others require none at all. What matters more in practice is whether the contractor is a registered Ameren Illinois Program Ally, because only an Ally can apply the rebate.
Energy code, permit and department details for O'Fallon come from the jurisdiction's own published pages and codified ordinances. Rebate figures come from the utility's own published incentive tables and carry the programme year they were verified in. Housing-age figures are ACS-derived medians republished by housing-data services, so treat them as approximate. Sources: ofallonillinois.org · codelibrary.amlegal.com · cdb.illinois.gov · ofallon.org · codelibrary.amlegal.com · codes.iccsafe.org · en.wikipedia.org · bestplaces.net · ameren.com · ameren.com · ameren.com · ameren.com · energy.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.