Attic insulation, air sealing, crawl spaces and walls for homes in O'Fallon — starting with a free assessment that measures the R-value you actually have. The typical home here dates to the late-1990s, and 5% of its 13,530 homes went up before 1940.
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 13,530 housing units here. 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 fiber 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.
The Census Bureau estimates 13,530 homes in O'Fallon. 731 of them (5%) went up before 1940 — 21st of 44 places on this site for pre-1940 housing. The 2000s were the busiest decade, adding 3,292 of today's homes (24%); 26% predate 1980 and 45% were built in 2000 or later. For St. Clair County as a whole the pre-1940 share is 13%, so O'Fallon sits below it.
Of 12,816 occupied homes, 58% heat with utility gas and 40% with electricity. Electric heat here is more common than across the county (33%), and ranks 10th of 44 on this site. That is why the assessment asks how the house is heated: the fuel and the name on the utility bill decide which program, if any, can pay toward the work.
70% of occupied homes are owner-occupied and 67% of all homes are single-family detached houses, with 423 mobile homes in the count. With houses like these, the conversations we expect most are about air sealing on newer houses that already have depth and an energy assessment before deciding what to fix first.
| Built | Homes | Share | |
|---|---|---|---|
| Before 1940 | 731 | 5% | |
| 1940s | 139 | 1% | |
| 1950s | 482 | 4% | |
| 1960s | 835 | 6% | |
| 1970s | 1,342 | 10% | |
| 1980s | 1,592 | 12% | |
| 1990s | 2,372 | 18% | |
| 2000s | 3,292 | 24% | |
| 2010s | 2,403 | 18% | |
| 2020 or later | 342 | 3% |
13,530 homes, Census ACS 2020–2024 (B25034, B25040, B25003); 12,816 occupied.
3,964 of O'Fallon's 13,530 homes (29%) were built in the 1980s and 1990s. The gaps in houses of that age are the parts nobody sealed: batts stuffed into the rim joist, bare spots over cathedral and tray ceilings, soffit vents buried with no baffles, can lights punched through the ceiling plane. A top-up plus a day of sealing usually covers it.
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.
That edition sets R-49 for the attic in new work, and R-20 + R-5 continuous, or R-13 + R-10 continuous for walls. The Department of Energy's recommendation for this climate zone runs higher: R60 for an attic with nothing in it, R49 where a few inches are already down.
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. 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, where Ameren Illinois publishes discounts the contractor applies on the invoice: $1.10 per square foot for attic or wall insulation, $2.50 per linear foot for rim joist, $3.75 per linear foot for crawl space wall and $0.70 per cubic foot per minute (CFM) of air leakage reduced for air sealing on Ameren Illinois' 2026 discount table. The discount comes off a registered Ameren Illinois Program Ally's price, once the program's home energy assessment confirms the house is eligible. Income-qualified households can apply to St. Clair County Community Action Agency (618-277-6790) for free weatherization instead. For every condition attached, see the St. Clair County page or the rebate guide.
Every layer of an insulated attic, explained
Elsewhere in St. Clair County we also cover:
Illinois sets the residential energy code from Springfield, so the attic figure is R-49 wherever it is published here; what changes is the permit desk, and none of the 4 publishes a permit rule for insulation.
| City | Energy code | Attic R required | Permit to insulate | Building dept. |
|---|---|---|---|---|
| O'Fallon | Illinois state code | R-49 | Not published | 618-624-4500 |
| 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 |
All 4 St. Clair County jurisdictions side by side →
O'Fallon enforces the Illinois state code, which sets R-49 for an attic in new work. That code also reaches work on existing houses, not only new ones.
O'Fallon publishes no rule either way, so we ask its building department at 618-624-4500 before the job is scheduled.
Ameren Illinois' 2026 table: $1.10 per square foot for attic insulation and $0.70 per cubic foot per minute (CFM) of air leakage reduced for air sealing, taken off the invoice by a registered Program Ally following a home energy assessment. The federal 25C credit ended for work placed in service after December 31, 2025.
The typical home in O'Fallon dates to around the late-1990s; the Census Bureau's 2020-2024 median year built is 1997. Of its 13,530 homes, 5% went up before 1940 and 45% in 2000 or later. Half the homes here are older than that and half newer; yours may sit at either end.
Code and permit facts checked September 10, 2026, rebates October 24, 2026; housing: Census ACS 2020–2024. Sources: ofallonillinois.org · codelibrary.amlegal.com (2) · cdb.illinois.gov · ofallon.org · codes.iccsafe.org · en.wikipedia.org · bestplaces.net · ameren.com (2, 3, 4) · energystar.gov · data.census.gov. These change; confirm with the building department at 618-624-4500 or your electric utility first.
Free assessment, a measured R-value, and photos you keep either way.
Book my free assessment →