Attic insulation, air sealing and crawl space work across St. Clair County. We cover 4 jurisdictions here, each with its own page: the energy code that applies, the attic R-value it requires, the permit position and the building department's number. Of the county's 115,630 homes, 13% were built before 1940, and 33% of occupied homes heat with electricity.
Click a city for its full code, permit and rebate detail.
| 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 |
| O'Fallon | Illinois state code | R-49 | Not published | 618-624-4500 |
| Swansea | Illinois state code | R-49 | Not published | 618-234-0044 |
All 4 publish an energy code edition we could name and all 4 a required R-value we could read; a blank in this table means the jurisdiction publishes nothing, not that we guessed.
Illinois writes one residential energy code for the whole state, and under 20 ILCS 3125 a town may make it neither weaker nor stronger, so the attic figure for new work is R-49 in every one of the 4 places we cover in St. Clair County. What changes from town to town is the permit desk.
The 4 places here with their own Census row hold 48,096 of St. Clair County's 115,630 homes (42%); the rest are in other municipalities or unincorporated areas.
One detail worth knowing before a job in each of them:
St. Clair County has 115,630 homes in the Census count, with a median year built of 1975. 55% were built before 1980 and 22% in 2000 or later; the single biggest decade was the 2000s, with 16,452 homes (14%). Of 100,846 occupied homes, 61% heat with utility gas and 33% with electricity.
Across the 4 places we cover here, Belleville has the oldest housing (26% before 1940) and O'Fallon the newest (45% built in 2000 or later). Belleville is the largest, at 20,212 homes, and O'Fallon has the most electric heat (40%).
| Place | Homes | Before 1940 | Median built | Electric heat | Owner-occupied |
|---|---|---|---|---|---|
| Belleville | 20,212 | 26% | 1962 | 32% | 60% |
| O'Fallon | 13,530 | 5% | 1997 | 40% | 70% |
| Fairview Heights | 8,179 | 4% | 1972 | 39% | 70% |
| Swansea | 6,175 | 5% | 1993 | 31% | 73% |
Against the eight-county metro on this site (1,177,290 homes), St. Clair County's pre-1940 share of 13% compares with 16%, and its electric-heat share of 33% with 32%. 4,656 occupied homes (5%) burn propane or other tank gas and 433 (0%) heat with wood, which matters because neither fuel comes with a gas utility's program attached. 68% of occupied homes are owner-occupied and 72% of all homes are single-family detached houses.
In St. Clair County that adds up to 41,328 homes from the 1950s to the 1970s, the usual candidates for an attic top-up; 25,986 built since 2000, where air sealing tends to matter more than extra depth.
| Built | Homes | Share | |
|---|---|---|---|
| Before 1940 | 15,578 | 13% | |
| 1940s | 6,628 | 6% | |
| 1950s | 15,055 | 13% | |
| 1960s | 13,973 | 12% | |
| 1970s | 12,300 | 11% | |
| 1980s | 12,538 | 11% | |
| 1990s | 13,572 | 12% | |
| 2000s | 16,452 | 14% | |
| 2010s | 8,494 | 7% | |
| 2020 or later | 1,040 | 1% |
Figures: U.S. Census Bureau, American Community Survey 2020–2024 5-year estimates (tables B25001, B25003, B25024, B25034, B25035, B25040). Survey estimates carry margins of error, widest in the smallest places. Census table.
St. Clair County 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. The rebate guide carries the full rules.
Nothing published settles it. None of the 4 jurisdictions we cover in St. Clair County lists insulation as permit work or as exempt, so the answer comes from a call to the building department before the job starts. Each town page gives that department's number.
R-49 for new work, in all 4 jurisdictions we cover in St. Clair County (Belleville, Fairview Heights, O'Fallon, Swansea). The Department of Energy's recommendation for an uninsulated attic in this climate zone is higher, at R60.
The Census Bureau's 2020–2024 estimates put 13% of St. Clair County's 115,630 homes before 1940 and 55% before 1980, with a median year built of 1975.
For most addresses across St. Clair County it is Ameren Illinois, whose 2026 table 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, taken off a registered Program Ally's price once the program's home energy assessment confirms the house is eligible.
Yes, through the Illinois low-income weatherization program, which St. Clair County Community Action Agency runs for St. Clair County (618-277-6790). If the household income qualifies, the work can be free rather than discounted.
Free assessment, a measured R-value, and photos you keep either way.
Book my free assessment →