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 early-2000s, and 1% of its 36,206 homes went up before 1940.
O'Fallon runs to about 31 square miles and roughly 94,074 people. It sits about 31 miles northwest of downtown St. Louis. Around 36,206 housing units sit inside those boundaries. The community dates back to 1912. We cover ZIP codes 63366 and 63368.
No completed job in O'Fallon is logged in our records yet, and this page will say so until one is.
The typical home in O'Fallon dates to around the early-2000s. 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.
First settled in 1856 and incorporated in 1912, O'Fallon's housing is dominated by rapid recent growth; the 1907-era St. Mary's Institute building (National Register, 2007) is a rare pre-20th-century holdover amid the newer subdivisions.
The Census Bureau estimates 36,206 homes in O'Fallon. 280 of them (1%) went up before 1940 — 37th of 44 places on this site for pre-1940 housing. The 1990s were the busiest decade, adding 11,382 of today's homes (31%); 9% predate 1980 and 50% were built in 2000 or later. For St. Charles County as a whole the pre-1940 share is 2%, so O'Fallon sits below it.
Of 35,471 occupied homes, 71% heat with utility gas and 27% with electricity. Electric heat here is less common than across the county (30%), and ranks 32nd of 44 on this site. Which utility program can contribute depends on the heating fuel and on who sends the bill, so that is the first thing an assessment pins down.
81% of occupied homes are owner-occupied and 75% of all homes are single-family detached houses, with 1,217 mobile homes in the count. On a housing mix like this, the likely starting points are air sealing on newer houses that already have depth and an energy assessment before deciding what to fix first.
| Built | Homes | Share | |
|---|---|---|---|
| Before 1940 | 280 | 1% | |
| 1940s | 123 | 0% | |
| 1950s | 666 | 2% | |
| 1960s | 1,150 | 3% | |
| 1970s | 1,119 | 3% | |
| 1980s | 3,468 | 10% | |
| 1990s | 11,382 | 31% | |
| 2000s | 10,946 | 30% | |
| 2010s | 5,726 | 16% | |
| 2020 or later | 1,346 | 4% |
36,206 homes, Census ACS 2020–2024 (B25034, B25040, B25003); 35,471 occupied.
18,018 of O'Fallon's 36,206 homes (50%) were built in 2000 or later. Depth is rarely what is missing in houses this new; air paths are: attached garages, bonus rooms over them, the chase around a high-efficiency flue, and the thin edge where blown fiber meets the wall plate. They usually earn their money back on sealing rather than on more fiber.
O'Fallon enforces the 2021 IECC. Section 500.120 of the O'Fallon, Missouri code adopts the International Energy Conservation Code, 2021 Edition, as the Energy Conservation Code of the City of O'Fallon. The adopting ordinance is Ordinance No. 7133, adopted 9-12-2024, the same ordinance that adopted the 2021 International Building Code with Appendix Chapter K. This is O'Fallon in St. Charles County, Missouri, not O'Fallon, Illinois.
Nothing O'Fallon publishes settles this. Its permit lists name the work types it cares about, and insulation is not among them in either direction. The Building and Permits page lists residential permits for new homes, decks, sheds, pools, basement finishes, additions and major alterations, and a small-projects list covering fences, decks, porches, sunrooms, patio covers, pergolas, sheds, pools, hot tubs, retaining walls, water heaters and HVAC replacement. So we ask the department directly before the first day on site, and whatever comes back, pulling it is our job rather than yours.
Not published for insulation contractors specifically. Section 605.020 requires that all merchants doing business within the City of O'Fallon purchase and display a City of O'Fallon business license, and Section 605.010 defines a merchant as a person, business or other legal entity doing business within the city. The code names no trade registration for insulation work.
The published number is 636-379-5660. The department page is here.
Permitting is on us, not on you.
O'Fallon is on the Missouri side, where most addresses buy their electricity from Ameren Missouri. Its Pay As You Save program publishes $0.20 per square foot on ceiling and attic insulation and $0.06 per square foot on air sealing for the 2026 program year; the incentive comes off the cost and the rest is repaid through a monthly charge on the bill. The conditions: an attic going from R-20 or below to at least R-38, air sealing that cuts measured leakage by 20 percent, and twelve months in the home. CommunitySavers can include attic insulation at no charge for income-eligible households. Income-qualified households can apply to North East Community Action Corp. (NECAC) (573-324-2231) for free weatherization instead. The St. Charles County page and the rebate guide carry the full rules.
Every layer of an insulated attic, explained
O'Fallon borders Lake St. Louis and St. Peters. These are the ones we also cover, each with its own energy code and permit rule:
Elsewhere in St. Charles County we also cover:
Missouri sets none of this at state level. Across the 6 places in this table there are 3 different adopted editions (2015 IECC, 2021 IECC and 2021 IRC ch. 11), and none of the 6 publishes a permit rule for insulation.
| City | Energy code | Attic R required | Permit to insulate | Building dept. |
|---|---|---|---|---|
| O'Fallon | 2021 IECC | — | Not published | 636-379-5660 |
| Lake St. Louis | 2015 IECC | R-49 | Not published | 636-625-1200 |
| St. Peters | 2021 IRC ch. 11 | R-38 | Not published | 636-477-6600 ext. 1670 |
| Cottleville | 2021 IRC ch. 11 | R-38 | Not published | 636-498-6565 |
| St. Charles | 2015 IECC | R-49 | Not published | 636-949-3200 |
| Wentzville | Not published | — | Not published | 636-327-5101 |
All 6 St. Charles County jurisdictions side by side →
O'Fallon does not publish the energy code edition it enforces, so we do not state a required figure here. The Department of Energy recommends R60 for an uninsulated attic in this climate zone and R49 where there are already a few inches.
Not on paper: O'Fallon names neither a permit nor an exemption for insulation, so one call to the department at 636-379-5660 settles it first.
For most Ameren Missouri customers, Pay As You Save: $0.20 per square foot on an attic starting at R-20 or lower and $0.06 per square foot on air sealing in 2026, the rest repaid on the bill. The federal 25C credit ended for work placed in service after December 31, 2025.
The typical home in O'Fallon dates to around the early-2000s; the Census Bureau's 2020-2024 median year built is 2000. Of its 36,206 homes, 1% went up before 1940 and 50% in 2000 or later. That is a median across the whole place, so your own house may be older or newer.
Code and permit facts checked September 10, 2026, rebates October 24, 2026; housing: Census ACS 2020–2024. Sources: ecode360.com (2, 3, 4) · ofallon.mo.us · ofallonmo.gov · 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 636-379-5660 or your electric utility first.
Free assessment: what depth you have now, what it has settled to, and what the sealing is worth.
Book my free assessment →