Insulation and air sealing for Unincorporated St. Louis County homes: attics, crawl spaces, rim joists and walls, starting with a free assessment of the R-value you actually have. Unincorporated St. Louis County enforces the 2015 IECC, which sets R-49 for a new attic.
This page covers the parts of the county that are not inside a city — where the county, not a municipality, runs building and permits. The Census does not publish population or housing totals for that remainder on its own, so this page does not quote any: the figures you will see elsewhere for “St. Louis County” include every municipality in it as well.
For St. Louis County as a whole, municipalities included, the Census Bureau's 2020–2024 estimates give a median year built of 1969, with 10% of its 446,192 homes built before 1940. The unincorporated remainder has no figures of its own.
We have not logged a completed job in Unincorporated St. Louis County yet. We would rather say so than put a number on this page that is not real.
The county's Historic Buildings Commission maintains a countywide National Register list, but districts are attributed to specific municipalities rather than the unincorporated area as a whole.
Unincorporated St. Louis County enforces the 2015 IECC. The State of Missouri's building-codes-by-county list on the Missouri Data Portal, which the Department of Natural Resources points to as the state's energy-code-by-jurisdiction source, lists St. Louis County as 2015 IECC, 2015 IRC and 2015 IBC. OneSTL, the regional plan site run by East-West Gateway Council of Governments, also states that St. Louis County adopted the 2015 IECC. The county's building code chapter is Chapter 1115 SLCRO and its residential code is Chapter 1116 SLCRO; both were most recently reissued by County Ordinance 27,654, approved December 18, 2019 and effective April 1, 2020, as confirmed by the cities of Berkeley, Breckenridge Hills and Wildwood, which adopt those county chapters by reference.
For new work, the attic number under that edition is R-49, with walls at R-20, or R-13 cavity plus R-5 continuous. Set that against the Department of Energy, which recommends R60 for an uninsulated attic in this climate zone and R49 for one that already has a few inches.
2015 IECC Table R402.1.2, climate zone 4 (except marine), as reproduced in the U.S. Department of Energy / PNNL Building America Solution Center comparison of 2009-2021 IECC and IRC minimum insulation requirements. These values follow from the 2015 IECC edition the state's list attributes to the county.
The St. Louis County Department of Public Works, Division of Code Enforcement publishes a list of 35 kinds of work that do not require a building permit. Item 35 exempts increasing attic insulation and installing exterior wall insulation under new or replacement siding. The same item states that a building permit is required to install insulation, furring strips and drywall on the room side or the interior basement side of exterior walls. Weatherstripping, caulking and air sealing are not named in the list.
St. Louis County licenses electrical, plumbing and mechanical contractors, and municipalities in the county require those trades to hold a current county license: Ballwin requires permit-pulling electrical, plumbing and mechanical contractors to hold an active St. Louis County license in their field, and University City issues electrical permits only to contractors licensed by St. Louis County.
The published number is 314-615-5184. The department page is here.
Permitting is on us, not on you.
Most Unincorporated St. Louis County addresses are Ameren Missouri electric customers, and Ameren Missouri pays toward insulation through Pay As You Save: $0.20 per square foot on the attic and $0.06 per square foot on air sealing in the 2026 program year, with the rest of the cost recovered through the electric bill. It counts only when the attic is at R-20 or below beforehand and R-38 or more afterward, when air sealing cuts measured leakage by 20 percent, and when the household has lived there for the past year; for income-eligible households, CommunitySavers can include attic insulation at no charge. Income-qualified households can apply to Community Action Agency of St. Louis County (CAASTLC) (314-863-0015) for free weatherization instead. The St. Louis County page and the rebate guide carry the full rules.
Every layer of an insulated attic, explained
Elsewhere in St. Louis County we also cover:
Missouri sets none of this at state level. Across the 7 places in this table there are 3 different adopted editions (2015 IECC, 2021 IECC and 2024 IECC), and 2 of the 7 publish a permit rule for insulation.
| City | Energy code | Attic R required | Permit to insulate | Building dept. |
|---|---|---|---|---|
| Unincorporated St. Louis County | 2015 IECC | R-49 | Not for an attic top-up | 314-615-5184 |
| Affton | 2015 IECC | R-49 | Not for an attic top-up | 314-615-5184 |
| Ballwin | Not published | — | Not published | 636-227-2129 |
| Brentwood | 2024 IECC | R-49 | Not published | 314-963-8602 |
| Chesterfield | Not published | — | Not published | 636-537-4000 |
| Clayton | 2021 IECC | R-60 | Not published | 314-290-8463 |
| Creve Coeur | 2015 IECC | R-49 | Not published | 314-872-2500 |
All 21 St. Louis County jurisdictions side by side →
Unincorporated St. Louis County enforces the 2015 IECC, which sets R-49 for an attic in new work. Whether it reaches a retrofit of an existing house is not published.
The St. Louis County Department of Public Works, Division of Code Enforcement publishes a list of 35 kinds of work that do not require a building permit. Item 35 exempts increasing attic insulation and installing exterior wall insulation under new or replacement siding. The same item states that a building permit is required to install insulation, furring strips and drywall on the room side or the interior basement side of exterior walls. Weatherstripping, caulking and air sealing are not named in the list.
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.
Code and permit facts checked September 10, 2026, rebates October 24, 2026; housing: Census ACS 2020–2024. Sources: data.mo.gov · dnr.mo.gov · onestl.org · cityofwildwood.com · ecode360.com (2, 3) · basc.pnnl.gov · manchestermo.gov · stlouiscountymo.gov (2, 3) · en.wikipedia.org · censusreporter.org · ameren.com (2, 3, 4) · energystar.gov. These change; confirm with the building department at 314-615-5184 or your electric utility first.
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