Crawl space insulation in St. Louis

What does crawl space insulation cost in St. Louis?

No price list here, because the number depends on what is under the house. The main drivers:

• The method. Floor insulation scales with floor area; a closed crawl space scales with the length of foundation wall and rim joist, plus ground cover and sealing.
• Cleanup first. Fallen or wet batts, old plastic and debris come out and are bagged before new work starts.
• Water. Any water getting in is traced in the report and dealt with before insulation goes in.
• Access and equipment. Low clearance and a small hatch slow the work, and ducts or a furnace down there change what closing it takes.

The free assessment covers all of it, and the proposal itemizes the sealing, the insulation, the target R-value and any utility money.

On Ameren Illinois' 2026 discount table, crawl space wall insulation earns $3.75 a linear foot and rim joist insulation $2.50 a linear foot. The discount comes off the contractor's price, only through a registered Ameren Illinois Program Ally and only after a home energy assessment shows the house qualifies; the crawl space line notes a ground vapor barrier may be required. Ameren Missouri's 2026 Pay As You Save sheet has no crawl space insulation line. Its air sealing incentive, $0.06 a square foot in 2026, lists crawl space penetrations among eligible areas, but it needs a 20 percent leakage reduction, tier 4 enrollment and 12 months living in the home; the balance goes on the electric bill as a monthly charge. Do not budget for the federal 25C credit either; nothing placed in service after 2025 qualifies.

How long does a crawl space job take?

Count on about an hour for the assessment. Plenty of fixes under a house are a day's work rather than a renovation; the schedule comes after we have been underneath, because what stretches a job is down there:

• water to handle first, from regrading to perimeter drainage where the crawl space floor sits below the yard;
• old batts, rotted wood or debris to clear;
• a gas furnace or water heater in the space, since DOE's steps call for sealed-combustion equipment before a crawl space is closed;
• the drying method the code requires for a closed space, which can mean another trade;
• in Illinois, the assessment and project application Ameren Illinois asks for before program work starts.

On the day, floors are protected, the space is photographed before and after, and the blower door runs before and after when the job calls for one.

What is included in a crawl space job?

Water and debris first, air sealing second, insulation last. A closed crawl space usually includes:

• Vent and moisture assessment, with any water source named in the report.
• Removal of old batts and debris, bagged and taken out.
• Air sealing at the subfloor, the sill plate and the rim joist.
• Closed-cell foam at the rim joist, explained below.
• A ground vapor retarder of 6-mil or thicker polyethylene, as Building America's crawl space guidance specifies, lapped and taped at the seams and sealed to the walls. The IRC asks for a 6-inch overlap; Building America recommends 12.
• Vents blocked and sealed.
• Foam on the foundation walls, board or spray, never fiber insulation, which DOE says collects and holds moisture against a crawl space wall.
• An ignition or thermal barrier over foam where code and product require one, such as gypsum board or an ignition-barrier coating.
• An insulated, weather-stripped access door.

A crawl space that stays vented gets a shorter list: the floor sealed from below, then new unfaced batts tight to the subfloor on supports so they cannot sag. Either way, photographs and the finished R-value come in writing.

Vented or encapsulated: which suits a St. Louis crawl space?

Building America's answer for this climate is encapsulated, with insulated walls, unless the house sits on flood-prone ground.

Vented, with the floor insulated. The code figure for a zone 4 floor over a crawl space is R-19 in every edition from 2009 to 2021, and ENERGY STAR recommends R19 for floors in zones 4A and 4B. The weak point is August. DOE's guide explains it: humid air comes in through the vents, batts between the joists are cool underneath and the joist edges cooler still, and once those surfaces drop below the dew point, water condenses and the damp batts insulate poorly. Lambert hits 90°F or hotter on 47.1 days in an average year (NOAA 1991–2020 normals), while air-conditioned rooms chill the floor from above.

Closed, with the walls insulated. For zone 4 crawl space walls, the 2009 through 2021 codes ask for R-10 continuous or R-13 in wall framing; ENERGY STAR's version is R10 insulative sheathing or an R13 batt. In practice that means foam. DOE's guide lists workable amounts: up to 2 inches of unfaced extruded polystyrene (XPS, R-5 per inch), 4 inches of unfaced expanded polystyrene, or 3 inches of medium-density closed-cell spray foam. Closing the space also brings any ducts and pipes down there inside the insulated shell.

Two limits: DOE's advice covers houses outside flood-prone areas, and a closed crawl space must meet the code conditions below.

Is the rim joist part of the job?

Yes, and it is often the first thing we fix. The rim joist sits on the sill plate around the top of the foundation, and air slips in where it meets the sill, the subfloor and the foundation wall. ENERGY STAR's list of common basement and crawl space leaks starts with the gaps between rim joists and under the sill plate.

DOE's guide says to air seal the rim and insulate it to the same R-value as the walls, with spray foam filling the joist bays for the most moisture control, or rigid foam cut to each bay and sealed at the edges. What it rules out is fiberglass pushed into the bays without sealing first. We use closed-cell foam at the rim. One caution from Building America: if the sill and rim may be wet from outside, check how they can dry before covering them, and leave them bare when in doubt.

In Illinois the rim has its own 2026 line, $2.50 per linear foot through a registered Program Ally for sealing and insulating the perimeter above basement and crawl space walls. Basements share the same board; see basement insulation.

Do you need a permit to encapsulate a crawl space here?

Rarely by any published rule, but we check before scheduling. Just three of the 45 jurisdictions we cover publish a rule that settles whether insulation work needs a permit, and none of them concerns crawl spaces. St. Louis County's no-permit list, which governs unincorporated St. Louis County and is the list Wildwood hands out, exempts adding attic insulation, requires a permit for insulating the room side of exterior walls, basement walls included, and says nothing about crawl spaces. Washington skips the permit for house remodeling that changes nothing structural, electrical or plumbing.

What a building department may care about is the switch from vented to unvented. In the IRC text DOE reprints (section R408.3, 2009 and 2012 editions), an unvented crawl space needs a continuous Class I vapor retarder over the exposed earth, joints overlapped 6 inches and sealed or taped, edges carried at least 6 inches up the stem wall and sealed. It also needs a drying method, such as a continuously running exhaust fan or a conditioned-air supply, sized at 1 cubic foot per minute per 50 square feet of crawl space floor, with the perimeter walls insulated. Your town's adopted edition sets the final wording, and DOE notes that some areas do not permit closed crawl spaces. If a permit is needed, we pull it with the job.

Why is the floor over my crawl space so cold?

Because the air under it is close to outdoor temperature, and some of it is rising into the house. A vented crawl space is built to let outside air through, and Lambert logs 84.9 nights a year that drop to freezing or lower (NOAA 1991–2020 normals). Batts that have sagged away from the subfloor make it worse: DOE's guide says pinched or drooping batts lose much of their R-value, because air loops through the gap above them.

The stack effect pulls that air upward. Warm air leaks out high in the house and is replaced from below, and Building America research cited by DOE found that 40 to 50 percent of the air in a typical home has been in the crawl space at one time. The usual fix starts with sealing the rim joist and the holes through the subfloor, then whichever insulation method suits the house.

What is different about crawl spaces in St. Louis?

• Two hard seasons. PNNL's county guide puts all eight metro counties in climate zone 4A, mixed-humid, and Lambert's 1991–2020 normals count 4,433 degree days of heating demand a year against 1,713 of cooling, so a crawl space here faces a cold floor and a muggy summer.
• Housing from every decade. Census figures (American Community Survey, 2020–2024) put the median housing unit in St. Louis County at 1969 and in St. Charles County at 1994; 68.6 percent of St. Louis County's units predate 1980.
• Rivers. Arnold, Eureka and Pacific have recorded Meramec River floods, and the Chesterfield Valley flooded when its levee failed in 1993. Near water, the close-the-vents advice needs a second look; see Arnold and Eureka.
• A code number that holds. Missouri towns adopt their own codes, but St. Peters, which amended its attic figure down to R-38, still lists crawl space walls at R-10 continuous or R-13. Metro East towns all follow the Illinois-amended 2024 IECC.
• Money that stops at the river. Per-foot crawl space discounts exist only in Illinois. Cuivre River Electric Cooperative, serving parts of St. Charles and Lincoln counties, ties its crawl space rebate to a ground source heat pump install and publishes no amount.

Common questions about crawl space insulation in St. Louis

Where we work

Crawl space insulation across greater St. Louis, from the city and St. Louis County out to St. Charles, Jefferson and Franklin Counties and across the river into the Metro East. Each city has its own page with the permit rule and code edition that apply there: