Spray Foam Insulation in St. Louis

Spray foam pays its way in a few spots in a St. Louis house: the rim joist on top of the foundation, crawl space and basement walls, the underside of a roof deck when an attic is brought inside, and wall cavities left open by a remodel. On an open attic floor, blown insulation does the job for much less.

Picking the foam is a moisture call as much as an R-value one, because the metro is climate zone 4A, mixed-humid. We start with a free assessment, measure the R-value you have and find where air moves, then name the foam, the thickness and any covering the code requires.

Open-cell or closed-cell: what is the difference?

Closed-cell is dense, rigid and slows water vapor. Open-cell is soft, light and lets vapor through. Both stop air. The federal figures:

R-value per inch: open-cell about R-3.5 to R-3.6, closed-cell about R-6 to R-6.5.
Density: open-cell about half a pound per cubic foot, closed-cell about 2 pounds.
Vapor: open-cell rates 5 to 10 perms at 5 inches thick; closed-cell rates under 1 perm at 2 inches, which makes it a vapor retarder, since the cutoff is 1 perm.
Liquid water: open-cell can soak it up and hold it; closed-cell does not absorb it.
Application: open-cell goes to full depth in one pass; closed-cell typically goes on 2 to 3 inches per pass.

Closed-cell costs more per inch because it expands far less, so each inch holds more material. Figures vary by product, so a quote should name the foam and its rated R-value.

Where does each kind belong in a St. Louis house?

Rim joists. Batts in these bays block little air, and DOE notes they typically go in with no air sealing at all. DOE's Building America guidance rates closed-cell preferred and open-cell acceptable here, and DOE's rim joist guide reserves extra vapor caution for zone 6 and colder. St. Louis is 4A.
Crawl space and basement walls. Closed-cell, not open-cell. DOE rates open-cell not acceptable below grade, where stray water can soak into it. ENERGY STAR's zone 4A target for these walls is R-10 of insulating sheathing or an R-13 batt.
Under the roof deck. For an attic being brought inside, or a cathedral ceiling, DOE prefers closed-cell in every climate and accepts open-cell in a cold one if indoor humidity is controlled. Either may need an ignition or thermal barrier, depending on the product.
Open wall cavities. In a gutted frame wall DOE rates both foams preferred for cold climates. Closed walls are covered on the wall insulation page.
The attic floor. Rarely. DOE notes open-cell's R per inch there is not much above far cheaper cellulose or fiberglass, so foam's job is sealing gaps under the blown layer; see attic insulation.

Installer spraying foam insulation onto rim joists in a crawl space lined with foil-faced boards

What does spray foam insulation cost in St. Louis?

This page carries no price, because two jobs both called spray foam can be far apart in material and labor. What sets the number:

Foam type and thickness. Closed-cell costs more per inch.
Surface. Rim joists run in linear feet; walls and roof decks in square feet.
Access. An open basement is easy. A low crawl space or a finished ceiling over the rim is not.
Prep. Old batts come out, and any leak, pest or wiring problem has to be dealt with before foam hides it.
Covering. Whether the code needs an ignition or thermal barrier over the foam.

The free assessment covers your current R-value, the air leaks, an infrared scan and photographs, written up for you to keep whether or not you hire us. The proposal that follows prices every item on its own line.

On the Illinois side, Ameren Illinois' 2026 table pays $2.50 a linear foot for rim joist insulation and $3.75 a linear foot for crawl space walls, the latter possibly needing a ground vapor barrier. Those discounts can only be applied by a contractor registered as an Ameren Illinois Program Ally, and they reduce the contractor's invoice rather than arriving as a check. On the Missouri side, Ameren's 2026 Pay As You Save sheet has neither line; its air sealing incentive of $0.06 per square foot covers crawl space penetrations among other spots, provided leakage drops at least 20 percent, for tier 4 participants who have been in the home a full twelve months. Cuivre River Electric, which serves parts of St. Charles County, offers basement and crawl space insulation money only with a ground source heat pump and publishes no amount. And the federal 25C credit closed at the end of 2025; property placed in service in 2026 does not qualify.

How long does it take, and do you have to leave the house?

The assessment takes about an hour. The install depends on how much foam goes in and how hard the spot is to reach: an open basement rim joist is a small job next to a tight crawl space or a roof deck over a finished room. Removing old insulation, moving belongings, building thick closed-cell layers pass by pass and adding a code covering all add time.

Plan to be away. EPA's guidance: occupants leave during installation and stay out until the foam has cured and the house has been ventilated and cleaned. Re-entry time depends on the formulation; EPA reports that some manufacturers recommend 24 hours for professional two-part foam, and that it varies. EPA advises getting that guidance from your contractor ahead of time, so ask for the figure for the exact product.

What is included in a spray foam job?

Existing insulation measured and photographed before any price.
Leaks located by infrared, with a blower door reading at the start and finish when the job warrants one.
Floors and stairs covered; old batts cleared from the bays.
Hazards first: vermiculite is referred out and left undisturbed, and live knob-and-tube wiring stops the work, since the National Electrical Code bars it from hollow spaces filled with foamed-in-place insulation around the conductors.
Foam sprayed to the thickness in the proposal, in passes the product allows; any covering the code calls for is written into the proposal.
Finished work photographed for your report; any permit the town requires pulled by us.

Does spray foam have to be covered? What the code says

In living space, yes. The International Residential Code's foam plastic section (R316 in the 2015 edition) requires a thermal barrier between foam and the interior; half-inch gypsum board is the common one. Two exceptions matter in a house:

Attics and crawl spaces entered only for repairs or maintenance may use a lighter ignition barrier, such as 1-1/2 inches of mineral fiber insulation, 1/4-inch wood structural panel or 3/8-inch gypsum board, or none if the foam passed the fire tests the code accepts instead.
Rim joists and sill plates may stay bare when the foam is no thicker than 3-1/4 inches, weighs 0.5 to 2.0 pounds per cubic foot, and rates a flame-spread index of 25 or less and a smoke-developed index of 450 or less.

An attic used for storage is not repairs-only space. Editions differ by town: the City of St. Louis enforces the 2018 IRC, Kirkwood and Clayton the 2021, Brentwood the 2024, and Swansea and Waterloo the 2006. DOE notes the bare-rim allowance covered only dense closed-cell foam until the 2009 IRC, which may matter in those two towns. We confirm the covering with your building department before writing the proposal.

Do you need a permit for spray foam here?

Mostly there is no written answer. Among the 45 jurisdictions on this site, 42 publish no rule we could find on whether insulation needs a permit. The other three:

Unincorporated St. Louis County exempts attic top-ups and wall insulation behind new siding, but wants a permit for insulation, furring and drywall on the inner face of an outside wall or a basement wall. Foam on a basement wall that will be framed and finished is in that group.
Wildwood uses the county's list.
Washington exempts residential remodels that leave structure, wiring and plumbing alone, or that cost under $1,000.

Elsewhere we call the building department before scheduling, and any permit is pulled as part of the job.

Why is the floor cold over the basement or crawl space?

Look first at the rim joist. That band of framing on top of the foundation is all seams, sill to foundation, sill to rim, rim to subfloor, and winter air moves through them while fiberglass pushed into the bays does little to stop it. Ameren Illinois defines its rim joist measure as sealing leaks and insulating the band where the house sits on its basement or crawl space walls: this exact spot. Foam sprayed into each bay seals and insulates in one step, and an infrared scan shows whether the cold edge is gone. Over a vented crawl space, the rest of the answer is on the crawl space insulation page.

What is different about spray foam in St. Louis?

Moisture moves both ways. The metro is climate zone 4A, mixed-humid. NOAA's 1991-2020 normals put Lambert at 4,433 heating degree days to 1,713 cooling degree days, with 84.9 nights at or below freezing and 47.1 days at 90°F or hotter in an average year. DOE's guidance says cold-weather moisture comes mainly from indoors and warm-weather moisture mainly from outside, and a St. Louis wall sees both.
The money depends on the river. A rim joist earns $2.50 a linear foot on Ameren Illinois' 2026 table through a registered Program Ally; Ameren Missouri's 2026 sheet has no rim joist line.
The rules depend on the town. Missouri leaves residential building codes to each city and county, so editions on that side run from the 2009 IRC in Creve Coeur to the 2024 in Brentwood. Illinois sets one energy code statewide, the 2024 IECC with state amendments since November 30, 2025, and its cities generally cannot loosen or tighten it for houses. See the code and rebate guide.

Common questions about spray foam insulation in St. Louis

Is closed-cell spray foam worth the extra cost?

Where water is possible, yes. Below grade and on foundation walls DOE's guidance rates closed-cell preferred and open-cell not acceptable, because closed-cell does not take up water and slows vapor. At the rim joist either works in our climate, and in a gutted frame wall DOE rates open-cell preferred too. Open-cell costs less per inch because it expands more.

Can spray foam be left exposed in my basement?

On the rim joist, yes, if it is 3-1/4 inches thick or less, 0.5 to 2.0 pounds per cubic foot, and within the code's flame-spread and smoke limits. On the walls of a basement you use, the residential code wants a thermal barrier such as half-inch drywall over it. Your town's edition settles the details.

How long do we stay out after spray foam goes in?

Until the foam has cured and the house has been aired out and cleaned, by EPA's guidance. EPA gives no single number; it reports that some manufacturers recommend 24 hours for professional two-part foam and that the time varies by product. Get the figure for the exact foam before the job is booked.

Does Ameren pay anything toward spray foam?

The programs pay by measure, not by material. Ameren Illinois' 2026 table sets $2.50 for each linear foot of rim joist and $3.75 for each linear foot of crawl space wall, applied by a registered Program Ally. Ameren Missouri lists neither for 2026; its Pay As You Save air sealing incentive of $0.06 per square foot needs a measured 20 percent leakage cut. No federal 25C credit applies to 2026 work.

Should I spray foam my attic floor?

Usually not. Blown cellulose or fiberglass gives far more R for the money there, and DOE's guidance says open-cell's R-value per inch is not much higher than those fills. On an attic floor, foam earns its place as a thin seal over gaps and top plates before the blown layer, or on the roof deck when the attic is brought inside.

Where we work

Spray foam 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: