Rim Joist Spray Foam: Sealing the Band Above the Foundation

Is rim joist spray foam for you?

A rim joist is the outer board that closes the ends of the floor joists, resting on the sill plate over the foundation. In many houses it is the one stretch of the thermal boundary you can see and touch from the basement, and it is usually the leakiest. Building Science Corporation, writing for the U.S. Department of Energy, calls it "a particularly troublesome detail" (Information Sheet 408). The reason is that several separate pieces meet there: the foundation, the sill plate, the rim board and the subfloor.

Why the rim leaks and sweats

Building Science Corporation says the inside face of a rim joist is a cold surface in winter, with a condensation risk, and that the concrete-to-sill-plate joint is often a source of significant air leakage. Its "critical seal" idea is simple: spray foam applied to the relevant surfaces so they join as one air barrier. The same sheet says a foam density of about 2 pounds per cubic foot (the closed-cell range) gives acceptable results, and that for air-barrier purposes the foam must connect from the foundation wall, up the rim joist, to the floor sheathing.

Source note: that sheet is a national DOE Building America publication, not a Kansas document. We use it for how air, wood and cold behave, which is the same in Johnson County as anywhere else. It does not tell you what your city requires.

What foam does at the rim:

• Seals the gap between foundation and sill plate, where cold air enters.
• Seals the rim board to the subfloor above.
• Keeps damp room air from reaching the cold board, because closed-cell foam is stuck to it.
• Adds insulation. Your foam product will have its own table, which we will show you.

Code path: the 3 1/4 inch exception

In a house, foam is normally separated from the room by a thermal barrier. In the 2018 International Residential Code, section R316.4 sets that barrier as 1/2-inch gypsum board, 23/32-inch structural wood panel, or a material passing the NFPA 275 tests. Section R316.5.11 is the exception for sill plates, headers and the perimeter joist space. It allows foam without that barrier if all three of these are true:

Our text comes from the Kansas copy of the 2018 IRC on the UpCodes viewer. The 2021 text, read on the Colorado copy, is the same. Closed-cell foam near 2 pounds per cubic foot sits inside the density band. The thickness and flame-spread numbers come from the specific product, which is why the next section matters more than the table.

What the exception does not say: it does not say your jurisdiction uses this edition, it does not say a product is approved at that thickness, and it does not tell you that nothing ever needs covering. The IBC version, used for buildings that are not houses, is narrower: it is limited to Type V construction and uses a density band of 1.5 to 2.0 pounds per cubic foot (IBC 2603.4.1.13). If your building is not a one- or two-family house, say so at the visit.

Which edition applies is local. Johnson County's page says the county adopted the 2018 International Codes on March 12, 2020, and that its commissioners will consider the 2024 codes on Nov. 12, 2026, effective Jan. 1, 2027 if adopted. Cities inside and outside the county adopt on their own schedule. Permit questions go to your building office; the pointer page is permits and adopted codes.

The foam's own report limit governs

A code section sets a ceiling. The product's evaluation report sets what that product was tested to. They are not the same number. In an ICC Evaluation Service report for one closed-cell foam (ESR-3228, reissued January 2026), the stated limit for sill plates, band joists and headers without a thermal barrier is 2 inches, in Type V construction. That is thinner than the code's 3 1/4.

So the working rule on our jobs is: the thinner of the code limit and the report limit. About 2 inches fits both. We never go past what the report for the foam in the drum allows.

Two practical points follow.

1. The report in the example above is for a different product from the one we will buy and is cited only to show that the product limit can be tighter than the code. We name no maker.
2. A foam that has no current report for rim joist use is not a foam we spray there.

How thick

Thickness at a rim joist is a balance between three limits and one goal.

Foam also goes on in passes. Maker data sheets we opened limit one pass to about 2 inches and ask for cooling time before a second pass; we follow the sheet for the product in use. Thicker is not better here: over 3 1/4 inches the barrier exception is gone, and deep passes bring heat. For the general thickness method, see how thick closed-cell foam should be.

Moisture and termite inspection notes

This is the part that separates a good rim job from a sealed-in problem.

• Dry wood first. Foam is not sprayed onto wet, frosted or dew-covered surfaces. We check the rim board and sill by eye before spraying.
• Find the water source. Gutters that dump at the foundation, a window well that fills, a plumbing leak above: these are fixed before foam, not covered by it.
• Rot and insects. Soft wood, frass or mud tubes are seen at the visit, and spraying stops until a pest professional or carpenter has dealt with them. We do not diagnose or treat pests.
• Termite inspection gap. The Department of Energy rim joist job aid says that where termite pressure exists, a 3-inch inspection gap is kept from the top of the insulation to the bottom of any wood so termites can be detected. The same aid says this varies by region. The report example above also says that in "very heavy" termite areas, foam must meet IBC 2603.8. Whether and how much your area is affected is not a fact we have.
• Do not spray what must breathe or burn. Electrical boxes, flues, chimneys, heaters and recessed lights are not foamed over, per the report example's limits on use.
• Penetrations. Wires and pipes through the rim are sealed as part of the air barrier; holes larger than a quarter inch are packed with a pest-proof material before sealing, per the same DOE job aid.

Covering the foam

For an unfinished basement, the exception above lets rim foam stay exposed at or under the lesser of the code and report limits, as long as the building official in your jurisdiction agrees. The Department of Energy rim joist job aid is stricter than the IRC text: it says foam over 3.25 inches, or in a permanently habitable space, needs a thermal barrier such as gypsum board, and it tells readers to consult local code officials if their area differs. When the rim is in finished living space, we follow that stricter reading.

The covering, where one is needed, is a drywall or coating question. We spray; we tell you at the visit whether your rim needs a covering. We do not install drywall. The full explanation of what covers foam and the questions to ask are on what must cover spray foam. The same covering question for the wall below the rim is on basement wall spray foam.

The day of the job

The U.S. EPA says building occupants and other workers should vacate during installation, and that re-entry should be restricted until the product has cured and the building has been ventilated and cleaned. It adds that some manufacturers recommend 24 hours, but the time may vary, so the data sheet for the foam in use sets ours. Only crew members in protective equipment are present during spraying. Pets and houseplants leave too; that is our addition, not the EPA's. Rim work is short compared with a whole attic, but the house is still not occupied during it.

Where we stand

Spray foam insulation is a new PolyJacks service line run by our own crew and rig. We have not yet finished an insulation job, so nothing here describes past work. We do not publish photos or counts we do not have.

FAQ

Does code let rim joist foam stay uncovered?

In the 2018 residential code, yes within three limits: no more than 3 1/4 inches, a density of 0.5 to 2.0 pounds per cubic foot, and a flame spread of 25 or less. The foam's report and your building office have the final say.

Is rim joist insulation the same as insulating the basement wall?

No. The rim sits above the foundation wall, is wood, and has its own exception. The wall below is concrete and falls under the thermal barrier rule. See basement wall spray foam.

Will you spray a rim joist that looks damp?

No. A damp or soft rim is fixed first; foam sealed over water holds the water against the wood.

Can I do the covering myself?

Where a covering is needed, who installs it is something we settle at the visit, and your building office decides what is accepted.

Book a free on-site measurement

A drive-by cannot show what is inside your basement, so the estimate starts with a visit. We look at the rim from the inside, check the wood, note the access and measure the length. You get a written estimate after the visit, valid for 2 months. The visit is free.

Book a free on-site measurement

Pick "Spray foam insulation" on the booking form. We work within about 35 miles of Spring Hill, KS.

Who wrote this: PolyJacks spray foam team. Last updated: October 27, 2026. How it was made: drafted with AI assistance from the sources below, then checked against each source.

Sources

• 2018 International Residential Code, Kansas copy (UpCodes), R316.4, R316.5.1, R316.5.11, opened 2026-10-26: up.codes (2021 text compared on up.codes; the same code in every state, so the text applies to Kansas, Missouri and everywhere the model code is adopted)
• Kansas Building Code 2018 (IBC), section 2603.4.1.13 and 2603.8 reference: up.codes
• Johnson County, KS, Building and Construction codes page: jocogov.org
• Building Science Corporation, Information Sheet 408, "Critical Seal (Spray Foam at Rim Joist)", for U.S. DOE Building America: www1.eere.energy.gov
• U.S. Department of Energy, Weatherization Installer Job Aid 12-2 "Insulate Rim Joist" (June 2024): energy.gov
• ICC Evaluation Service report ESR-3228 (example of a product report limit; not our product): icc-es.org
• U.S. EPA (archive), "Vacate and Safe Re-Entry Time for Spray Polyurethane Foam Application": archive.epa.gov