Best by project · Written September 8, 2026 · Based on the 2021 IRC; your local code may differ
Can this foam legally stay bare? Four numbers on the can decide it.
The rim joist, or band joist, is the strip of framing at the edge of each floor where the joists sit on the sill. It is the leakiest part of most older houses, and it has its own prescriptive exception in the residential code: foam plastic spray applied to sill plates and headers may be left without a thermal barrier where it stays inside four numeric limits.
The four limits are on the foam, not on the house: a maximum installed thickness of 3-1/4 in, a density between 0.5 and 2.0 pounds per cubic foot, a flame spread index of 25 or less, and a smoke-developed index of 450 or less. The code section does not say this, but it matters: a surface-burning result only describes the thickness and setup it was tested at, so read the report as well as the number. A can that publishes none of those figures cannot be shown to qualify.
The assembly decides the product
IRC R316.5.11 allows spray foam on sill plates and headers without a thermal barrier where installed thickness, density and both surface-burning indices are within its limits. Every one of those four is a number the maker has to publish before the exception can be shown to apply.
Each bay is a small box, 9-1/4 to 11-7/8 in tall by the joist spacing, with the mudsill, the treated board bolted flat to the foundation, and the subfloor above. Foam reaches the corners a batt never does, which is why the rim joist is the classic DIY foam job.
A rim joist that shows staining or rot has a water problem; foam over it hides it. Dry it and fix the cause first.
What the code says
Section numbers are from the 2021 International Residential Code. Your city or county adopts one edition of the code and changes parts of it, and your local building official has the final word. A thermal barrier and an ignition barrier are different things, and the glossary explains both.
Products that document this job
Listed by product family. Each line says why the product is on this list, always from something the maker published. Follow the link for the documents, the questions still open, and dated prices.
Quantity, worked once
Board feet are square feet times inches. The kit figures are the makers’ theoretical yields, which every maker that defines the term says are laboratory numbers; plan on less.
Buy one 24-can kit or one Froth-Pak 630 and expect to come up a little short of the theoretical figure. The DAP route takes about 26 cans and costs more. Two inches leaves room under the 3-1/4 in ceiling for a second pass.
Run your own numbers in the calculatorQuestions
Sometimes, and only where it stays inside the four limits in IRC R316.5.11: installed thickness of 3-1/4 in or less, density of 0.5 to 2.0 pounds per cubic foot, flame spread index of 25 or less, smoke-developed index of 450 or less. Those are the maker’s numbers to publish, and the surface-burning result describes the thickness it was tested at. A can that prints none of them can still be used; it just cannot be shown to qualify for the exception.
Two inches is the common target: it air seals, it approaches the recommended wall R-value at the makers’ claimed R-5.66 per inch, and it stays under the 3-1/4 in limit in the exception with room for a second pass. Check the label for the maximum thickness per pass.
None of the single-can kits prints all four. HandiFoam FR prints a density and both indices at 2 in. DAP prints a density of 1.4 pounds per cubic foot and an E84 result at 2.75 in with a smoke index of 450, which is exactly the limit in the section. The gap between what each maker prints and what the exception asks for is on the product pages.
Step-by-step how-to and the full safety picture live on our sister site: Choosing the foam family by job, at Spray Foam Guides. This page connects the job to specific products and to what their makers publish. We have not sprayed or tested any of it, and none of it stands in for the label, the safety data sheet or what your local inspector tells you.