Does IRC preclude use of spray foam to insulate rim bands in unfinished basement? (re: ignition barrier)

My new home, currently under construction, has a full basement, half of which will remain unfinished, i.e., no drywall or ceilings. We intend to use that area for storage. My specs call for 2" foil-faced polyiso on the walls (below grade) and 6" oc-SPF in the rim band area (mostly above grade). Spraying the rim band is obviously a LOT easier than cutting & piecing foam board between the floor trusses and over the double end pins.

Since there's no ceiling, an alternate thermal and ignition barrier is required in the unfinished area. It's clear from the code and spray foam ESR reports that an approved intumescent coating satisfies the thermal barrier requirement. However, the same does not appear to be the case for the ignition barrier requirement.

In particular, the ESR reports for the products being considered only address the use of an alternative ignition barrier (i.e., coating) in crawl spaces (or attics) with service access only (i.e., 'no storage is permitted'). Also, the air in that area cannot be circulated to other parts of the building."  My HVAC design calls for return air to pass through the basement, including unfinished area.

If I want to keep this area unfinished (omitting the ceiling greatly facilitates future access to pipes, wires, dampers, etc), it seems I have two choices: either cover the spray foam with an approved material such as hardboard (re: R316.5.4.3) or cut-and-piece foil-faced polyiso into the rim band. Either option would be a pain in the butt.  I checked the ESR's for several different SPF products and all have the same limitations where there's no prescriptive ignition barrier.

Am I interpreting the ignition barrier requirements correctly?


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Depending on the type of floor joists you may already be required to cover the floor joists.  There are spray-on ignition barriers as well.

R316.5.11 says not more than 3-1/4 inches of certain density foam and some other verbiage need not be covered in the perimeter joist space.

use a spray foam that is approved as an ignition barrier … and while you are at it use a foam that has low Global Warming Potential, something like ; that's not an endorsement but an example. There is probably a thickness limit to the amount of foam that is approved for, maybe 4" in that products case but if you use closed cell that's plenty of r-value for rimjoist.

If you are up against a thickness limit then you can always add rock wool or equivalent to boost your r-value.

I also believe that the exception Peter refers to in R316.5.11 is an exception to the thermal barrier requirement. So if you don't meet those criteria for an exception (thickness of foam, density, or flame spread) then you just install the thermal barrier and you meet code so that would be acceptable in your example. Of course, I'm not your fire code official (or anyone's) so that is who knows for sure.

I also thought the Section R316.5.11 exception only applied to the thermal barrier requirement. After all, it specifically refers to the thermal barrier.  But as it turns out, I was confused about the relationship between thermal and ignition barriers. As it turns out, I was asking the wrong question. Allow me to explain...

I spoke with a senior technical rep at one of the foam mfrs whose job it is to know these things. He pointed out that the requirements I'm hung up on when using an intumescent coating as an ignition barrier (as described in his product's evaluation report) were specifically referring to spaces that are isolated from living space (i.e., attics & crawlspaces) and thus don't require a thermal barrier.

He explained that a thermal barrier is a higher hurdle than an ignition barrier (someone reading the evaluation report is presumed to know this). This relationship is evidenced by the intumescent coating thickness required to satisfy the former vs. the latter. Ignition barriers only come into play in isolated spaces where there's no requirement for a thermal barrier. In a non-isolated space such as my unfinished basement, I *must* have a thermal barrier, so I can ignore the ignition barrier requirements.

Bottom line, I can specify 6 inches of oc-SPF with an approved coating at a thickness that satisfies the thermal barrier requirement, which I already knew I could do.  OR, I can use the R316.5.16 exception and not apply a coating as long as I don't exceed the 3-1/4-inch depth limitation. In that case, I would need to use 2 lb foam to achieve my design R-value for the rim band. So it becomes a cost trade at that point, which only my installer can answer.

Thanks for the replies!


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