Rim joist thermal barrier
The rim joist in my unfinished basement is insulated with 3 inches of rigid polyiso along with one part spray foam to air seal around the edges. My understanding is that building code exempts up to 3 1/4 inches of spray foam in this area from the thermal barrier requirement, but that this exclusion does not apply to rigid foam boards. I have seen some postings on this site (http://www.qa-greenbuildingadvisor.com/community/forum/building-code-questions/36030/spray-foam-ignition-barrier) which suggest that installing R15 mineral wool over the rigid foam would meet the thermal barrier requirement, but I have not been able to find much in the way of actual documentation to support this assertion. Will the mineral wool actually meet code? What would be a code compliant way of installing 1/2″ gypsum over the rigid foam? The one part spray foam applied around the edges of the rigid foam has not created a ‘flat’ surface amenable to gluing the gypsum directly to the foam. Can I glue the gypsum to some sort of 1/4″ to 1/2″ spacer material (maybe more gypsum) and then glue the spacer to the central area of the rigid foam, thus avoiding the unevenly foamed edges?
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Replies
Michael,
Like you, I have not found any references in the code that allow the use of mineral wool insulation as a thermal barrier. (It can, however, in some cases be used as an ignition barrier.)
The IRC exception for rim joists (R316.5.11) applies only to spray-applied foam, not to rigid foam.
Here's what I have written on this issue:
"In section R316.4, the IRC defines a thermal barrier as 1/2-inch gypsum wallboard (a prescriptive thermal barrier) or a material that is equal in fire resistance to 1/2-inch gypsum wallboard. The required test to determine whether a material is equivalent to 1/2-inch drywall is the 15-minute fire test; for this reason, this type of thermal barrier is sometimes called a 15-minute barrier. Examples of materials that have passed the 15-minute fire test and are therefore considered equivalent to 1/2-inch drywall include certain types of spray-applied cementitious materials, spray-applied cellulose materials, and cement-based plaster."
The easiest way to proceed in your case is to use 1/2-inch drywall as a thermal barrier. You can use any kind of shim you want -- including wood scraps or drywall scraps -- behind the 1/2-inch drywall to make a co-planar surface. Either glue or screws can be used to attach the 1/2-inch drywall.
For more information, see Thermal Barriers and Ignition Barriers for Spray Foam.
Drywall sounds like the simplest solution. But just out of curiosity, could the foam be painted with an intumescent paint?
Does anyone know if 2x material would count as a 15 minute thermal barrier? I am thinking of using 2x8 blocking to cover the rigid foam on my rim joists. I would think it would be okay, given 3/4" plywood is approved, but I can't find any references.
Thanks,
Dave
It's not explicitly approved, and I can't find examples where framing lumber has gone through NFPA 275 testing. Your local code enforcement official might be willing to allow it as a judgement call, but otherwise I'd say no.
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